Biol. Rev. (1980), 55, pp. 65-92 Printed in Great Britain

65

SOLUTE MOVEMENT IN SUBMERGED ANGIOSPERMS

BY PATRICK DENNY Department of Botany and Biochemistry, Westfield College, The University of London,

London, NW3 7ST, England

(Received 17 October 1979)

CONTENTS I. Introduction . . . . . . . . . . . . . . 65

11. Morphological and anatomical characters . . . . . . . . . 66 111. Evidence for absorption of solutes in the natural environment . . . . . 68

(i) Plant distribution . . . . . . . . . . . . . 68 (a) The iduence of waters . . . . . . . . . . . 68 (b) The influence of sediments . . . . . . . . . . 71 (c) Discussion . . . . . . . . . . . . . 72

(ii) Elemental composition of plants and bio-accumulation of toxins IV. Experimental evidence for absorption and translocation . . . . . . 74

(ii) Direct evidence for absorption . . . . . . . . . . 75 (a) Uptake into shoots and leaves . . . . . . . . . . 75 (6) Uptake into roots . . . . . . . . . . . . 78

V. Long distance transport . . . . . . . . . . . . 78 (ii) Discussion . . . . . . . . . . . . . 82

VI. General discussion . . . . . . . . . . . . . 84 VII. Summary . . . . . . . . . . . . . . 85

VIII. Acknowledgements . . . . . . . . . . . . . 86

. . . . 72

(i) Transplant and culture experiments . . . . . . . . . 74

(i) Translocation of nutrient ions, . . . . . . . . . . 79 (iii) Translocation of heavy metals and herbicides. . . . . . . . 83

IX. References . . . . . . . . . . . . . . 86

I. INTRODUCTION

In lakes and rivers the photic zone, outside the emergent plant fringe, is usually occupied by floating-leaved and submerged, aquatic vascular plants. With the excep- tion of free-floating species such as Ceratophyllum demersum L. the submerged taxa are anchored to the substratum by their roots and have their shoots suspended in the water. This allows access to two independent sources of solutes; those in the water and those in the sediment. Nutrients such as nitrogen and phosphorus from aerobic sediment may not be readily available to water-suspended organisms (Fitzgerald, 1970) and nutrient supplies to rooted submerged macrophytes may depend upon the absorptive capactites of both the roots and the shoots. However, which tissue pre- dominates in absorption is a subject of conflicting opinions. Pearsall (1920), in his studies of the English Lake District, concluded that the main factor affecting the

0006-3231/80/0000-1850 $10.00 @ 1980 Cambridge Philosophical Society

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66 P. DENNY distribution of water-plants within a lake was the sediment, and correlations between concentrations of solutes in plants and soils suggested that absorption was through the roots. Haslam (1978) considers roots to be better adapted than shoots for uptake. Spence (1967), in his survey of the Scottish aquatic vegetation, could find no causal connection between distribution of species and the type of sediment, although he accepted that muds may provide nutrients; others (Olsen, 1953; Sutcliffe, 1959, 1962) have assumed that, as conducting tissue is reduced in submerged plants, absorption must be through the shoots.

Why do such differing opinions exist? There are several reasons. Water-plants have received less attention than terrestrial species and are often difficult experimental material. Submerged plants have little secondary thickening and rely upon their pneumatic skeleton, provided by aerenchyma, for support. Whole plants, manipulated and subjected to experimentation in the laboratory, require most delicate handling. Cultures are prone to infections and the conditions therein often provide more favourable environments for epiphytic and planktonic algae than for higher plants. Algicides, such as copper sulphate pentahydrate, are detrimental to higher plants, even in low concentrations (Ryan & Riemer, 1975), and cannot be used satisfactorily. Axenic culturing techniques (Gerloff & Krombholz, 1966; Hillman, 1961; Rimon & Galun, 1968) are suitable for small plants but there are limits to the size of plant that can be handled conveniently.

Consequently there is a dearth of experiments on whole plants under controlled conditions simulating the natural environment. More frequently information has been obtained by experiments on plant parts (e.g. Ingold, 1936; Arisz, 1953; Helder, 1975~) or whole plants under artificial conditions (Frank & Hodgson, 1964; DeMarte & Hartman, 1974; Welsh & Denny, 1979a), or from correlating environmental factors with plant growth or distribution (Pearsall, 1920; Misra, 1938; Denny, 1972a). With emphasis recently on pollution and weed control, data are increasingly available on accumulation of heavy metals, herbicides, pesticides, and other toxins in water- plants, but the sites and mechanisms of absorption are rarely considered.

This review attempts to assemble the information which may contribute to a clearer understanding of absorption, translocation, accumulation and excretion of solutes by submerged, rooted aquatic vascular plants. The information will be considered in three sections: ( I ) morphological and anatomical evidence; (2) field surveys; (3) physiological evidence at cell, tissue, and whole plant levels.

11. MORPHOLOGICAL AND ANATOMICAL CHARACTERS

The morphological and anatomical features of aquatic angiosperms have been fully described andillustrated by Arber (1920), and their diversity in relation to their ecology has been discussed by Sculthorpe (1967) and Hutchinson (1975). Important adapta- tions of submerged taxa are summarized by Spence (1964) and include: small rooting systems ; reduced water-conducting tissue, thin leaves and cuticles, or absence of cuticles.

The widespread reduction of root systems in submerged plants was taken by early

Solute movement in submerged angiosperms 67 botanists (Schenck, 1886 ; Strasburger, 1891) to indicate their under-utilization owing to absorption through the whole surface of the plant, and the misconception that root hairs are frequently absent, has led to the assumption that the roots are solely organs of anchorage (Brown, 1913; Sutcliffe, 1959). However, there is abundant evi- dence of hairs on the roots in most taxa. (Shannon (1953), for example, examined 209 species, representing 105 genera and 54 families, and found hairs on 195 species) and there is no demonstrative evidence that the hairs cannot participate in solute absorp- tion. The structure and function of roots in aquatic vascular plants is discussed further by Bristow (1975).

The leaves of submerged water-plants are usually only two or three cell layers thick and thus over two-thirds of the cells are in direct contact with the external solution. Some leaves are highly divided or filiform. Others are cylindrical in cross section with a large central air-space. The air-spaces not only provide a pneumatic skeleton (Williams & Barber, 1961) and flotation for the plant but also increase the volume/weight ratio of the tissue. These adaptations have the effect of increasing the surface-area/volume ratio for gas exchange and potential solute flux, and make leaves morphologically very suitable for absorption. A cuticle provides a resistance to solute flux and, contrary to popular belief, electron-microscope studies have revealed a cuticular layer over all leaf tissue examined. In Potamogeton pectinatus L. it is very distinct, about 0.1 pm thick (Sharpe & Denny, 1976), and in P. crispus L. it is 0.05 pm. Water permeation through such cuticles has been elegantly demonstrated in isolated cuticular membranes from P. lucens L. and has been found to be approximately three orders of magnitude greater than through those from terrestrial species (Schonherr, 1976). The chemical composition of P. lucens cuticular membranes is different from that of terrestrial species, but whether the high permeability is enhanced by the absence of waxes or not, was not resolved. Sharpe (1976) suspected a difference in composition of cuticles from the upper and lower leaf surfaces of Potamogeton polygonifolius Pourr. and concluded that waxes may be reduced in quantity or absent from the lower leaf surfaces which are out of contact with air.

A reduction in xylem elements could restrict or inhibit movement of solutes between roots and shoots, and the lack of evaporation from the leaves, raises doubts about the existence of any upward flowing water stream. Stocking (1956), in his review, concluded that there was a slow movement of water up the plant : but the early experiments, such as those of Thoday & Sykes (1909), were necessarily simple, and the results are open to question. Root pressure rather than active excretion of water from hydathodes was considered to be the more probable driving force (Wilson, 1947). With the advent of radio-isotope tracers, evidence for translocation of sub- stances within submerged plants has been strengthened, but whether it occurs via the conducting tissue has not been verified. For example, although rates of movement of 32P have been correlated with vascular development in some potamogetons (Welsh & Denny, 1979a) the location of the isotope was not established to be solely within the vascular system.

In conclusion, there appears to be no obvious morphological or anatomical reason why shoots or roots should not absorb solutes. Whether absorption actually occurs

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68 P. DENNY through the roots and/or shoots, and whether translocation of solutes is normal, can now be considered.

111. EVIDENCE FOR ABSORPTION OF SOLUTES IN THE NATURAL ENVIRONMENT

(i) Plant distribution Environmental components liable to affect the zonation and distribution of plants

are: depth of water; wind exposure; light; temperature; solutes; and type of sediment. The interrelationship of these various factors with vegetation has been discussed broadly by Pearsall (1917), Tutin (1941), Tansley (1949), Spence (1964; 1967) and others. Transects through the vegetation zones from shore to open water usually show a build-up of organic matter and silt in the sediment with increasing depth, and a consequent increase in fertility, and in the ratio of fine:coarse particles (Pearsall, 1920). The deep-water zone is low in light flux and receives light deficient in certain wave- lengths (Spence, 1976); its water may be considerably cooler, particularly if the vegetation is dense (Dale & Gillespie, 1976). In this multi-factorial system the combina- tion of several components probably determines whether a particular plant occurs at a particular site. Obviously light is a primary factor. It determines the ultimate depth at which a plant can grow (Pearsall, 1920), and affects the zonation of the vegetation (Crum & Bachmann, 1973). Wave action and turbidity explain the paucity of aquatic macrophytes in some lakes (Schiemer & Prosser, 1976) and the deposition of silt on submerged shoots can limit the distribution of some species, especially those with fine-leaves.

The role of water and sediment chemistry in plant distribution will be considered in detail. If one or other has a dominant effect it may be argued that the plant derives its nutrition from either the water or the sediment, and the shoot or root absorption respectively may be indicated.

(a) The infuence of waters In his classic study of the Scottish vegetation, West (1905) appreciated that some

species had preferences for particular types of water, but since his concern was mainly with the broad description of vegetation, the later survey undertaken by Moyle (1945) can be considered as a more relevant starting point. Moyle recorded the vegetation and collected representative water samples from 225 lakes throughout Minnesota, USA. He examined the relationship between the chemical nature of the waters and the distribution of aquatic plants and found that on this basis the plants fell into three groups: (i) the soft-water flora, (ii) the hard-water flora, and (iii) the alkali-water flora. Although the three floras overlapped and some species were ubiquitous (e.g. Ceratophyllum demersum), certain taxa were confined to a particular type of water. For example, Lobelia dortmanna L. was confined to soft-waters with total alkalinity < 40 ppm calcium carbonate; Potamogeton pusillus L. to hard waters with total alkalinity > 90 ppm calcium carbonate; Ruppia occidentalis S. Wats. and Najas marina L. to alkali waters with sulphate concentrations > 125 ppm. This led

Solute movement in submerged angiosperms 69

Table I . The tolerance of eutrophic species of submerged andjoating-leaved water-plants to impoverished conditions. (After Seddon, 1972)

Usual minimum tolerance I ,

Hardness ratio Calculated trophic Conductivity (Ca+ Mg): index (conductivity

Taxa ,US cm- (Na + K) x hardness ratio) Potamogeton pectinatus 200 Myriophyllum spicatum 200

Ceratqphyllum demersum I00 Potamogeton perfoliatus I 0 0 Ranunculus aquatilis 60 Elodea canadensis 5 0

Potamogeton crispus 150

5 0 5 0 30 2.5 2.5 20

20

I000 1000

450 250

250 I20

I 0 0

Moyle to conclude: Although water chemistry appears to be the most important single factor influencing the general distribution of aquatic plants in Minnesota, field observations show that the type of bottom soil and the physical nature of the body of water greatly influence the local distribution of species within its range of chemical tolerance.

Similar extensive surveys of lakes in North America (Metcalf, 1931 ; Swindale & Curtis, 1957); Scotland (Spence, 1964, 1967); Wales (Seddon, 1972); and Lower Saxony (Wiegleb, I 978) substantially support Moyles conclusion. Data have also shown that some species are confined to either nutrient-rich or nutrient-poor waters : other species can be arranged in an order depending upon their tolerance of impover- ished conditions (Seddon, 1972). This is a factor assessed from a hardness ratio (Ca + Mg) : (Na + K), and conductivity; the multiple of which produces a trophic index. In Table I selected data from Seddons chemical isoclines show the tolerance of eutrophic species towards impoverished conditions.

Various attributes of the water which have correlated positively with plant distribu- tion include: total salts (Metcalf, 1931), conductivity (Swindale & Curtis, 1957; Seddon, 1g72), alkalinity (Spence, 1964, 1967), and phosphate concentrations (Jones & Cullimore, 1973). Correlations with conductivity are sometimes erratic, for example in maritime situations of variable chloride concentrations (Spence, I 967), and alkalinity has been favoured as a good indicator of expected lake flora (Spence, 1964, 1967). More particularly, the correlations between plant distribution and concentrations of inorganic carbon and calcium in the waters may be important (Wiegleb, 1978). Concentrations of phosphates in waters may also affect plant distribution. In the QuAppelle Lakes of Canada the three most abundant plant species were Cerato- phyllum demersum, Potamogeton pectinatus and P. richardsonii (Benn.) Rydb. (Jones & Cullimore, 1973). Ceratophyllum was present only where total phosphorus < 0.5 ppm, but Potamogeton pectinatus was increasingly abundant as total phosphorus rose from 0-1 to 0.6 ppm. Potamogeton richardsonii showed the reverse trend and was dominant in waters with 0.1 ppm phosphorus, but declined as the level rose to 0-7 ppm. The

P. DENNY authors claimed that they could anticipate the concentration of phosphorus by noting the dominant species.

Some plants have been suggested as indicators of water quality; Lemna gibba L., for example, prefers phosphate- and ammonia-rich waters with an alkalinity exceeding 1.5 meq dm-3, while Ceratophyllum demersum indicates nitrate-rich waters. Potamo- geton lucens is intolerant of ammonia-rich waters and Myriophyllum verticillatum L. predominates in carbon dioxide-rich but nitrogen- and phosphate-deficient waters (Wiegleb, 1978). In fact, plants and vegetation as indicators of water eutrophication and pollution have attracted wide attention (Lind & Cottam, 1969; Volker & Smith, 1965; Kurimo, 1970; Litav & Agami, 1976; Kohler, Wonneberger & Zeltner, 1973; Dale & Miller, 1978). Kurimo graded 50 species according to the pollution from wood- processing factories and domestic sewage. Some plants (Ceratophyllum demersum and Myriophyllum verticillatum) benefited from the sewage enrichment but not from the wood-processing wastes ; others (Potamogeton praelongus Wulf., Lobelia dortmanna, Myriophyllum alternijorium D.C.) preferred waters of a fairly low nutrient content. The submerged flora of the Moosach river system near Munich could be graded according to ammonium-nitrogen and orthophosphate concentrations in the water. Potamogeton coloratus Hornem., was intolerant of high concentrations of either ; Potamogeton densus L., P. natans L. and P. perfoliatus L. occurred where concentra- tions were intermediate and Callitriche obtusangula Le Gall, indicated waters polluted with phosphate and ammonium in the range 0-4 to 1-2 ppm (Kohler et al . , 1973).

However, plant indicators may not always be reliable. In standing-crop estimates along a short stretch of chalk stream enriched by sewage, Owens & Edwards (1961) reported a distinct change in dominant associations of species but this was correlated with total radiation rather than water pollution. They found no obvious correlation with the substratum (Edwards & Owens, 1960). Likewise, the excessive growth of water-weeds in the Holston River, Tennessee, could not be clearly linked to the large amounts of nutrients in the water (Peltier & Welch, 1969).

A particularly relevant study in the present context relates to Sydling Water, an unpolluted chalk stream in Dorset, England (Casey & Westlake, 1974). The submerged vegetation is dominated by Ranunculus penicillatus vax. calcareus (R. W. Butcher) C. D. K. Cook. Casey & Westlake measured the biomass of Ranunculus between two sample stations every two weeks during the growing season, and by evaluating the concentrations of nitrogen, phosphorus and potassium in the tissue, were able to calculate the amount of nutrients taken up to compensate for the increased biomass. They also compared the concentrations of nutrients in the stream water at the two stations and estimated the total loss of nutrients between the stations (Table 2). Generally, the decrease in potassium through-put along that section of stream was of the same order of magnitude as the requirements for increased biomass of vegetation. Their estimated value for phosphorus in the plants was probably low (see Casey & Downing, 1976) and allowing for this, the reduction of phosphorus in the water was also of the same order of magnitude as the estimated accumulation by Ranunculus. The data for nitrogen were very variable and inconclusive. The authors emphasized the difficulty of correlating plant growth with nutrient through-put but their data

Solute movement in submerged angiosperms 71

Table 2. Comparison of difleyences in through-put and estimated accumulation of nitrogen, phosphorus and potassium by Ranunculus during the period of maximum growth, in a stream 10 m broad. (After Casey & Westlake, 1974)

Total decrease in through-put between

two stations

Accumulation in kg per 16000 m2 assuming a daily dry-weight change of

A r \

Nutrient ion 1600 m apart 1.5 g m--2 d-l 5 . 0 g m--2 d- Nitrate -38.1 72 240 Potassium -47.3 51.8 173 Phosphate - 10.3 29 9.6

* Omitting data from 2 abnormal days.

could indicate that during periods of rapid growth the shoots of Ranunculuspenicillatus removed detectable amounts of nutrients from the stream water.

(b) The injluence of sediments Early authors (Snell, 1908; Brown, 191 I ; Rickett, 1922, 1924) suggested that

different taxa of water-plants have preferences for different substrata; Rickett (1922) for example, tabulated the preference of species for muddy or sandy bottoms in Lake Mendota, but Pearsalls work (1917, 1920) perhaps is the most familiar. In his survey of Esthwaite Water, Pearsall(1917) sampled vegetation from sheltered and wind-exposed shores and identified three plant communities: (i) communities in the region of direct wave action, (ii) communities below the direct effect of wave action, and (iii) communities between the two. Within the first community he distinguished a plant succession which changed with change in substratum from gravel to black organic mud. Similar correlations between substrata and plant succession in the other two communities led him to conclude: In the distribution of these plant communities, silt and organic content of the soil appear to be of greatest importance, although light intensity and shelter have also considerable influence.

Comparing the vegetation from a group of lakes, Pearsall (1920) considered light, temperature, and aeration to be of minor importance in the distribution of rooted vegetation within the photic zone. Within a single lake he could find no difference in water chemistry (circulation and mixing would be a normal feature within the photic zone) and again concluded that the distribution of plants was controlled by variation in composition of the lake shores and soils. I t should be emphasized that Pearsalls concern was the study of the plant associations within each lake and not between different lakes (Seddon, 1972). Spence (1967) considered that on occasions Pearsalls sample size was small, and correlations between species and substratum may have been by chance. Even accepting the correlations, he argued that this does not necessarily imply a causal connection. In his survey of Scottish aquatic vegetation, Spence stressed the importance of soils but felt that it was still to be proved that any species is causally associated with one circumscribed type of soil. In an account of Lake Bunyonyi, Uganda, he did suggest a correlation between soil phosphate and species distribution but this was not substantiated in later surveys (Denny, 1972b, 1973).

72 P. DENNY Good correlations between flora and soil characters including particle size, organic

matter, pH and some ions have been found by several authors (Swindale & Curtis, 1957; Lind & Cottam, 1969), and the plant communities in streams and rivers have been identified with particular types of sediment (Haslam, 1975, 1978), but the textures of soils in relation to the rooting depths of the plants are also important. The delicate rhizomes of Potamogeton perfoliatus for example, cannot penetrate hard, sandy bottoms and tend to be confined to the top few centimetres of soft mud. Potamogeton lucm has robust rhizomes which penetrate the lake sediment to a depth of 20 cm, and are less sensitive to texture. Potamogeton l u c m , therefore, has a wider selection of sediments in which to become established (Ozimek, Prejs & Prejs, 1976).

(c) Discussion Evidence may appear contradictory in that both substratum and lake water are

claimed to control the distribution of water plants. This dilemma is resolved to some extent by Swindale & Curtis (1957, p. 402). Various investigators in the past have correlated the distribution of aquatic plant species with certain environmental gradi- ents. Manifestly, however, species are not distributed in a regular progression along the entire range of any single ecological factor; nor does a factor always have the same effect on the same species. An ecologically sounder concept is that distribution is affected by a complex of interacting environmental factors. Some of them, when present in excess or limiting amounts, may be expected to have critical influence. The existence, or not, of particular taxa in different waters has been strongly correlated with the tolerance limits of those taxa to chemical factors within the water. Similar correlations have been indicated for sediments. Within a lake, where water chemistry may be assumed to be relatively uniform, correlations between substrata and species distribution have also been demonstrated. None of this evidence detracts from Moyles (1945) or Spences (1967) conclusions; but as Spence pointed out, correlations need not imply causal connections.

(ii) Elemental composition of plants and bio-accumulation of toxins There are now considerable data on the elemental constitution of water-plants.

(Allenby, 1968; Boyd, 1969; Riemer & Toth, 1969; Korelyakova, 1970; Adams & Cole, 1973; Lathwell, Bouldin & Goyette, 1973; Cowgill, 1973a, b, 1974) but until sufficient information on plants, waters, and sediments, is available from bodies of water throughout the world, comparisons are of limited value. Critical tissue concen- trations (i.e. minimum tissue content of an element in a particular species necessary for maximum growth) have been investigated in a range of water-plants by Gerloff & Krombholz (1966). They found nitrogen levels ranged from 0.78 to 4.28% and phosphorus from 0.10 to 0-70%, but plant yields were unaffected above 1 3 % nitrogen and 0 - 1 3 % phosphorus. From field studies Gerloff & Krombholz implied a relationship between concentrations of nutrients in the lake waters and plant tissues. Tissue concentrations above 1 . 3 % for nitrogen and 0 . 1 3 % for phosphorus suggested luxury consumption; those nutrients then not being limiting in the water. Correlations between concentrations of mineral ions in tissues and waters have also been noted by

Solute movement in submerged angwsperms 73 Adams, Mackenzie, Cole & Price (1971) but in other cases (e.g. Casey & Downing, 1976) correlations were not found, although they were sought.

Pesticides, herbicides, heavy metals and other toxins in the aquatic environment, and their transfer through the food web are the focus of majormonitoringprogrammes. Data on submerged plants have shown that the plants take up and accumulate toxins in various amounts (Cope, 1961; Croker & Wilson, 1965; Vrochinskiy, 1970) but whether absorption occurs through the roots or shoots is not always considered (e.g. Devlin & Yaklich, 1971 ; Devlin, 1974). This is regrettable since organo-chloride insecticides such as DDT and polychlorinated biphenyls (PCB) for example, are persistent, relatively insoluble in water, and tend to sink and accumulate in the bottom sediments (Crump-Wiesner, Feltz & Yates, 1974). Sedimented pollutants apparently lost to the ecosystem could be recycled if aquatic macrophytes absorb them through their roots and translocate them into their shoots. Aquatic vegetation in the Camargue is known to accumulate chlorinated pesticides and PCBs (Vaquer, 1973). Levels of BHC (hexachlorocyclohexane) and DDT are generally highest in May/ June when systematic spraying against chironomids occurs. Vaquer concluded that accumulation in the submerged plants was due to rapid active absorption by the shoots: but he gave no direct evidence for active uptake. Rooted, floating-leaved plants are also known to accumulate DDT and for Nymphaea alba L., predominant root absorption with some absorption through the leaves has been suggested (Vro- chinskiy, Grib & Grib, 1970).

Investigations on metal accumulation in aquatic vegetation produces further opinions as to the sites of absorption. Analyses for rare-earth metals in plants, plant parts, waters, and sediments indicated that metals were removed from sediments by roots of emergent plants to such an extent that concentrations in the sediments were detectably reduced (Cowgill, 1973 a, b, 1974). Submerged rooted vegetation did not produce such large reductions and had lower plant : sediment metal concentration ratios. Cowgill presumed therefore that submerged rooted plants had some potential for the extraction of metals from waters as well as sediments, and rootless plants extracted metals rapidly only from the water. This would support the hypothesis that shoot absorption increases with progression towards submergence and simplicity of structure (Denny, 1972~) .

Submerged plants from arsenic-rich hot springs in New Zealand contained variable but high concentrations of arsenic compared with emergent plants (Fish, 1963 ; Reay, 1972). In several submerged species correlations between concentrations in plants and waters existed, but not between plants and sediments; again implying absorption and accumulation from the water by the shoots. The preferential absorption of particular metals by submerged macrophytes has been noted by others (Dietz, 1973; Guilizzoni, 1975; Baudo & Varini, 1976), but Dietz (1973) could find no correlation between the concentrations of lead, nickel or zinc in the plants and waters from the River Ruhr, West Germany.

In the English Lake District close to mining catchment areas, concentrations of lead and copper in shoots of submerged plants were found to be highly correlated with concentrations in the sediments (Welsh & Denny, 1976; and in press). Samples

74 P. DENNY of lake water were low in these metals and no correlation between the concentrations in the water and shoots was observed. These results implied a pathway of metal absorption via the roots with subsequent translocation to the shoots. However, in the light of experimental evidence (see later) it was concluded that although copper was probably transported in this way, the high concentrations of lead in the shoots was the result of adsorption on the shoot tissue of metals released intermittently from the sediments.

In an attempt to determine the sites of absorption Mayes, McIntosh & Anderson (1 977) adopted field techniques incorporating transplant experiments (cf. Misra, 1938) in two lakes; one rich in cadmium and lead, the other not. Cuttings of Elodea canadensis Rich in Michx. from an uncontaminated lake were planted in containers of sediment from each lake, were deposited in the lakes, and were cropped at regular intervals over a nine-week period. The authors concluded that rooted vascular hydro- phytes were capable of accumulating metals from both the surrounding water and the sediment in which they were rooted, and that the metals were transported from an- chored segments upwards into the shoots and leaves. However, this study was not without shortcomings. Firstly, roots and shoots were not separated for chemical analyses. Although roots were only a very small portion of the biomass, complete removal of contamination from the roots is practically impossible (Welsh, 19729, and artificially inflated tissue concentrations from metal-rich sediments may have resulted. Secondly, some transplanted Elodea was lost in the lake, . ..probably a result of wave action washing out the contents of the containers. Sharpe & Denny (1976) and Welsh (1978) have observed very rapid adsorption of lead onto leaves of Potamogeton sp. Thus the wave action in the experiments of Mayes et al. may have been of sufficient magnitude and duration for the shoots to adsorb metals from the disturbed muds.

IV. EXPERIMENTAL EVIDENCE FOR ABSORPTION AND TRANSLOCATION

(i) Transplant and culture experiments Observations on the patterns of lake vegetation have encouraged workers to test

possible causes of plant distribution by culture and transplant experiments. Classic experiments were performed by Pond (1905) using two aquaria filled with tap-water and with either a mud or clean sand substratum. Plants of Ranunnclus trichophyllus Chaix were suspended in each tank and others were planted in the substratum. Those rooted in substratum grew better than the suspended plants, and those rooted in mud grew considerably better than those in sand. There was very little difference between those suspended over mud or sand, which suggested that the liquid medium had little control over growth. It is unfortunate that Pond used tap-water rather than a full nutrient medium as tap-water is often nutrient-deficient. If he had added soil extract to his tap-water, the results might have been different: some plants only thrive in media without substratum if they contain extracts of organic matter (Bot- tomley, 1917, 1920; Ashby, 1929; and others). Pond undoubtedly favoured the idea of solute absorption via the roots of water-plants; a view supported by Misra (1938). Misra planted seedlings of Potamogeton perfoliatus from Lake Windermere into

Solute movement in submerged angiosperms 75 containers of different substrata obtained from the lake, and re-submerged them. The seedlings grew best in the mud in which they were found naturally. Similar results were obtained with Potamogeton alpinus Balb. and Sparganium minimum Wallr.

Submerged plants have been cultured in outdoor artificial ponds to study the effects of either water enrichment or soil enrichment on their growth. Experiments involving water enrichment, however, have been plagued by algal infections which affect macrophyte growth and may obscure growth differences genuinely due to the experi- mental treatments (e.g. Mulligan & Baranowski, 1969 ; Mulligan, Baranowski & Johnson, 1976; Ryan, Riemer &Toth, 1972). Ryan et al. (1972) arranged twelve plastic pools in a random-block design and made various applications of nitrogen, phosphorus and potassium to the water. Elodea canadensis, Myriophyllum spicatum L., and Pota- mogeton pulcher Tuckerm (a floating-leaved species), were planted in each pool and their yields were measured over a two-year period. There was a significant lowering of phosphorus in the sediments of the controls (untreated pools) which, the authors concluded, was due to absorption of phosphorus by the roots of plants in the low- phosphorus conditions. High levels of water enrichment lasted for only short periods of time, but may have been sufficient for ion absorption by shoots of Elodea and Myriophyllum and account for the observed tissue enrichment. Nitrogen, phosphorus and potassium levels in the Potamogeton tissue were not significantly higher in enriched treatments in the first year, and this plant may have had an alternative uptake through the roots.

In Uganda, I designed replicate experiments in artificial ponds in which various submerged and floating-leaved taxa were grown for 8-15 weeks in either sand or nutrient-rich mud (Denny, 1972~) . The ponds were filled with natural pond-water and all plant shoots were considered to be surrounded by the same medium. Unlike Pond's (1905) experiments, the medium was nutrient-rich and the free-floating plants, used as indicators, remained actively dividing and healthy throughout the experiments. A comparison of growth rates showed that all taxa grew better on mud than sand and the substratum had an increasingly greater influence on the yield of those taxa with a more specialized anatomy. There was a trend froma greater dependence upon roots for solute absorption in rooted, floating-leaved taxa, to a lesser dependence in submerged taxa. This was associated with a trend towards simplicity of shoot structure, and a decreasing root/shoot ratio on sandy, nutrient-poor substrata. A division of labour between roots and shoots was considered probable with the possibility of either of them acting alone, depending upon the anatomy and morphology of the taxon.

(ii) Direct evidence for absorption Literature on absorption of solutes into shoot and leaf tissues of submerged plants

is extensive as the material is a convenient physiological tool for uptake studies. In contrast, there is a dearth of experiments on the root tissues of aquatic plants.

(a) Uptake into shoots and leaves Early experiments on shoots and leaves were carried out by a number of workers

(Rosenfels, 1935; Ingold, 1936; Gessner & Kaukal, 1952; Olsen, 1953) but those of

76 P. DENNY Arisz and his associates are perhaps the most exhaustive (Arisz, 1953, 1960, 1961 a, b, 1963, 1964; Arisz & Sol, 1956; Winter, 1961), and a brief account of them is pertinent. Excised leaves of Vallisneria spiralis L. were arranged across several com- partments of a separation apparatus. Where the leaf passed from one compartment into the next it was enclosed by a watertight seal. The tissue in the first compartment, Arisz called the absorbing zone, and the adjacent compartment(s) the free zone(s). By quantitative chemical analyses of ions in tissues from different compartments, and by different treatments of the absorbing and free zones, he traced the absorption, translocation and accumulation of ions.

The mechanism of movement postulated by Arisz (1961a), and discussed in the light of a study on cation uptake by Winter (1961), can be summarized thus:

(i) A passive penetration of ions (mostly exchange of cations) into the peripheral region. Winter (1961) in his study on rubidium ion movement found this initial uptake to be into the Apparent Free Space (AFS), i.e. the volume of the tissue freely accessible to the diffusion of solutes (Briggs & Robertson, 1957). The Apparent Free Space is composed of two fractions: the Water Free Space (WFS) in which only water, molecules and freely mobile ions are involved; and the Donnan Free Space (DFS), in which mobile cations from the external solution and indiffusible anions especially associated with the cell wall are distributed according to Donnan equilibria (Briggs & Robertson, 1957). Winter confirmed that the uptake into the AFS of VaZZisneria leaves included both the WFS and the DFS and concluded that cation exchange sites were located in the cell walls. Such sites have been identified also by Sharpe & Denny ( I 976) in electron-microscope studies on Potamogeton pectinatus leaf cells. The Apparent Free Space has been estimated by Kylin (1957) and Winter (1961) to be in the order of 5 to 7'5% in Vallisneria leaf tissue, and 20% in leaf tissue of Pota- mogeton schweinfurthii A. Benn. (Denny, 1966) ;

(ii) The active uptake of ions into the cytoplasm, the movements of different ions being independent.

(iii) The active secretion of ions into the vacuole from the cytoplasm. (iv) The translocation of ions in the symplasm ; an active process by which ions are

transferred in the cytoplasm from cell to cell via the plasmadesmata. As labelled ions apparently moved as well in the parenchyma cells as in the bundles, and as ions were equally distributed over different tissues, Arisz (1961 b) stated that symplasmic transport does not discriminate between sieve tubes and less specialized tissue. Sutcliffe (1959), who generally supported Arisz's view, took issue with this final point, as he had observed by autoradiography, movement of a6Rb in the phloem only of Vallisneria leaves.

The arguments used by Arisz in support of active transport are questionable (Dainty, 1961). The observation that a particular ion moves into or out from a tissue only when the cell is utilizing energy does not prove that energy is being expended on its transfer. Active transport can only be concluded if that ion is moved against an electrochemical potential gradient. Electrochemical potential gradients of ions have been studied in the leaf cells of Potamogeton schweinfurthii and no ion appeared to be in passive electrochemical equilibrium across the membranes (Denny & Weeks, 1968).

Solute movement in submerged angiosperms 77 The anions chloride, phosphate and sulphate, were considered to be actively transported into the cells since the net passive physical driving force upon each acted strongly outwards. The forces determining cation movement were less certain. The cations potassium, sodium, calcium and magnesium might have been actively exported from the cells or the tissue may have approached cation flux equilibrium only slowly. Kinetic studies of phosphate and sulphate uptake into leaves of Elodea densa (Planch.) Casp., have indicated that at least three different processes are involved, namely a diffusion through unstirred layers; an active uptake at low and medium external ion concentrations; and a partly active influx at high external ion concentrations (Jeschke & Simonis, 1965). The first diffusive phase is rate-limiting at external concentrations below about 0.1 ,UM. The effects of external concentrations on ion uptake into root and leaf tissues, and the identification of separate mechanisms at low (0-ooz to 0.5 mM) and high (I to 50 mM) salt concentrations are now well estab- lished, and are fully discussed by Epstein( 1976). Uptake of chloride and rubidium ions in leaves of Vallisneria (Prins, 1973) and of chloride in Elodea (Jeschke, 1972) show a close relationship with the action spectrum of photosynthesis. Ion influxes thus appear to be linked to reactions in photosynthesis; but there is also evidence that they can be supplied with energy from respiration. Prins concluded that cyclic photo- phosphorylation drove uptake of the chloride ion in Vallkeria and that a contribution from photosystem I1 was also evident, whilst Jeschke (1972) favoured pseudocyclic photophosphorylation for potassium and chloride influxes in Elodea. Jeschke con- sidered that potassium and chloride influxes were partially coupled. A fuller discussion on ion fluxes and energy supply in water-plant leaves has been given by Jeschke

Many submerged angiosperms have a unique ability of transferring ions from one surface of the leaf to the other through the cells of the leaf lamina. This directional transport, commonly called polar transport, is linked with carbon supply and photo- synthesis but the mechanism is not fully understood. Originally, it was proposed that bicarbonate ions were absorbed in the light and cations entered passively with them through the abaxial (morphologically lower) surface. During photosynthesis hydroxyl and carbonate ions left through the adaxial (morphologically upper) surface together with cations (Arens, 1936). Later, absorption of ions through both leaf surfaces seemed more likely (Steeman Nielsen, 1947, 195 I). In photosynthesis, electrogenic active influx and efflux pumps for bicarbonate and hydroxyl ions respectively have been proposed by Denny & Weeks (1970), but others have favoured passive and/or active proton flux to maintain electrochemical balance (e.g. Bentrup, Gratz & Ubehauen, 1973).

Suggested forces for polar transport include: active transport of hydroxyl ions through the adaxial leaf surface (Steeman Nielsen, 1947); and cation influx and efflux pumps (Lowenhaupt, 1 9 5 8 ~ ~ b), but active cation transport has gained little support. More recently, movement of potassium and rubidium has been shown to be largely passive and linked to an active bicarbonate and hydroxyl flux (Helder & Boerma, 1972, 1973) although under certain conditions active polar transport of cations can occur (Helder, 1g75a, b).

(1976).

78 P. DENNY This section must leave no doubt that fluxes of ions occur between the bathing

media and submerged angiosperm leaves, and that the cuticles and cell walls provide little resistance to movement. Active uptake is driven by ATP and NADPH provided by photosynthesis, and probably by respiration too. Several major anions are actively pumped into the cells, but cation fluxes may be either active or passive. One or more electrogenic pumps including an electrogenic proton eMux pump is indicated. The polar transport of ions from the abaxial to adaxial leaf surfaces is associated withphoto- synthesis, uptake of bicarbonate ion, and fluxes of hydrogen and/or hydroxyl ions. The actual mechanisms which ultimately bring about the deposit of calcium carbonate on the adaxial leaf surfaces are not understood. There is evidence that active anion uptake and transport are involved, and cation movement is probably largely passive. Passive polar transport of cations may occur through the apoplast, i.e. the non-living tissue outside the plasmalemma, whilst some cations may be passed actively through the symplasm.

(b ) Uptake into roots If a solute is applied to the root portion of a submerged angiosperm and it is

translocated in the plant to the shoots, it may be assumed that uptake through the roots has taken place. However, the roots may provide nothing more than a matrix for the flow of substances to the shoots as a wick provides a means of translocation of oil to a flame. In order to show that roots participate actively in ion absorption and translocation, criteria similar to those discussed for leaf tissue must be used. Recent review articles on absorption by roots of terrestrial plants are provided by Pitman (1976) and Anderson (1976). The available data support active ion uptake mechanisms for all anions but the actual process differs for different ions. A diffusion potential and an electrogenic potential are implicated for the movement of solutes into the xylem sap (Anderson, 1976).

There is no published work on the roots of submerged angiosperms but there are some data from the floating-leaved Potamogeton nutuns (Shepherd & Bowling, 1973). Electrochemical studies on the roots of P. natans indicated active accumulation of sodium from loch water. Potassium, chloride and nitrate ions were also actively accumulated whereas calcium and magnesium seemed to be in approximate flux equilibrium. It is possible that roots of totally submerged potamogetons may have similar attributes but this needs to be tested.

V. LONG DISTANCE TRANSPORT

Experiments on translocation in submerged whole plants have been based on the practice of exposing either shoots or roots to a substance, and after a specific time, locating its presence in other parts of the plant. As both parts of the plant have to be submerged, two chambers separated by a watertight partition are generally used and the plant is arranged so that its stem passes through the partition. These experiments rely upon a perfect seal around the stem without damage to the tissue, but this has proved to be a major problem.

Solute movement in submerged angiosperms 79 Frank & Hodgson (1964) were amongst the first to devise a two chambered system.

They positioned Potamogeton pectinatus in a glass funnel so that the shoots were contained in the funnel and the plant stem was sealed into the neck of the funnel with the paraffin wax, eicosane. The root portion extended down beyond the funnel stem into the second chamber. Radio-isotope-labelled tracer was then supplied to one or other of the chambers. They assumed that as no appreciable activity was detected in the untreated compartment, the seal did not leak. This assumption is false. Tissues of aquatic plants can absorb and concentrate ions rapidly from very low concentrations in water (Sharpe & Denny, 1976; Welsh, 1978), and labelled solution could have leaked through the seal without detection and have been adsorbed onto tissue in the untreated chamber. The Frank & Hodgson-type apparatus was rejected by DeMarte & Hartman (1974) in favour of an idea from Littlefield & Forsberg (1965) such that roots and shoots were isolated from each other by placing the roots in a beaker containing medium; and nearly submerging the beaker in a larger container of solution. The shoots extended over the lip of the beaker into the surrounding medium. Tygon tubing packed with lanolin enclosed the part of the stem between the two compartments.

The type of material used for the seal between the plant and the two-chamber partition is critical, and is the weakest link in these arrangements. Silicone grease is a favourite (e.g. Funderburk & Lawrence, 1963a, b ; Thomas & Seaman, 1968) but it can seriously damage delicate tissues (Denny, 1966). Bristow & Whitcombe (1971) reported successful use of the plasticine-like material, Terostat Type VII, in amodified Frank & Hodgson apparatus, and others have used eicosane (Frank & Hodgson, 1964; Sutton & Bingham, 1968 ; Gentner, 1977). Whilst in short-term experiments these various sealing compounds may be more or less effective, over longer periods they can sometimes cause damage and rotting to the stem, and leakage. Potters clay has been used with good results, the clay forming an effective and natural seal around the stem (Welsh, 1978; Welsh & Denny, 19794 b). Leaks can be detected by placing non-experimental water-plant tissue such as Lemna trisulca L. or Sphagnum spp. close to the seal in the untreated chamber.

Partition-type experiments on translocation are considered in two categories (i) nutrient ions, especially phosphorus and nitrogen ; (ii) heavy metals and herbicides.

(i) Translocation of nutrient .ions There is a shortage of sound data on translocation of nutrient cations because doubt-

ful experimental conditions and the absence of tests for contamination and leakage between compartments invalidate various reports. Experiments on plants of Myrio- phyllum exalbescens Fern. rooted in sand or muck and treated with high specific activity 59Fe or GCa in the substratum, suggested absorption and translocation of 45Ca from sand in the light but no translocation from muck (DeMarte & Hartman, 1974). The results from treatments with 59Fe were peculiar and mainly showed absorption and translocation from the roots in the dark. With treatment of the shoots, absorption of 45Ca was relatively rapid (< 15 minutes) whilst absorption of 59Fe continued for up to 60minutes. Good evidence for translocation from the

80 P. DENNY shoots to the roots was lacking. 59Fe supplied to Vallisneria spiralis in a Frank & Hodgson funnel-type apparatus allegedly showed significantly more ' absorption' of iron by roots than shoots, and some translocation; but amounts were small (Gentner, 1977). When root to shoot transport was recorded, about 27% of the iron apparently was excreted by the shoots (measured by increase in activity of the non- treated solution), but how excretion was distinguished from contamination was not explained.

Phosphate is an important nutrient anion, and there is a convenient radio-isotope of phosphorus, 32P, which has been used extensively in the study of absorption and translocation in terrestrial plants (e.g. Biddulph, Biddulph, Cory & Koontz, 1958 ; Marshall, 1967). Many similar experiments on submerged plants have been thwarted by the difficulties of working in the aqueous phase and the interpretation of results is far from simple. For example, plants of Heteranthera dubia (Jacq.) Macm. treated with "P solution indicated translocation of the isotope in both directions with greater movement in the acropetal (upward) direction (Funderburk & Lawrence, 1963 b), whilst predominant basipetal (downward) translocation, and indeed excretion of shoot-absorbed 32P by the roots, has been suggested for Vallisneria (Gentner, 1977): but neither experiment is without the suspicion of leakage or contamination. Seadler & Alldridge (1977) could locate only very slight movement of 32P in root and leaf tissues of Najas minor All. which they presumed to be from cell to cell. The vascular system in Najas is poorly developed and Seadler & Alldridge suggested that the lack of translocation may be due to the anatomical characteristics of the plant ; but they found it difficult to reconcile this with results obtained by Bristow & Whitcombe (1971) from experiments on plants of Myriophyllum and Elodea in a modified Frank & Hodgson apparatus. These experiments indicated mainly absorption by the roots, followed by translocation. When the roots were removed from lower parts of the plants, transloca- tion in Myriophyllam brasiliense Cambess. and Elodea densa was substantially reduced, although M . spicatum L. var. exalbescens (Fernald) Jepson did not show this reduction. In long-term experiments (10 days) over 90% of the total phosphate in the axillary shoots of M . brasiliense was derived from the roots or stem bases. The absorptive capacity of the roots was several-fold that of the shoots (the high retention capacity of tracer phosphorus by roots of terrestrial plants is well documented, Biddulph et al. 1958). Bristow & Whitcombe accepted the possibility of an absorption pathway via the bases of stems as well as the roots, especially as roots sometimes decayed during experiments, but they postulated acropetal translocation as the main direction of transport. Myriophyllum brasiliense showed acropetal movement most clearly; per- haps because of its semi-emergent habit and well-developed vascular system.

Hydrilla verticillata Royle and Myriophyllum spicatum also appear to have a predominantly acropetal translocation system for 32P (Bole & Allan, 1978). In experi- ments lasting 28 days shoots of Hydrilla had accumulated about 38% of their total phosphorus from the interstitial fluid of the mud, via the roots. Myriophyllum spicatum produced similar results in 40-day experiments. Different concentrations of phosphate in the water surrounding the shoots did not affect substantially the amount of Z Z P absorbed from the mud but higher concentrations in the water produced higher total

Solute movement in submerged angiosperms 81 phosphorus in the plants. The concentration of total phosphorus in the Myriophyllum increased almost linearly with increasing phosphorus concentration in the water from 0.015 to 2.0 pg P cm-3. The absorption of phosphorus by the roots seemed to be independent of the phosphorus demand of the plant, and evidence from DeMarte & Hartman (1974) raises the possibility of excess phosphorus being excreted by the shoots. Results from studies on the marine eel-grass, Zostera marina L., could plausibly support a hypothesis of absorption of phosphate by the roots; translocation; and excretion from the shoots (McRoy & Barsdate, 1970; McRoy, Barsdate & Nebert, 1972) ; but possible cross contamination between root- and shoot-bathing media was not critically investigated.

Recently Welsh & Denny (1979 a) followed the translocation of 32P in two submerged species of Potamogeton and, using a three-compartmented apparatus, overcame the problems of possible cross-contamination of the shoot and root media. Results showed that there was very little acropetal translocation of 32P in P. pectinatus in five hours but some movement had occurred by 24 hours. Leaf treatments produced negligible movement in one hour but after five hours basipetal and acropetal movement, con- sistent with phloem translocation, had occurred. With incubation periods of 24 and 48 hours a pattern of widespread 32P translocation from the treated area was evident. The major movement was into rhizomes and young, secondary shoots; but not roots. The transport by-passed mature leaves. Comparable results were found in plants of P. crispus but translocation under similar conditions was more rapid. In long periods of treatment (6-10 days), where plants were rooted in 32P-labelled mud separated from

. the shoot medium by an impervious clay seal, both species exhibited root absorption and acropetal translocation. Approximately ten times more 32P had been transported to the shoots of Potamogeton crispus than to those of P. pectinatus in equivalent times.

Three features are of particular interest in these experiments: (i) translocation of 32P was faster in P. crispus than in P. pectinatus; (ii) the amount of 32P translocated from roots to shoots in 6 days was greater in P. crispus; (iii) there was more acropetal than basipetal movement in P . crispus whereas the opposite was true for P. pectinatus. This distinction between species was correlated with differing vascular systems. Potamogetm crispuS retains three groups of vascular bundles within the stem whilst, at any one level, P. pectinatus has only a single xylem element surrounded by a ring of phloem. The ecological distribution of the two species is different. Potamogeton pectinatus tends to grow in eutrophic waters, P. crispus in more oligotrophic conditions. The observed difference in translocation between the two species, and the greater vascular system in P. crispus are consistent with these ecological findings. Potamogeton pectinatus is rarely found in low-phosphate water and therefore its phosphate require- ments may be obtained mainly via the shoots. The shoots of P. crispus, on the other hand, may not be able to obtain sufficient phosphate from the more oligotrophic water and efficient absorption and redistribution from the roots may be necessary.

Studies on the absorption and translocation of nitrogen have been made possible by utilizing lSN-labelled compounds but analytical techniques are expensive and require sophisticated apparatus, so the number of experiments is limited. Some of the most elegant experiments are described by Nichols & Keeney (1976a, b). Extensive

6 B R E 55

82 P. DENNY field observations by these authors revealed that differences in available nitrogen in the sediment from different sites correlated with the total nitrogen concentrations in Myriophyllum spicatum rooted in these sediments. The correlation indicated a possible dependence of Myriophyllum on the sediment for its nitrogen supply. To test their hypothesis they placed Myriophyllum plants in two-compartmented boxes containing sediments, and applied either 16N-labelled or unlabelled ammonium sulphate to the sediments. The shoot compartments were continuously irrigated with water contain- ing ammonium sulphate.Those plants rooted in labelled sediment were irrigated with unlabelled solution, and those rooted in unlabelled sediment had l5N-labelled ammonium sulphate solution passing through their shoot compartment. Under these conditions, although uptake of nitrogen occurred in both roots and shoots, foliar uptake supplied about twice as much nitrogen to new shoots and five times as much nitrogen to old shoots as did root uptake. Foliar uptake supplied very little nitrogen to the lower shoots and roots, and basipetal translocation was small. When plants were given solutions deficient in nitrogen, they were apparently able to obtain their entire nitrogen requirements from the sediment. The authors concluded that although M . spicatum had a greater ability for nitrogen absorption by the shoots than by the roots, when this source was limited, root absorption could supply its requirements. Nichols & Keeney found little evidence of rapid basipetal or acropetal translocation, or of root-absorbed nitrogen being excreted by shoots. In contrast, McRoy & Goering (1974) applied 15N-labelled ammonium and nitrate to attached roots of Zostera marina and, after eight hours, found nitrogen from both sources in the roots; in the shoots, and in epiphytes attached to the shoots. They concluded that Zostera and most other sea-grasses obtain their nutrients from the sediments : acropetal translocation is normal, and some of the translocate is generally released from the shoots.

(ii) Discussion In these experiments on movement of phosphorus and nitrogen there are three

separate processes : absorption, translocation and excretion. Strong evidence has been presented for absorption by both roots and shoots but the evidence for acropetal and basipetal translocation is not always convincing. Few authors have proved indubitably that label in the tissue of the label-free compartment was translocated from the other compartment through the tissue rather than by contamination of the label-free medium.

Data presented by authors as evidence for excretion are even more anomalous. All too frequently it has been assumed that if label has been found in label-free medium, or in detector plants, or in epiphytes attached to shoots, then label must have been excreted by the plants. Gentner removed water samples from the unlabelled compart- ment at the end of his experiments to detect leaks and/or excretion (Gentner, 1977, p. 268). He assumed (p. 271) that excretion had taken place! Although McRoy and his collaborators are ardent propounders of cycling of nutrients from the sediment by pumping through the plant and excretion from the shoots, their data do not exclude the possibility of leakage. Most observations suggest that there is some translocation, and that excretion under particular conditions may occur in some plants,

Solute movement in submerged angiospevms 83 but whether excretion is as extensive and as widespread as some authors believe, remains to be tested.

(iii) Translocations of heavy metals and herbicides Although results show that under experimental conditions metals are often accumu-

lated in water-plants to concentrations above those of the external media (e.g. Sutton & Blackburn, 1971), very few experiments have identified translocation of metals within the plants. Myriophyllum spicatum was shown to accumulate mercury when grown in sediments containing either organic or inorganic mercury compounds (Dolar, Keeney & Chesters, 1971) but no attempt was made to separate the sediment from the culture medium and, contrary to the authors beliefs, translocation cannot be assumed. Results from ecological studies in lead-enriched lakes (see p. 73) pointed to lead absorption by the roots of submerged plants, and translocation to the shoots (Welsh & Denny, 1976), but when carefully tested in laboratory experiments very little translocation was observed (Welsh, 1978; Welsh & Denny, 1979b; and in press). However, there was good acropetal translocation of copper, particularly to young metabolically-active sites. This was more evident in plants of Potamogeton crispus than of P. pectinatus and may be related to a better developed vascular system in the former (Welsh & Denny, 1 9 7 9 ~ ; and see p. 81) .

Some useful data have been obtained from studies using 14C-labelled herbicides. Funderburk & Lawrence ( I 963 b) observed that Heteranthera dubia transported the xylem-mobile herbicide Simazine, slowly and equally in both directions and implied symplasmic translocation. When Thomas & Seaman (1968) applied xylem-mobile 14C herbicides, including Simazine and Atrazine to the roots of Potamogeton nodosus Poir., no movement was observed, nor did it occur when an Atrazine inoculation was made in the mid-leaf area. Sutton & Bingham (1968) likewise observed no transloca- tion of root-applied Simazine in Potamogeton crispus. Application of Endothal to roots of P. nodom produced a similar response (Thomas & Seaman, 1968) but when applied to submerged leaves it was distributed into the youngest leaves of primary plants, and into developing rhizomes and shoots of secondary plants, but not into mature or partially senescent leaves. Thomas & Seaman explained their results by accrediting translocation of Endothal to the phloem, and indeed, similarities can be seen between Endothal movement in P. nodosus and 32P movement in P. schweinfurthii (Denny, 1966) and P . pectinatus (Welsh & Denny, 1 9 7 9 ~ ) . As Potamogeton nodosus had an observable, although somewhat reduced xylem tissue, they attributed the lack of movement of Atrazine in xylem to the absence of a transpiration stream. Extrapolation of Thomas & Seamans proposals leads to the surprising conclusion that submerged water-plants translocate nearly all solutes in the phloem.

Frank & Hodgson (1964) found slight acropetal translocation of the phloem-mobile herbicide, Fenac, in Potamogeton pectinatus with tubers, but transport was reduced when tubers were removed. They observed no basipetal translocation, although basi- petal movement of phloem-translocated 2,4-D has been reported in the same species (Aldrich & Otto, 1959). Sutton and his co-workers investigated the inter-relationship between various herbicides and heavy metal uptake (Sutton, Weldon & Blackburn,

6-2

84 P. DENNY 1970; Sutton, Hailer, Steward & Blackburn, 197z), but their results did not provide information on long-distance transport.

VI. GENERAL DISCUSSION Data have been presented on the ecology and physiology of submerged

angiosperms and it would be desirable to formulate a clear-cut hypothesis for absorp- tion, translocation and excretion of solutes in such plants. Unfortunately this cannot be done. However, to draw together various aspects of the problem it may be helpful to summarize the case history for phosphorus as this element has been more extens- ively studied than most in the fresh-water environment.

A study of Canadian lakes and a survey of the Moosach river system suggested that species could be arranged in an ecological series which correlated with the phosphorus concentrations in the waters (Jones & Cullimore, 1973; Kohler et al. 1973) but Owens & Edwards (1961) could find no such correlations in their investiga- tions. In an English stream the phosphorus requirements of Ranunculus penicillatus were balanced by the measured loss of phosphorus from the water (Casey & Westlake, 1974) and in experimental tanks, where phosphorus was limited in the water, plants may have obtained a supply from the sediment (Ryan et al., 1972).

A correlation between the concentrations of phosphorus in shoot tissue and those in water has been used as an argument for ion absorption by shoots (Adams et al., 1971 ; Ryan et al., 1972) and when the phosphorus concentrations of shoot tissues fall below the critical level required to sustain growth, phosphate concentrations in the water have been considered to be the limiting factor (Gerloff & Krombholz, 1966).

Root absorption and acropetal translocation of 32P have been demonstrated in Myriophyllum bradieme, M . spicatum, Elodea canadensis, and Hydrilla verticillata (Bristow & Whitcombe, 1971 ; Bole & Allan, 1978). Very little basipetal movement was observed. DeMarte & Hartman (1974) reported acropetal translocation of =P in Myriophyllum exalbescens but also found that 32P from treated leaves was detectable in roots within 15 minutes. In Heterunthera dubia acropetal movement also seemed to predominate, although basipetal translocation could occur (Funderburk & Lawrence,

In submerged potamogetons, phosphorus seems to be very mobile. Widespread translocation of 32P from treated attached leaves has been shown for Potamogeton schweinfurthii (Denny, 1966), P. pectinatus and P. crispus (Welsh & Denny, 197ga), and over long periods, acropetal movement of root-absorbed 32P has been detected. Mobility is generally more rapid in P. crispus than in P. pectinatus. Experiments on Nujas minor revealed no translocation of 32P (Seadler & Alldridge, 1977) but in Vallis- neria movement has been observed in both directions, and Gentner (1977) even claimed that shoot-absorbed 32P was excreted by the roots.

Excretion of root-absorbed 32P by the shoots has been proposed for Myriophyllum exalbescens (DeMarte & Hartman, 1974) and for Zostera marina (McRoy & Barsdate, 1970). Evidence from Bole & Allan (1978) on Myriophyllum spicatum suggests that acropetal translocation of phosphorus may be independent of demand. Thus if

19634.

Solute movement in submerged angiosperms 85 acropetal translocation is unregulated, excess phosphorus may be lost from plant shoots. The pattern of mobility of a2P in submerged plants is in keeping with phloem translocation and possible transfer between xylem and phloem (Welsh & Denny,

In conclusion, a tentative hypothesis of solute movement can be formulated as

(i) Rooted submerged angiosperms can absorb solutes into their roots and shoots. (ii Most submerged plants utilize acropetal and basipetal translocation pathways. (iii) The amount of acropetal and basipetal translocation depends upon: (a) the

solute in question, (b) the anatomy and morphology of the plant, (c) the need for redistribution of the solutes, and (d) the physiological state of the plant.

19794.

follows :

(iv) From (iiib and d), different taxa must be expected to behave differently. (v) Different solutes may move independently in the plant. (vi) On occasions some solutes may be excreted by the plant tissues. Most of the above criteria are well established for ion transport in terrestrial plants.

However, in submerged plants the transpiration stream is absent and all reported translocation rates are relatively slow. In some instances, the movement appears to be extremely slow or non-existent.

If these statements are correct then the anatomy, morphology, physiological state and tissue requirements of some plants may be expected to favour shoot absorption whilst those of other plants would favour root absorption. At opposite extremes, translocation may hardly occur at all and, of course, it must be confined to shoots in the relatively few taxa which do not have roots. There is no reason to suppose that all solutes behave in the same manner, and whereas some may be transported in a predominantly acropetal or basipetal direction, others may have a general distribution. Excretion of solutes may occur sometimes but evidence is not yet conclusive. The diversity of behaviour of water-plants would explain much of the apparently con- flicting evidence from field observations and laboratory studies.

No longer should we be asking Do submerged angiosperms obtain solutes mainly through their shoots or through their roots? but What conditions govern root and/or shoot absorption, and by what mechanism(s) does translocation occur? Is translocation confined mainly to the conducting tissues of the phloem and/or xylem; or does symplasmic movement predominate? Does apoplastic movement occur to any great extent? How significant is solute excretion? When these questions are answered we may understand a little more about solute movement in submerged aquatic angio- sperms.

VII. SUMMARY

I. The roles of shoots and roots in the absorption of solutes, and in their trans- location within submerged aquatic angiosperms are contentious.

2. Correlations between the distribution of taxa and the chemistry of the water or sediment, and correlations between the concentrations of solutes in plants, waters and sediments, have been presented by various authors as evidence for the absorption of solutes from water or sediment. However, apparently conflicting data and diversity of opinions are numerous and large, and correlations can be misleading.

86 P. DENNY 3. Field data indicate that some taxa are directly affected by the quality of the

water in which they grow while others indubitably show a relationship between their growth and the underlying sediments. Experiments in artificial ponds substantiate both observations and suggest that solutes can be absorbed from either or both media. 4. Most taxa have the morphological adaptations necessary both for absorption

from the sediment (i.e. roots and root hairs), and from the waters (i.e. thin leaves with very thin cuticular membranes). They also have the facilities for translocation, although phloem and xylem tissues are frequently reduced.

5. Physiological evidence for direct absorption of solutes into leaf tissue is indis- putable. The mechanism of uptake has in some instances been shown to be an active process. Direct absorption of solutes by root tissue has been less investigated but observed acropetal translocation suggests absorption by the roots in those plants that have roots.

6. Many experiments have been described which purport to show acropetal and/or basipetal translocation of solutes. Generally, these experiments have been based on the separation of zones of shoot-treatment and root-treatment in plants suspended between two compartments. However, a leak-proof seal separating experimental solutions has not always been proved and this has led to doubts as to whether tracer substances have leaked through the apparently watertight seals. Nevertheless, the weight of evidence indicates that plants obtain solutes both through their roots and shoots, and both acropetal and basipetal translocation can occur. Whilst some solutes are readily absorbed and are freely mobile within the plant, others seem relatively immobile. Some solutes may be mainly translocated by the phloem, others may travel in the xylem, the symplasm, or perhaps the apoplast.

7 . The direction of movement depends upon various factors including the structure of the plant, the requirements of its tissues, the availability of the solutes, and the type of solute.

8. The suggested excretion of inorganic solutes by submerged angiosperms needs further investigation.

V I I I . ACKNOWLEDGEMENTS

The foundations of this review were laid during the tenure of a Visiting Professorship in the Department of Botany and Genetics, The University of Guelph. I am most grateful to my colleagues there for providing such an encouraging atmosphere. My thanks are also due to Dr Peter Welsh who has patiently helped to mould my ideas over several years of collaboration. Dr J. A. Wallwork kindly made helpful criticisms of the manuscript.

IX. REFERENCES ADAMS, F. S. & COLE, H. (1973). Element constitution of selected aquatic plants from Pennsylvania:

Submerged and floating-leaved species and rooted emergent species. Environmental PolZution 5 (z) , 117-147.

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