tolerance ofalbizia zygia (dc) macbride, a shade tree for cocoa, to soil acidity and water stresses

Sci Food Agric 1996,70, 389-395

Tolerance of Albizia zygia (DC) Macbride, a Shade Tree for Cocoa, to Soil Acidity and Water Stresses Zewge Teklehaimanot* and Gilbert Anim-Kwapong School of Agricultural and Forest Sciences, University of Wales Bangor, Gwynedd, LL57 2UW, UK (Received 10 March 1995; revised version received 28 June 1995; accepted 3 October 1995)

Abstract: This paper reports the results of the study carried out to assess the tolerance to soil acidity and drought of Albizia zygia (DC) Macbride, which is one of the West African native tree species growing wild and used to provide shade for cocoa, with a high potential for amelioration of degraded cocoa soils. The results of the present study indicate that A zygia is tolerant to soil acidity. The species also exhibited characteristics of drought avoidance. Stomata1 control was found to serve as a protective mechanism against excessive water loss when the seedlings were exposed to drought. The relationship between leaf water content and xylem pressure potential also indicated an ability of the species to maintain a given level of water tension throughout a certain degree of tissue dehydration. The high root : shoot ratio obtained for A zygia in this experiment suggests that the species is adapted to a resource limited environment. The increased magnitude of instantaneous relative growth rate in well-watered plants compared to water-stressed plants also indicates the overriding effect of changes in the instantaneous unit leaf rate on growth in A zygia subjected to different watering regimes.

Key words: soil moisture stress, soil acidity, tolerance, Albizia zygia, root, shoot.

INTRODUCTION due to high loss of nutrients in the soil and drought (Ahenkorah et al 1974; Beer 1987). Low pH in cocoa

Ayanlaja (1983) observed in Nigeria that soils carrying soils increases the availability of some major nutrients cocoa have lower pH values compared to those in adja- such as Cu, Fe and Mn to harmful concentrations and cent counterpart forest soils. Ahenkorah (1984) similarly reduces the availability of some major nutrients such as observed in Ghana that soil pH declined under cocoa N and P. Such a nutrient imbalance has been reported relative to adjacent forest soil. This is because when the to contribute appreciably to the poor cocoa seedling forest is felled or thinned and cocoa is planted, the establishment observed in old cocoa soils (Ayanlaja organic matter content of the forest soil diminishes 1983). According to Ahenkorah et a1 (1987), the absence under cocoa since the reduced density of the vegetation of fertiliser especially phosphatic fertiliser will accen- under cocoa is unable to replenish the soil organic tuate the decline in productivity in less than 10 years of matter as rapidly as it decomposes. Thus, the soils intensive cropping of cocoa in the absence of shade under cocoa become less fertile and acidic. These soils trees. Therefore, there is the need to rehabilitate cocoa also become susceptible to drought once the forest is soils using shade trees. Tolerance to soil acidity and cleared for cocoa cultivation as the moisture holding drought are considered major criteria for selecting can- capacity of cocoa soil is dependent on its organic matter didate shade tree species for rehabilitation of degraded content (Havord 1955; Ahenkorah 1984). cocoa soils.

Although the complete removal of shade in cocoa is Amelioration of soil acidity problems, especially in known to maximise yield in the short term, the the subsoil, is difficult and generally not economically increased yield is followed by a decline in productivity feasible (Foy et al 1974). An alternative approach, which

may be more cost-effective, is to plant shade tree species * To whom correspondence should be addressed. which are tolerant to acidic soils and drought (Sartain

J Sci Food Agric 0022-5142/96/$09.00 0 1996 SCI. Printed in Great Britain 389

390 Z Teklehaimanot, G Anim-Kwapong

and Kamprath 1978). The role of trees in improving soil physical and chemical properties in agriculture by maintaining soil organic matter levels is well docu- mented (Young 1989). There is also considerable know- ledge on the function of shade trees in cocoa (Beer 1987, 1988). One of the West African native tree species which grows wild and used to provide shade for cocoa is Albizia zygia (DC) Macbride, syn Albizia brownei (Walp) Oliv, which can accumulate appreciable amount of organic matter from litter and fix nitrogen through biological nitrogen fixation both of which contribute to soil amelioration. However, no work has been reported in the literature on the capability of the species to toler- ate acidic soils and drought which are prevalent in cocoa soils.

The aim of the present study was, therefore, to evalu- ate the potential of A zygia for amelioration of degraded cocoa soils by examining its early growth patterns as influenced by low soil pH and moisture stress.

EXPERIMENTAL

Seedling production and experimental design

The experiment was conducted under tropicalized glass- house condition during March to August 1993 at the Penn-y-Ffridd Field Station (University of Wales, Bangor, UK).

Seeds of A zygia collected from cocoa fields at Cocoa Research Institute of Ghana (CRIG), Tafo were germi- nated in trays containing moist sand in the greenhouse. Four weeks after germination, the seedlings were trans- planted into 2 litre pots containing 1.5 kg of potting mixture (John Innes No 1 Compost, Humax L & P Peat Limited, Carlisle, UK). Prior to transplanting, the pH of the potting mixture was adjusted with hydro- chloric acid (HCl) solution to a target pH of 4.5 and 5.5 which is the pH range that occurs in degraded soils of abandoned cocoa farms in West Africa. There were in total two soil pH treatments plus a control (pH 6.8). All treatments were inoculated with 6 ml of liquid culture of Bradyrhizobium sp (Cowpea miscellany, RCR 3824 or CB 756) in two splits of 3 ml at transplanting and a week later. Two weeks after transplanting, two phostrogen plant food tablets (Phostrogen Limited, Clwyd, UK) were implanted in each pot. Two tablets were again implanted at 4-weekly intervals. This was done in order to replenish the store of nutrients in the potting mixture. Four weeks after transplanting, each of the soil pH treatments was sub-divided into two parts, each consisting of two pots. One part, watered daily to field capacity was designated unstressed while the stress treatment was watered to field capacity once every 2 weeks over a period of 10 weeks (five cycles of stressed/ unstressed periods). The seedlings were raised in relatively large plots (Zlitres), with the growth medium

wetted to field capacity from above and below through perforations at the bottom of the pots following the method described by Pennypacker et a1 (1990). The growing medium for drought stress, was allowed to dry out gradually under greenhouse condition, after initially wetting to field capacity. This technique allowed for the gradual development of drought stress (a gradual decline in soil water potential) thus enabling pot grown plants to acclimate to the stress. The experiment was laid out in a randomised complete block design arranged in a 3 x 2 factorial with three replications. Pots were re-arranged (re-randomised) every week.

Data collection

All measurements were taken at the end of every cycle (at 2 weeks intervals, five cycles).

Midday diffusion resistance was measured with an automatic porometer (MK 3, Delta-T Devices, UK) on the abaxial surfaces of exposed mature leaves of two plants in each treatment. Stomata1 conductance was then calculated as the inverse of diffusion resistance. Transpiration rate values were finally derived according to Fick's law as the product of stomata1 conductance and the absolute humidity difference between leaf and air measured during the leaf diffusion resistance mea- surement (Sterne et all977).

Relative water content of leaves was measured after whole leaves were allowed to saturate with water for 4 h at room temperature following the method described by Barrs (1968). The xylem pressure potential of the shoots of two plants per treatment were determined by the pressure chamber method following Ritchie and Hinck- ley (1975). Detached leaves from six replicate plants per treatment destructively harvested were used for the determination of total leaf area of plant. The determination was done using an electronic leaf area meter (Model AAM-7-1974, Hayashi Denco Co Ltd, Tokyo, Japan). Soil moisture potential of the stressed and unstressed treatments were measured at the end of every cycle using the filter paper method described by Fawcett and Collis-George (1967). The determination was done on soils (growing medium) in which plants have been harvested for the above measurements.

Growth analysis

The functional approach to growth analysis was followed in analysing the primary data on leaf area and leaf, stem and root dry weights (Hughes and Freeman 1967; Hunt 1990). Leave, stem, and root dry weights were obtained by destructively harvesting six replicate plants per treatment at the end of each cycle, and drying at 70°C for 24 h after which they were weighed. The primary data on leaves and stem (shoot) and root dry weights were logarithmically transformed to render the

Tolerance of Albizia zygia to acidity and water stresses 39 1

TABLE 1 Significance levels (P values) of the effects of soil pH and water regime treatments on water relations and

root : shoot ratio of Albizia zygia

Treatment Stomatal Transpiration Relative Xylem Root : shoot conductance rate water content pressure ratio

potential

First cycle Soil pH 0.10 0.5 0.6 0.86 0.88 Water regime (W) 0.001 0.001 0.001 0.00 1 0.001 pH x W 0.21 0.21 0.35 0.08 0.4

Second cycle Soil pH 0.710 0.4 0.7 0.06 0.4

pHxW 0.903 0.49 0.25 0.40 0.7 Water regime (W) 0.001 0.001 0.001 0.002 0.001

Third cycle Soil pH 0.86 0.45 0.8 0.36 0.9

pH x W 0.36 0.6 0.5 0.26 0.6 Water regime (W) 0.001 0.001 0.001 0.00 1 0.002

Fourth cycle Soil pH 0.4 0.4 0.94 0.46 0.58 Water regime (W) 0.001 0.001 0.001 0.001 0.001 pH x W 0.636 0.86 0-62 0.17 0.54

Fifth cycle Soil pH 0.9 0.8 0.27 0.53 0.17 Water regime (W) 0.001 0.001 0.001 0.001 0.001 pH x W 0.3 0.07 0.78 0.10 0.2

variability in the data more homogenous with time. From these data, root : shoot ratios were calculated. Cubic polynomial curves were fitted to the natural logarithms of the primary data on total dry weight (root and shoot) and leaf area to derive values of relative growth rate (RGR), leaf area ratio (LAR) and unit leaf

effects. Correlation analysis was also carried out to find out relationships between the parameters measured.

RESULTS

Water relations rate (ULR).

The results of the experiment showing the effects of soil Statistical analysis

The data were subjected to a two-way analysis of variance to determine significant levels of treatment

pH and water stress on all water related parameters measured are given in Tables 1 and 2.

Stomatal conductance, transpiration, relative water content and xylem pressure potential were significantly

TABLE 2 Mean values of water relations and root : shoot ratio of Albizia zygia measured over five cycles of drought

Parameters Water pH stress (MPa) 4.5 5.5 6.8

(1) Stomatal conductance - 0.6 .0.34 f 0.02 0.36 _+ 0.02 0.35 f 0.02 (cm s- ' f SE) - 0.02 0.60 & 0.05 0.58 * 0.05 0.59 f 0.04

2.9 f 0.2 3.1 & 0.1 3.0 f 0.2 (2) Transpiration rate - 0.6 (g cm-' s - ' f SE) - 0.02 5.9 f 0.2 6.0 & 0.1 6.1 f 0.2

(3) Relative water content - 0.6 77 f 2.5 76 k 3.0 78 f 3.0 (% & SE) - 0.02 89 f 4.0 91 k 3.5 90 f 3.0

(4) Xylem pressure potential -0.6 -0.70 f 0.02 -0.69 k 0.02 -0.71 f 0.01 (MPa f SE) -0.02 -0.40 & 0.02 -0.41 _+ 0.01 -0.39 f 0.02

( 5 ) Root : shoot ratio - 0.6 0.67 k 0.02 0.68 f 0.02 0.69 f 0.01 (ratio +_ SE) - 0.02 0.40 0.01 0.39 f 0.02 0.41 f 0.01

Mean

0.35 f 0.02 0.59 f 0.05 3.0 f 0.2 6.0 f 0.1 17 f 3.0 90 f 3.0

-0.70 f 0.01 -0.40 & 0.01

0.68 k 0.01 0-40 f 0.01


TABLE 3 Correlations (r') between stomata1 conductance, relative water

content and xylem pressure potential

Stomatal Relative Xylem conductance water pressure

content potential ~

Stomatal conductance - 0.83 0.65 - 0.73 Relative water content _.

( P < 0.01) affected by soil moisture regime throughout the time course of the experiment. However, soil pH and the interaction between the two main effects of water regime and soil pH had no significant effects ( P > 0.05) on any of the water relation parameters.

Stomatal conductance showed positive linear correlations with relative water content and xylem pressure potential. The relative water content also correlated positively with xylem pressure potential (Table 3).

' O 1

-2 O J

-l -1.5 i -2.0 J

1 0 1

Fig 1. The interaction of soil pH and water regime on the time course of instantaneous relative growth rate of Albizia zygia. Bars are the 95% CI for six replicate plants. The solid line and broken line represent soil moisture potentials of

- 0.6 MPa and - 0.02 MPa, respectively.

'10'' 1.01

3 -o.si I Weeks

pH 6 . 8 E 5 6 7

Weeks " 0.01

a 3

Fig 2. The interaction of soil pH and water regime on the time course of instantaneous unit leaf rate of Albizia zygia. The bars are the 95% CI for six replicate plants. The solid line and broken line represent soil moisture potentials of

- 0.6 MPa and - 0.02 MPa, respectively.

Growth analysis

In Tables 1 and 2, it is also shown that moisture regime significantly ( P < 0.01) affected root : shoot ratio. Soil pH and its interaction with water regime had no significant effects ( P > 0.05) on root : shoot ratio.

Although there were no consistent significant effects due to moisture regime, well-watered plants showed a slight tendency towards a higher instantaneous RGR. The fitted curves for all treatments followed a similar sigmoidal pattern (Fig 1).

Instantaneous ULR followed a trend similar to the instantaneous relative growth rate. However, unlike RGR, ULR was more consistently and significantly influenced by moisture stress throughout the time course of the experiment ( P < 0.001) (Fig 2).

Whereas the fitted curves for RGR and ULR followed similar sigmoidal patterns with maximum points coinciding at the same time, this was not so for the

Tolerance of Albizia zygia to acidity and water stresses 393

110

-- 16

2 % a 5 90

B IS f ” 1

drought than ULR in this experiment although the effect of water stress and pH on LAR was statistically significant ( P -= 0.001).

pH 4.5

DISCUSSION

i=- 70 -

0 1 2 3 L 5 6 7 8 9 10 Wccks

80

pH 5 . 5

pH 6 .8

f 2 ’“1

0 1 2 3 L 5 6 7 8 9 10 Weeks

Fig 3. The interaction of soil pH and water regime on the time course of instantaneous leaf area ratio of Albizia zygia. Bars are the 95% CI for six replicate plants. The solid line and broken line represent soil moisture potentials of -0.6 MPa

and -0.02 MPa, respectively.

instantaneous leaf area ratio (LAR), particularly the curves fitted for well-watered plants (Fig 3).

The correlation analysis (Table 4) between RGR, ULR and LAR indicate that LAR was less sensitive to

Effects of soil acidity on the water relations and growth of Albizia zygia

The non-significant effect of substrate pH on all the reported parameters measured on A zygia in this experiment suggests that the species is tolerant to acidic substrate and capable of maintaining satisfactory growth at low soil pH levels comparable to those in this experiment. However, this evidence should be treated with caution since hydrogen ion concentration per se is not the only factor in the soil acidity complex capable of influencing plant growth. On the other hand, however, an associate species of the same genus, Albizia adianthi- folia is reported to be adapted to acid soils (von Carlo- witz 1991).

Effects of water stress on the water relations of Albizia zygia

The relationship between midday stomatal conductance and transpiration as observed in this experiment is consistent with the concept of stomata serving as an overriding protective mechanism against excessive water loss.

The average stomatal conductance value of 0.59 cm s - l obtained for plants growing at the soil moisture potential of -0-02 MPa was comparable to the values reported for unstressed tropical woody plants. For example, Osonubi and Fasehun (1987) reported an average midday stomatal conductance value of 0.50 cm s - for unstressed seedlings of Parkia biglobosa.

The relationship between xylem pressure potential and transpiration rate also observed in this experiment suggests the possibility of an internal redistribution of water in the tissues of A zygia seedlings (Ritchie and Hinckley 1975). The relationship between relative water content and xylem pressure potential as also observed

TABLE 4 Correlations between RGR, ULR and LAR

Treatment ULR LAR

r2 P-value r2 P-value

pH 4.5, stressed RGR 99.7 <0.01 95.2 >0*05

pH 5.5, stressed RGR 99.4 <0*01 42.0 >0.05

pH 6.8, stressed RGR 100 <0.01 44.0 >0,05

pH 4.5, unstressed RGR 99.9 <0.01 67.8 >0.05




in this experiment is another indication of the ability of the species to maintain a given level of water stress under a certain degree of tissue dehydration (Jordan and Ritchie 1971). These findings are in close agreement with results reported in the literature for drought resist- ant plant species. For example, an association between low stomatal conductance and high xylem pressure potential has been found in several North American desert living shrubs by Nelsen et a1 (1984). Bradbury (1990) also found that Sesbania sesban and Acacia nilotica responded to water stress by reducing stomatal conductance, so that leaf water content and xylem pressure potential were maintained at values similar to those of the non-stressed plants.

Effects of water stress on the growth of Albiziu zygiu

The root : shoot ratio obtained for well-watered A zygia plants of 0.40 & 0.03 is high compared to the range of values of 0.15-0.30 reported for some unstressed tropical deciduous tree seedlings (Huante et al 1992). It is apparent that the seedlings allocated a substantial amount of dry-matter to the roots, hence the increased root : shoot ratio. The high root : shoot ratio obtained for A zygia in this experiment suggests that the species is adapted to a resource-limited environment (Chapin 1983). The effect of soil moisture stress in increasing root : shoot ratio of seedlings points to the fact that root growth progressed faster than shoot growth. This is a tolerance mechanism that brings the roots into contact with large volume of soil.

A comparison of the nature of the fitted curves for the instantaneous RGR, ULR and LAR (Figs 1-3) and the correlation analysis between these three growth com- ponents (Table 4) suggest that LAR is relatively less sensitive than ULR to drought.

The instantaneous relative growth rate values of 0.59 and 0.36 g g-' week-' obtained for well-watered and water-stressed plants, respectively, are within the range of 0.053-0.822 g g-' week-' reported for both tem- perate and tropical woody seedlings (Jarvis and Jarvis 1964; Grime and Hunt 1975; Huante et a1 1992). These slow growth rates in tree seedlings have been attributed to the expenditure on woody tissue, a process concomi- tant with a slow rate of leaf expansion.

The peak instantaneous unit leaf rates of 3.3 x lop3 g cm-' week-' (33-0 g m-' week-') and 6.3 x lo-' g cm-' week-' (63.0 g cm-' week-') recorded for moisture-stressed and unstressed A zygia plants, respectively, are also well within the range of 20-60 g m-' week-' recorded for tropical woody plants (Okali 1971 ; Whitmore 1984).

Soil moisture stress increased the magnitude of the instantaneous LAR over that of unstressed A zygia. This result agrees with findings for Sesbania sesban and Acacia nilotica where soil moisture stress significantly increased the leaf area ratio (Bradbury 1990).

The effect of water stress on RGR did not result from a decrease in leaf area per unit of whole plant dryweight since LAR of water stressed plants was larger than plants in the absence of water stress. Water stress reduced RGR by lowering ULR, indicating that net carbon assimilation was primarily affected. In other words, the decline in RGR in water stressed plants was accompanied by a fall in ULR, indicating that a decrease in photosynthetic activity on a leaf area basis was responsible for the fall in RGR.

Applying stress to the roots of A zygia apparently reduced growth through reduction of assimilation by leaves. The reduction in growth rate and transpiration in A zygia as a result of the imposition of stress to the root medium is in agreement with findings for many other plants (Stocker 1960; Okali and Dodoo 1973). This is an indication that the factors affecting rate of assimilation in leaves greatly influenced growth responses in A zygia.

The results obtained from this experiment suggest a very sensitive stomatal control mechanism in the seedlings of A zygia which enables it to avoid the development of internal water deficit. However, when stomata closes, protecting the plant against water loss, they simultaneously restrict carbon assimilation by the plant; hence a reduction in the growth rate of water- stressed plants.

CONCLUSIONS

Results obtained from the present study indicate tolerance of A zygia to substrate acidity. The species also exhibited characteristics of drought avoidance. That is, under moisture stress, the seedlings maintained a high midday xylem pressure potential while midday stomatal conductance and transpiration were lowered relative to the unstressed situation.

Some organs of A zygia seedlings exhibited a high level of plasticity. For example, a small reduction in leaf area in relation to a large reduction in the total biomass of the seedling under drought stress, resulted in a high leaf area ratio. This is one of the most effective mechanisms by which mesophytic plants can adapt to dry environments (Begg 1980). The plasticity exhibited in root-shoot partitioning of assimilates in response to resource availability is also an important means by which the seedlings of A zygia adjusted to the drought stress. Plasticity in root-shoot partitioning is of ecological significance as examined by Grime (1974). By divert- ing more assimilates to the root, rooting density and depth is increased. An increased depth and density of roots is considered a major mechanism for maintaining water uptake, which in turn maintains turgor pressure.

Results from the present study also indicated that the reduction in seedling relative growth rate under moisture stress was due mainly to a reduction in unit leaf

Tolerance of Albizia zygia to acidity and water stresses 395

rate which implies that under these circumstances, net carbon assimilation was greatly affected.

To understand the fullest impact of water stress on the photosynthetic capacity of the species, further studies is recommended on the effects of soil water stress on diurnal changes in net carbon assimilation, xylem pressure potential, leaf relative water content and stomata1 conductance. The establishment through further research of the probable existence of diurnal relationships between xylem pressure potential, stomatal conductance and leaf relative water content will elucidate any internal mechanisms of adaptation to water stress by A zygia.

The results of the present study in general show the tolerance to acidic soil and the resistance to drought by A zygia. These are indications of the species potential for soil amelioration of degraded cocoa soils.

ACKNOWLEDGEMENT

Dr Frank Minchin of the Institute of Grassland and Environmental Research of AFRC in Aberystwyth is gratefully acknowledged for providing the Brady- rhizobium inoculum. The greenhouse observations were made by one of the authors (GAK) in the course of the work supported by the Overseas Development Admin- istration of the United Kingdom scholarship, for which the PhD of the University of Wales was awarded.

REFERENCES

Ahenkorah Y 1984 The influence of environment on growth and production of the cocoa tree: soils and nutrition. In: Proc Seventh International Cocoa Conference. The Cocoa Producers’ Alliance, Lagos, Nigeria. pp 167-176.

Ahenkorah Y, Akrofi G S, Adri A K 1974 The end of the cocoa shade and manurial experiment at the Cocoa Research Institute of Ghana. J Hort Sci 49 43-51.

Ahenkorah Y, Halm B J, Appiah M R 1987 Twenty years results of cocoa shade and manurial experiment at the Cocoa Research Institute of Ghana. Exper Agric 23(1)

Ayanlaja S A 1983 Rehabilitation of cocoa (Theobroma cacao L.) in Nigeria: Major problems and possible solutions I. Causes of difficulty of seedling establishment. Plant Soil

Barrs H D 1968 Determination of water deficit in plant tissues. In: Water Dejcits and Plant Growth (Vol I), ed Koz- lowski T T. Academic Press, New York, USA, pp 236-268.

Beer J 1987 Advantages, disadvantages and desirable characteristics of shade trees for cocoa, coffee and tea. Agrofor Sys 5 3-13.

Beer J 1988 Litter production and nutrient cycling in coffee (CofSea arabica) or cocoa (Theobrorna cacao) plantation with shade trees. Agrofor Sys 7 103-114.

Begg J E 1980 Morphological adaptations of leaves to water stress. In: Adaptation of Plants to Water and High Tem-

31-39.

73(3) 403-409.

perature Stress, eds Turner N C & Kramer P J. Wiley- Interscience, New York, USA, pp 33-42.

Bradbury M 1990 The effect of water stress on growth and dry matter distribution in juvenile Sesbania sesban and Acacia nilotica. J Arid Envir 18 325-333.

Chapin F S 1983 Adaptation of selected trees and grasses to low availability of phosphorus. Plant Soil 72 283-297.

Fawcett R G, Collis-George N 1967 A filter paper method for determining the moisture characteristics of soil. Aust J Exp Agri Anim Husb 7 162-167.

Foy C D, Lafever H H, Schwartz J W, Fleming A L 1974 Aluminium tolerance of wheat cultivars related to region of origin. Agron J 66 751-757.

Grime J P 1974 Vegetation classification by reference to stra- tegies. Nature 250 26-33.

Grime J P, Hunt R 1975 Relative growth rates: its range and adaptive significance in a local flora. J Ecol63 393-422.

Havord G 1955 Soil conditions for cocoa and their amend- ment for maximum yield. In: Report on Cocoa Conference. The Cocoa Chocolate and Confectionary Alliance, London.

Huante P, Rincon E, Gavito M 1992 Root system of seedlings of seven tree species from a tropical dry forest in Mexico. Tree 6 77-82.

Hughes G E, Freeman P R 1967 Growth analysis using fre- quent small harvests. J Appl Ecol4 553-560.

Hunt R 1990 Basic Growth Analysis. Unwin Hyman, London, UK.

Jarvis P G, Jarvis M G 1964 Growth rates of woody plants. Physiol Plant 17 654-666.

Jordan W R, Ritchie J T 1971 Influence of soil water stress on evaporation, root absorption, and internal water status of cotton. Plant Physiol48 783-788.

Nilsen E T, Sharifi M R, Rundel P W 1984 Comparative water relations of phreatophytes in the Sonoran desert of California. Ecology 63 767-778.

Okali D U U 1971 Rates of dry matter production in some tropical forest tree seedlings. Ann Bot 35 87-97.

Okali D U U, Dodoo G 1973 Seedling growth and transpiration of two West African Mahogany species in relation to water stress in the root medium. J Ecol61(2) 421-438.

Osonubi 0, Fasehun F E 1987 Adaptation to soil drying in woody seedlings of African locust bean (Parkia biglobosa (Jacq.) Benth.). Tree Physiol3 321-330.

Pennypacker B W, Leath K T, Stout W L, Hill R R 1990 Techniques for stimulating field drought stress in the greenhouse. Agron J 82 951-957.

Ritchie G A, Hinckley T M 1975 The pressure chamber as an instrument for ecological research. Adv Ecol Res 9 165-254.

Sartain J B, Kamprath E J 1978 Aluminium tolerance of soybean cultivars based on root elongation in solution culture compared with growth in acid soil. Agron J 70

Sterne R E, Kaufmann M R, Zentmyer C A 1977 Environ- mental effects of transpiration and leaf water potential in avocado. Physiol Plant 41 1-6.

Stocker 0 1960 Physiological and morphological changes in plants due to water deficiency. UNESCO Arid Zone Res 15

von Carlowitz P G 1991 Multipurpose Trees and Shrubs. Sources of Seeds and Inoculants. ICRAF, Nairobi, Kenya.

Whitmore T C 1984 Tropical Rain Forest of the Far East (2nd edn). Oxford University Press, Oxford, UK.

Young A 1989 Agroforestry for Soil Conservation. CAB Int, Wallingford, Oxon, UK.

pp 35-40.

17-20.

63-104.

tolerance ofalbizia zygia (dc) macbride, a shade tree for cocoa, to soil acidity and water stresses

Documents