pectin changes in samples containing poplar cambium and inner bark in relation to the seasonal cycle

Planta (1994)193:446-454 P l a n t ~

~ Springer-Verlag 1994

Pectin changes in samples containing poplar cambium and inner bark in relation to the seasonal cycle Mathias BaYer 1, Ren6e Goldberg ~, Anne-Marie Catesson 1, Mich~le Liberman ~, Nadia Bonchemal 2, V6ronique Michon 2, Catherine Herv~ du Penhoat 2

Laboratoire des Biomembranes et Surfaces Cellulaires V6g6tales URA 311, Ecole Normale Sup6rieure, 46 rue d'Ulm, F-75230 Paris Cedex 05, France 2 D~partement de Chimie, URA 1679, Ecole Normale Sup6rieure, 24 rue Lhomond, F-75231 Paris Cedex 05, France

Received: 16 October 1993 / Accepted: 26 November 1993

Abstract. Biochemical changes occurring during the tran- sition between meristematic activity and rest were studied in samples containing cambial cells and their phloem derivatives from Populus x euramericana. Uronic acids represented around 9% of the cell-wall dry matter in spring and 7% in summer and winter. In contrast, a higher content of methylated galacturonic acids was observed during the rest period. The degree of esterification increased from 2% in spring to 35% in winter, indicating an important accumulation of acidic pectins during the active season although the cation content was always very low. Nuclear magnetic resonance spectroscopy of neutral polysaccharides solubilized with boiling water showed that in winter arabinans and xylans were the main carbohydrates. By contrast, in spring and in summer the xylans were very scarce, arabinans being the major neutral polysaccharide, indicating that important modifications occur during the autumn. Histochemical observations of material treated with hot water and ED TA confirmed the low relative pectin content during the rest period. Calcium ions, detected as antimonate salt were scarce. In the cambium, they were located mainly in cell junctions whereas in phloem derivatives these cations were distributed throughout the whole cell wall.

Key words: Bark - Cambium Pectins - Poplar - Season

Abbreviations: Araf= arabinofuranose; COSY = 2 D homonuclear correlation spectroscopy; NMR=nuclear magnetic resonance spectroscopy; NOESY=2 D nuclear Overhauser effect spectroscopy; PATAg-periodic acid thiosemicarbazide silver proteinate; PME=pectin methylesterase; R wall=radial wall; T wall = tangential wall; Xylp = xylopyranose

Correspondence to: R. Goldberg, Enzymologie en milieu structur6, Institut Jacques Monod, Tour 43, 2 Place Jussieu, F-75251 Paris Cedex 05, France; FAX: 33(1)44275994

Introduction

The regulation of cambial activity which controls wood production and diametral growth of trees still requires clarification (Little and Savidge 1987; Lachaud 1989; Creber and Chaloner 1990; Savidge 1990; Aloni 1992). While the seasonal variations of the characteristic fea- tures of cambial cells are well documented (see Catesson 1990 for a review), at least in trees which grow in temperate climates, cell-wall structure and composition have been studied almost solely in actively dividing cambia. Northcote and his group were amongst the first to per- form detailed analyses of cambial cell walls (see North- cote 1963) and to underline the importance of pectin me- tabolism in these cells. Since then, they have focused most of their work on the relationships between differentiation and the control of cell-wall biogenesis (see Northcote 1984, 1985, 1989 and references therein). The important contributions of S imson and Timell (1978a-d), as well as more recent studies (Edashige et al. 1992), have also been concerned with the cambial zone of actively growing trees. These reports showed that in the cambial zone the relative amounts of cellulose, hemicelluloses and pectins vary with the species. Values given for the pectic fraction range from 7% (ash cambium, Northcote 1963) to 50% (aspen cambium, Simson and Timell 1978a) of total polysaccharides. Such discrepancies may arise, at least in part, from the extraction technique used. In addition, cytochemical investigations (Roland 1978; Catesson and Roland 1981) revealed early changes in pectin localization following cell division, which shows the heterogeneity of cambial cell walls. Recently formed tangential walls (T walls), rich in methylated pectins, did not have any recognizable middle lamella while radial walls (R walls) had a well-defined middle lamella rich in acidic pectin. Besides the pectins, xyloglucans were also localized cytochemically in the primary wall of cambial derivatives (Baba et al. 1992).

In temperate climates, cambial activity follows the seasonal cycle. Changes in wall thickness accompanying the onset of the rest period and the resumption of

M. Baier et al. : Seasonal changes of pectins in poplar cambium 447

meris temat ic activity were described some sixty years ago by Kerr and Bailey (1934) bu t detailed invest igat ions were restricted to a few cytochemical da ta (Catesson 1980; Rao 1985; F u n a d a and Catesson 1991). This lack of i n fo rma t ion is regret table since cell wall c o m p o n e n t s are now k n o w n to play a role in p lan t growth and de- ve lopment (Ald ing ton et al. 1991). The present s tudy was ini t ia ted in order to shed light on these seasonal changes th rough parallel biochemical , cytochemical a nd biophysical approaches. Focus was centered on pectins, the s t ructure of which is known ' to influence cell-wall p H and in t u rn cell-wall extensibility. The results repor ted here were ob ta ined on a deciduous ha rdwood , Popu- lus x euramericana.

Materials and methods

Plant material. Material was collected from two to three-year-old branches of poplars (Populus x euramericana, cv. 1214, Betulaceae) grown in the garden of our institute. Collections were made at three different times during a year: in spring (middle of April), late summer (first week of September) and winter (middle of December). All samples from a given collection date were pooled together. All biochemical analysis were carried out three times.

Analysis of components extracted from cell walls. The outer bark and most of the phloem were removed in order to expose the latest- formed phloem layers. These phloem layers were scraped from the lignified xylem together with the cambium and lyophilized. In spring, the first differentiating xylem cells were also included. At each stage of the collection, the nature of the removed tissues was checked with a light microscope on transverse sections obtained from the extremity of the sample. Cell walls were isolated (Goldberg et al. 1986) and their dry weight determined. Two pectic fractions were extracted successively with boiling water (two extractions at pH 5.5) and 1% EDTA (6 h at 60 ~ C). The eounterions initially present in the EDTA extract were exchanged with protons, a process which allows removal of EDTA molecules by dialysis (Goldberg et aL 1989). The pectic fractions were then reduced to a small volume by concentration. Uronic acids and total carbohydrates were estimated according to previously described procedures (Goldberg et al. 1986). Neutral sugars were separated through HPLC following acid hydrolysis with 1 N H2SO4. Hydrolysate was neutralized with strontium carbonate, dried under vacuum, sus- pended in 1 ml ultrapure water and filtrated on a 0.45-gin mem- brane (MiUipore, Eschborn, Germany). Neutral sugars present in the hydrolysate were separated using an SP 8750 liquid chromato- graph (Spectra Physics, Darmstadt-Kranichstein, Germany) equip- ped with a NH2-column. Sugars were eluted with a mixture of acetonitrile-water (75/25) and detected with a Spectra-Physics SP 6040 refractive-index detector connected to a SP 4290 integrator.

Estimation of pectin-methylesterase activity. Peetin-methylesterase activity was measured titrimetrically by following the increase in free carboxyl groups as previously described (Goldberg et al. 1992).

Nuclear magnetic resonance ( NMR) spectroscopy. A Bruker (Wis- sembourg, France) AM-400 spectrometer operating in the Fourier transform mode at 400.13 MHz for tH and 100.57 MHz for x3C was used. Samples were dissolved in DzO and dimethylsulfoxide (DMSO) was the internal reference (6C 39.5, 6H 2.72). The spectral window for the 1H-NMR spectra in Fig. 2A was 10 ppm for 16 - 103 data points with a pulse width of 8 gs (45 ~ and an acquisition time of 1.02 s. The 13C-NMR spectra were recorded with complete proton decoupling and a pulse width of 8 gs (90~ The acquisition time was 1.11 s with a 3-s delay between each scan.

Double-quantum-filtered phase-sensitive two-dimensional (2 D) homonuclear correlation spectroscopy (COSY) NMR experiments

(Piantini et aL 1982) were performed using a (90~176 - (90~ t2) sequence. A delay of 0.1 s was introduced between the last two 90 ~ pulses for the 2 D nuclear Overhauser effect spectroscopy (NOESY; Bodenhausen et al. 1984) NMR spectrum. Long-range COSY spectra were acquired with the (90~ - (90~ t2) sequence and a z-value of 0.100 s. The spectral width in F1 and F2 was about 2000 (4000) Hz for the COSY (NOESY) spectra; the number of data points in F2 was 1024, and 512 increments were recorded. The 90 ~ pulse was 17 I~s and the total acquisition time was about 20 h. Before Fourier transformation, the data were multiplied with a II/2 shifted squared sine bell (a sine bell in the case of the COSY (Bax and Freeman 1981) experiment]. Zero filling was applied in F1.

Preparation for electron microscopy and cytochemical staining. Small pieces of tissues including the cambial region were fixed for 1.5 h at room temperature in 4% glutaraldehyde in 0.1 M sodium cacodylate buffer (pH 7.2). Samples were treated according to one of the following procedures: (i) overnight incubation at room temperature in 1% EDTA; (ii) 2 h incubation in boiling water; (iii) overnight incubation in cacodylate buffer for control specimens.

Tissue blocks were post-fixed for 1 h with 1% osmium tetroxide in the same buffer, dehydrated through a graded alcohol-propylene oxide series, and embedded in Araldite. Ultrathin transverse sections were stained with periodic acid thiosemicarbazide silver proteinate (PATAg) to visualize polysaccharides (Thi6ry 1967) and observed with either a Philips 300 or a Philips 400 (Eindhoven, The Netherlands) electron microscope at 80 kV.

Calcium localization at the electron-microscope level. Calcium bound to the cell walls was localized in samples by the antimonate- precipitation technique (Wick and Hepler 1980; Slocum and Roux 1982) according to the procedure previously used for Fraxinus cambium (Funada and Catesson 1991).

Results

Composition o f the pectins extracted from bark tissues. The wall mater ia l was sequent ia l ly extracted with boi l ing water at acidic p H (a round p H 5.0) and with ho t 1% E D T A at 60 ~ C. These successive t rea tments have been shown to solubilize more pectins t han a single 1,2-cy- c lohexanediaminete t race t ic acid t r ea tmen t with ( C D T A )

70 [ ] ARA

60 [] C-,AL [] O.~_U

50 [] RHA

4o [] XYL

--- so L

'2 _ 1 Sp S~n W[

Fig. 1. Changes in the water-soluble neutral carbohydrates in the cell-walls of the inner bark and cambium of poplar during the seasonal cycle. Sp, Sm and Wr indicate spring, summer and winter. The different neutral sugars released by acid hydrolysis are ex- pressed as percent arabinose (ARA); galactose (GAL); glucose (GL U); rhamnose (RHA); xylose (XYL)

448 M. BaYer et all: Seasonal changes of pectins in poplar cambium

1 H

Sp A2

Sm

Wr Y ~ ~ / ~ . i . . . . I . . . . r . . . . i . . . . i . . . . r . . . . i . . . . E . . . . i . . . .

5.0 4.0 3.0 2.0 PPM

13 c

Sp

A3 A1 A4A21 IA5

.,~.~.~.L~..,,a,., ~, " ~ r r ~ " ~ ~nIl~lT

Wr

X2

. / 100 80 60 40

PPM

Fig. 2. Proton-nuclear magnetic resonance (400 MH2) and l a C - N M R (100 MH2) spectra of water-soluble neutral polysaccharides from the inner bark of poplar. Material collected in spring (Sp), summer (Sin) and winter (Wr), A1 and X1 indicate H-1 of arabinosyl and xylosyl residues in the ~H spectrum and C-1 of arabinosyl and xylosyl residues in the laC spectrum, etc.

(Goldberg et al. 1989). The respective amounts of uronic acids (esterified and non esterified), neutral sugars and divalent cations (calcium and magnesium) are reported in Table 1. Substantial changes occurred during the an- nual cycle. The active period (spring) was characterized by a high level of non-esterified galacturonans, a very low level of methylated galacturonans [which corresponds to a degree of esterification (DE) around 2%, Table 2] and a relatively low content of neutral sugars. In contrast, during the rest period, the pectic fractions were enriched in neutral sugars and impoverished in galacturonic acids. Moreover, the galacturonans were more methylated than in spring. In order to understand this increase, the activity of cell-wall pectin methylesterase known, to be involved in demethylation occurring "in muro", was also

Table 1. Composition of the pectins extracted from bark tissues of poplar in spring (Sp), late summer (Sm) and winter (Wr). Neutral sugars and uronic acids as ~tmol - g - 1 cell walls; cations as ~teq �9 g - 1 cell walls

Sp Sm Wr

Neutral sugars 372 323 526

Uronic acids Esterified 10 92 104 Unesterified 463 222 190

Cations Ca 2 + 1.2 a 3.8 Mg 2+ 0.2 2.8 4.6

a The sample was inadvertently destroyed. As a result, the calcium evaluation could not be performed

M. Baier et al. : Seasonal changes of pectins in poplar cambium 449

Table 2. Activity of cell-wall pectin methylesterase (PME) and degree of esterification (DE) in bark tissues of poplar during spring (Sp), late summer (Sm) and winter (Wr)

Sp Sm Wr

PME geq �9 min- ~ �9 g- 1 cell walls 30 15 22

DE% 2 29 35

estimated (Table 2). This activity was at its highest during the active period, which might explain the low DE value noticed for spring pectins. All through the year, the cation contents (Table 1) remained particularly low com- pared with data obtained from primary walls. Contents were, however, slightly higher in winter than in spring.

Carbohydrate composition of the pectic fractions. Most neutral polymers associated with the pectins were extracted with the boiling-water treatment. The sugars released after acid hydrolysis of water-soluble pectins were identified by HPLC (Fig. 1). Arabinose was the major monomer recovered in spring and at the end of summer. In contrast, during winter, equal amounts of arabinose and xylose were observed. In order to isolate

the neutral polysaccharides extracted with the pectins during the boiling-water treatment, the water-soluble fractions were submitted to anion-exchange chromatog- raphy on diethylaminoethyl (DEAE) Sepharose CL 6B. The unbound material containing the neutral polysaccharides, was collected, reduced to a small volume and submitted to N M R analysis.

Nuclear magnetic resonance spectroscopy. Both 1H and 13C N M R spectra were recorded for all fractions of unbound neutral polymers and these spectra are given in Fig. 2, with the exception of the 13C spectrum of the summer wall material. In this latter case, the signal-to- noise ratio was very low due to the limited amount of sample available.

The major component of the samples obtained during the active period could be identified as ~-Arabinofura- nose (~-Araf) by comparison with both ~H- and ~3C- chemical-shift data reported for the 5-1inked a-Araf residue of pectins isolated from Vigna radiata (Herv6 du Penhoat et al. 1987; Table 3). The proton assignments were confirmed with a long-range COSY spectrum. Mul- tiple signals are observed for all carbons suggesting that the arabinan is highly branched. Similar chemical-shift ranges have been reported (Capmek et al 1983) for the

Fig. 3A-B. Transverse sections of the cambial zone (CZ) and adja- cent tissues of poplar during the active season (May); PATAg staining. A Unextracted control. B Material treated with EDTA,

showing swelling of the middle lamella (*) in very young vessel derivatives and differentiated phloem cells. ST, sieve tube; Is, fu- ture vessel. Scale bar = 5 gm

450 M. Ba'ier et al. : Seasonal changes of pectins in poplar cambium

Fig. 4A-C. Transverse sections of the cambial zone of poplar during the active season (May). A, B Treatment with EDTA; PATAg staining. A Higher magnification of boxed area of Fig. 3B; the cell walls of phloem derivatives are scarcely extracted. B The radial-wall middle lamella of cambial derivatives towards the xylem is swollen

and strongly extracted. C Localization of calcium bound to the cell wall with the antimonate precipitation technique. Dense precipitate in the cambial zone (CZ) and xylem derivatives (X). Diffuse precipitate in radial (R) walls of phloem derivatives (P). Scale bar = 1 gm

carbon signals of the L-arabinan from the roots of Al- thaea officinalis which contains terminal, 5-1inked, 2,5- linked and 3,5-1inked residues: C-1 (107.6-108.7), C-2 81.4-88.2), C-3 (76.4-84.3), C-4 (82.5-85.2) and C-5 (62.3-68.0).

Several signals which could not be attributed to the arabinosyl residues were also present in the 13C spectrum of the summer sample: 62.3, 62.7, 64.1, 68.5, 70.2, 71.4, 73.8 and 78.5 ppm. However, the chemical shifts of almost all of the signals in the anomeric region of the carbon spectrum are near 109 ppm. Moreover, the inten- sity of the 1 H - N M R signals in the anomeric region of 13-glycosyl sugars in Fig. 2A-1 is very weak. This would appear to indicate that some of the glucose residues (the second most abundant neutral sugar according to chemical analysis) also adopts the a-furanose form. The carbon chemical shifts described (Bock and Pedersen 1983) for a-methyl-glucofuranose (a-Me-Glcf) are 110.0, 80.6, 75.8, 82.3, 70.7, and 64.7 ppm for C-1 to C-6, respectively.

In contrast, the major component of the samples from the rest period was a 13(1-4)-xylan (Table 3). The proton assignments which were obtained from double-quantum- filtered COSY and NOESY spectra, are analogous to those reported for 13-methyl-xylopyranose (13-Me-Xylp; Bock and Thogersen 1982). The carbon chemical shifts of this residue are almost identical to those of the 13(1-4)- xylan isolated from wood (Mendonga-Previato et al. 1979). Comparison of the 13C spectra of the unbound neutral polymers during the rest and active periods in- dicates that a considerable proport ion of a-arabinan is also present in the sample from the rest period. Numer- ous weak signals which could not be identified were also observed (72.4, 72.97, 73.38, 74.84, 76.03, 76.77, 76.97, 102.98, 140.16, 105.93, and 175.22 ppm).

Subtractive localization of pectins. Observations of the cambial zone before and after cell-wall extraction allow- ed localization at the ultrastructural level, of the pectic substances analyzed above. During the period of meriste-

M. Baier et al. : Seasonal changes of pectins in poplar cambium

matic activity (Figs. 3, 4), the cambial zone comprised four to eight cell layers with thin R walls and very thin, newly formed T walls. Phloem derivatives were characterized by early thickening of both R and T walls. In contrast, the walls remained thin in enlarging xylem derivatives and the cells could be distorted or partly crushed during embedding and sectioning (Fig, 3). As previously observed (Benayoun et al. 1981; Czaninski et al. 1982), neither hot water nor EDTA had an effect on lignifying cell walls. Hot-water treatment often disrupted the fragile walls of the cambial zone and its xylem derivatives, separating the differentiated xylem from the phloem (not shown). Treatment with EDTA alone was not very effective in the R walls of cambial derivatives towards the phloem but extracted most of the middle lamella in the R walls of the first xylem derivatives (Fig. 3B; compare Fig. 4A and 4B). Extraction and swelling of the middle lamella were observed in R and T walls of phloem and enlarging xylem cells where the swelling was often extensive (Fig. 3B).

In late summer and winter, during the resting season (Fig. 5), the cambial zone was reduced to one to two layers of cells with thickened R walls, the middle lamella of which was irregularly extracted with EDTA. For the most part, T walls were unaffected by that treatment. Phloem cell walls were swollen and their middle lamella completely extracted as observed in spring. Hot-water

451

treatment solubilized most of the middle lamella and part of the primary wall both in cambium and phloem cells.

Thus, with the exception of the conspicuous R-wall thickening, few differences were observed between active and resting cambia. The heterogeneity between R and T walls reported previously (Catesson and Roland 1981) persisted even in resting cells although it disappeared gradually with maturation of cambial derivatives. Pectin localization was similar whatever the season. Hot-water- soluble polysaccharides were present both in the primary walls and middle lamellae, both in young T walls as well as in older T or R walls. On the other hand, uronic acids which could be extracted with EDTA were localized almost exclusively in the middle lamellae, which promp- ted us to localize wall-bound calcium ions.

Calcium localization in the cell walls. When blocks of tissues were fixed in the presence of potassium antimonate, an electron-dense precipitate occurred in the cell walls of unextracted specimens. This precipitate was solubilized when ultrathin sections were treated with EDTA and precipitate formation was prevented by prior treatment of tissues with EDTA. This suggests that the electron-dense precipitate consisted of calcium antimonate and agrees with the results of a secondary ion mass spectrometry (SIMS) investigation (A. Jauneau, Univer- sity of Rouen, France, personal communication). In the

Fig. 5A-B. Transverse section of the resting cambial zone (CZ) of poplar in September; PATAg staining. A Unextracted control; B EDTA treatment. The middle lamella is more strongly extracted

in phloem cell walls (arrows). PP, phloem parenchyma; ST, sieve tube; V, vessel. Scale bar=2 lam

452 M. Baier et al. : Seasonal changes of pectins in poplar cambium

Table 3. The 400-MHz IH and 100-MHz 13C chemical shift (~) data" of sugars identified in unbound neutral polymers isolated from poplar inner bark

Sugar and IH 13C position ~ (lit.) b Mul t ip l ic i ty g (lit.) c

ct-Araf a

1 5.1-5.2 ( 5 . 1 2 ) 109.08-109.4 (109.0) br s

2 4.15 (4.16) 82.4-83.87 (82.3) br s

3 4.00-4.02 (4.04) 78.13 (78.2) br m

4 4.2-4.3 (4.24) 85.58-85.88 (83.7) hr m

5 3.72, 3.84 (3.81, 3.91) 62.3, 62.7, 67 (68.2) dd (12, 5), m

[3-Xylp e

1 4.5 (4.21) 103.2 (103.1) br d

2 3.12 3.14 (3.14) 74.24 (74.3) br t

3 3.59 (3.33) 75.2 (75.3) t (9)

4 3.8 (3.51) 77.91 (78.0) 5 3.4, 4.1 (3.21, 3.88) 64.52 (64.5)

a Key: br d = broad doublet; br m = broad multiplet; br s = broad singulet; dd=doublet of doublets; t=triplet. In D20 at 296 ~ K referenced to internal dimethylsulfoxide (cSH 2.72 and 6c 39.5 ppm) b See Herv6 du Penhoat et al. (1987) for cc-Araf and Bock and Pedersen (1983) for [3-Xylp respectively c See Herv~ du Penhoat et al. (1987) for c~-Araf and Mendonga- Previato (1979) for [3-Xylp respectively d From spring sample e From winter sample

cambial zone, the precipitate was localized almost exclusively in the R walls, mainly at cell junctions (Fig. 4C). In the phloem, it was more scattered but present in most walls. This distribution is similar to the distribution previously observed for calcium ant imonate in Fraxinus (Funada and Catesson 1991).

Few differences were observed in unextracted cambium and phloem in relation with the seasonal cycle. In contrast, partial solubilization of cell walls with hot water prevented precipitate format ion during the active season but not during the rest period (not shown). This could be due to the very low amount of calcium present in the cell walls in spring (Table 1). These results demon- strate the sensitivity of the ant imonate technique since the calcium content represents only 1-4 % of cell-wall dry weight, a very low value in comparison with data obtained f rom other materials (Goldberg et al. 1986).

Discussion

The pectic fraction represents 11-13% of total cell-wall polysaccharides isolated f rom the cambial zone and young phloem of Populus x euramericana. These values agree with those given by Nor thcote (1963) for other tree species (7 20%) but differ f rom those reported by Simson

and Timell (1978a). According to these authors the cambial cell walls of aspen, Populus tremuloides, contained nearly 50% of pectic material. However this high value corresponded to the amount of material extracted f rom the tissues by successive treatments with H 2 0 at 80 ~ C, 0.5% ammonium oxalate at 80 ~ C, 10% sodium carbonate and 1% KOH. The two last extractants also solubilized significant amounts of hemicelluloses which might explain the yield reported by Simson and Timell (1978a). In any case, the uronic acids did not account for more than 12% of the cambial cell walls. Moreover, pre- cise comparisons between chemical analyses performed on the cambial zone are difficult since it is nearly impossible to isolate pure cambial tissue in sufficient amount. The samples studied here also contained several layers of recent phloem cells and, in the case of spring samples, enlarging xylem derivatives were also present. Cyto- chemical observations of extracted and non-extracted material once more underlined cell-wall heterogeneity in the cambium itself. As previously noted (Roland 1978; Catesson and Roland 1981), cambial T walls were more resistant to extractants, especially to EDTA, than R walls. These differences disappeared with derivative differentiation and cell wall extraction always seemed more pronounced in the phloem than in the cambium. The main seasonal changes concerned the sensitivity of cambial cell walls to hot water which was much higher in spring than during the resting season. In contrast to other species such as sycamore (Catesson 1990), few differences were observed in relation to E D T A treatment between active and resting cambial cells of poplar, although the amount of uronic acids sharply decreased with the cessation of meristematic activity.

In fact, the relative amount of pectins remained more or less constant all year round (11% of total polysaccharides in winter, 13 % in spring) but the composit ion of the pectic fraction varied appreciably with the seasons. Spring cell walls were characterized by a high degree of acidity: unesterified uronic acids were abundant and cations were scarce. The relative amounts of uronic acids and arabinose were similar to those found in aspen pectin by Simson and Timell (1978d) but the respective amounts of galactose and rhamnose were noticeably lower in Populus x euramericana. The presence of a high amount of a highly branched arabinan in the middle lamella of phloem walls may explain the extensive swelling of these walls after extraction either with E D T A (see also Cates- son 1982) or hot water, it may also explain the notable fragility of the R walls of active cambial cells after hot- water treatment. During summer, when meristematic activity gradually slowed down, the amounts of uronic acids and pectin-methylesterase activity decreased in parallel to an increase in the degree of esterification. The onset of winter rest was characterized by an increase in neutral sugars, mostly due to a fourfold increase in xylose residues.

Seasonal changes in apoplasmic pH contrast with those described for cytoplasmic pHs in Castanea sativa stems (Pezet-Si Mohammed 1987). In this instance, the internal pH of bark and cambial cells was maximal during the growth period. Inverse pH variations on both


sides o f the p lasma m e m b r a n e emphasize the seasonal changes o f the t r ansmembrane electrochemical gradient and in turn the suitability o f the condi t ions for the active m e m b r a n e t ranspor t o f auxin (see L a c ha ud 1989). More- over, due to the high p ropor t i on o f unesterified pectins dur ing the active period, it can be assumed that the apoplasmic p H is lower in spring than in winter. Accord- ing to the acid g rowth theory (Rayle and Cleland 1977), a low p H would be part icularly suitable for the cell-wall enzyme activities involved in cell-walt loosening and in turn for cell expansion.

It is interesting to compare the increase in xylans fol lowing the cessation o fmer i s t emat ic activity observed in poplar cambium with the results obta ined by Nor th - cote and his co-workers using differentiating vascular cells. Successive induct ion o f a rabinan-synthetase and xylan-synthetase activities was observed dur ing bean xy- logenesis (Bolwell and Nor thco t e 1981, 1983). The first type o f activity occurred dur ing the period o f cell division and growth and the second one was observed dur ing the period o f secondary cell-wall thickening. Similarly, in Acer cambial derivatives, the onset o f secondary thickening was correlated to the cessation o f po lyga lac tu ronan- synthetase activity (Bolwell et al. 1985) and to an increase in xylan-synthetase activity (Dalessandro and Nor thco t e 1981). A n increase in xylan-synthetase activity was also described by Suzuki et al. (1991) in differentiating tra- cheids o f Zinnia. Thus, the cessation o f cell divisions in the cambial zone is closely fol lowed by the switching on of xylan synthesis whether the cambial cell differentiates into xylem or enters into quiescence.

In conclusion, qualitative and quant i ta t ive pectin modif icat ions m a y represent early markers o f bo th the determinat ion o f cambial derivatives and the transi t ion f rom activity to rest or vice versa.

We acknowledge support from the French Ministry of Research and Technology and also from the European Program Eureka 447/ Eurosilva.

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