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Aquaporin water channels (AQPs) constitute a large family of transmembrane proteins present throughout all kingdoms of life. They play key roles in the flux of water and many solutes across the membranes. The AQP diversity, protein features, and biological functions of silver birch are still unknown. A genome analysis of Betula pendula identified 33 putative genes encoding full-length AQP sequences (BpeAQPs). They are grouped into five subfamilies, representing ten plasma membrane intrinsic proteins (PIPs), eight tonoplast intrinsic proteins (TIPs), eight NOD26-like intrinsic proteins (NIPs), four X intrinsic proteins (XIPs), and three small basic intrinsic proteins (SIPs). The BpeAQP gene structure is conserved within each subfamily, with exon numbers ranging from one to five. The predictions of the aromatic/arginine selectivity filter (ar/R), Froger’s positions, specificity-determining positions, and 2D and 3D biochemical properties indicate noticeable transport specificities to various non-aqueous substrates between members and/or subfamilies. Nevertheless, overall, the BpePIPs display mostly hydrophilic ar/R selective filter and lining-pore residues, whereas the BpeTIP, BpeNIP, BpeSIP, and BpeXIP subfamilies mostly contain hydrophobic permeation signatures. Transcriptional expression analyses indicate that 23 BpeAQP genes are transcribed, including five organ-related expressions. Surprisingly, no significant transcriptional expression is monitored in leaves in response to cold stress (6 °C), although interesting trends can be distinguished and will be discussed, notably in relation to the plasticity of this pioneer species, B. pendula. The current study presents the first detailed genome-wide analysis of the AQP gene family in a Betulaceae species, and our results lay a foundation for a better understanding of the specific functions of the BpeAQP genes in the responses of the silver birch trees to cold stress.

Leaf hydraulic conductance (Kleaf) and capacitance (Cleaf) are among the key parameters in plant-water regulation. Understanding the responses of these hydraulic traits to drought conditions remains a challenge for describing comprehensive plant-water relationships. The ability of an organism to resist and/or tolerate embolism events, which may occur at high negative pressure caused by hydric stress, relies on how well it can sustain a hydraulic system in a dynamic equilibrium. Populus deltoides is a water-saving tree species with a stomatal conductance that declines rapidly with reduced water availability. Under unfavorable conditions, the stomatal control of transpiration is known to be closely coordinated with a loss of plant hydraulic functioning that can ultimately result in hydraulic failure through xylem embolism, notably in leaves. The effects of drought on leaf hydraulics are also related to regulation in water permeases such as the aquaporins. To describe the responses linked to leaf hydraulics under severe drought and rewatering conditions, water-stressed poplars were monitored daily on an ecophysiological and a molecular scale. A structural and expression analysis on a set of aquaporins was carried out in parallel by in situ hybridization analysis and quantitative PCR. In complement, water distribution in water-challenged leaves was investigated using X-ray microtomography. A general depression of leaf hydraulic conductance and relative water content occurred under drought, but was reversed when plants were rewatered. More interestingly, (i) extreme leaf water deficiency led to marked xylem and lamina embolism, but a degree of hydric integrity in the midrib extraxylem territories and the bundle sheath of the minor veins was maintained, and (ii) the sub-tissue water allocation correlated well with an over-accumulation of several PIP and TIP aquaporins. Our multi-facet molecular ecophysiological approach revealed that leaves were able to secure a certain level of hydric status, in particular in cell territories near the “living ribs”, which provided rapid hydric adjustment responses once favorable conditions were restored. These findings contribute to an integrated approach to leaf hydraulics, thus favoring a better understanding of the cell mechanisms involved in tree vulnerability to climate changes.

Climate change is expected to increase drought frequency and intensity which will threaten plant growth and survival. In such fluctuating environments, perennial plants respond with hydraulic and biomass adjustments, resulting in either tolerant or avoidant strategies. Plants' response to stress relies on their phenotypic plasticity. The goal of this study was to explore physiology of young Populus nigra in the context of a time-limited and progressive water deficit in regard to their growth and stress response strategies. Fourteen French 1-year-old black poplar genotypes, geographically contrasted, were subjected to withholding water during 8 days until severe water stress. Water fluxes (i.e. leaf water potentials and stomatal conductance) were analyzed together with growth (i.e. radial and longitudinal branch growth, leaf senescence and leaf production). Phenotypic plasticity was calculated for each trait and response strategies to drought were deciphered for each genotype. Black poplar genotypes permanently were dealing with a continuum of adjusted water fluxes and growth between two extreme strategies, tolerance and avoidance. Branch growth, leaf number and leaf hydraulic potential traits had contrasted plasticities, allowing genotype characterization. The most tolerant genotype to water deficit, which maintained growth, had the lowest global phenotypic plasticity. Conversely, the most sensitive and avoidant genotype ceased growth until the season's end, had the highest plasticity level. All the remaining black poplar genotypes were close to avoidance with average levels of traits plasticity. These results underpinned the role of plasticity in black poplar response to drought and calls for its wider use into research on plants' responses to stress.

Les hauts-marais - ou tourbieres ombrotrophes bombees - sont des ecosystemes caracterises par des vegetations turfigenes qui reposent sur un histosol hydromorphe formant un tertre independant des nappes locales. Leurs interets biologique, historique et paleoecologique ont suscite des demarches de preservation, menant a l’elaboration de diagnostics et d’actions de gestion. Pour etre pertinents, ces derniers doivent s’appuyer sur un modele conceptuel de fonctionnement. L’objectif de ce travail est donc de proposer un modele de developpement des hauts-marais a partir d’une synthese de la litterature recente. Il en ressort que le haut-marais est un ecosysteme auto-organise, fortement dependant du role des sphaignes dans l’elaboration des premiers elements de l’ecosysteme, la conservation de la matiere organique et le maintien d’un milieu hostile aux competiteurs potentiels comme les arbres. Les etudes paleoecologiques montrent que les vegetations les plus turfigenes correspondent a celles croissant sur les buttes, avec Sphagnum austinii (disparu de France), S. fuscum et S. capillifolium, ou celles des banquettes a S. magellanicum et S. rubellum. La forme generale du haut-marais resulte d’un equilibre avec le volume d’eau circulant dans le systeme. Sa microtopographie de surface est dependante des phenomenes de competition entre les differentes sphaignes et du volume d’eau circulant dans le systeme. Vegetations, forme generale et microtopographie resultent de phenomenes complexes faisant intervenir les relations entre les plantes, le sol et l’eau. En integrant l’ensemble de ces donnees, le diagnostic fonctionnel, mais aussi l’elaboration et le suivi des actions de gestion, n’en seront que plus efficaces.

Summary In winter and spring, walnut trees exhibit variations of sugar content in the vascular sap. According to their location, the vessel-associated cells (VACs, also called contact cells) could control nutrient exchanges between the storage parenchyma and the xylem vessels. According to the literature, the recovery of sap (influx) occurs at the VAC/vessel interface via an H+/sugar symport that depends on the transmembrane pH gradient generated by the plasma membrane H+-ATPase. The aim of this study was to investigate the ATPase activity, using a perfusion technique that allows the use of several effectors: carbonyl cyanide m-chlorophenylhydrazone (CCCP) and fusicoccin (FC). During winter, the uncoupler CCCP revealed a low pH gradient between the xylem vessels and the vessel-associated cells. Under these conditions, FC, an activator of the H+-ATPase, had no effect on the pH of the perfusion solution, suggesting that the enzyme could be lightly active. In contrast, close to bud break, a high pH gradient was revealed by the use of CCCP, and an acidification of the perfusion solution was observed in the presence of FC. Moreover, cytochemical investigation showed high activity of two respiratory enzymes located in mitochondria: NAD-dependent isocitrate dehydrogenase and succinate dehydrogenase. The hypothesis is that in spring this high respiratory activity of VACs provides a consequent increase in available ATP that can be utilized by the plasma membrane H+-ATPase.

Because of the potential involvement of adenosine in the winter re-acquisition of nucleotide synthesis capability of strawberry plants (Fragaria × ananassa Duch., Elsanta), the properties and the time-course activity of an adenosine kinase (EC 2.7.1.20) and an adenine phosphoribosyltransferase (APRTase, EC 2.4.2.7) of the adenosine recycling pathway were characterized. The results showed an increase in APRTase activity during winter re-acquisition of nucleotide synthesis capability and an increase in adenosine kinase activity during spring growth of strawberry plants. Western blot analysis, performed with polyclonal rabbit antibodies raised against peach bud adenosine kinase, showed a concomitant rise in the strawberry enzyme. These results suggest that APRTase activity could be a good marker of the break of strawberry plant dormancy and adenosine kinase a marker of subsequent spring growth.

At the end of summer, the diminution of photoperiod and temperature cause a decrease of vegetative growth and the dormancy of strawberry plants. Although the decrease in vegetative growth can be measured morphologically, no test is able to evaluate the decrease in growth potential (i.e. during the dormancy induction) nor its possible influence on vegetative growth. On the one hand, to estimate this influence biometrically, we have correlated photoperiod and temperature decreases with the vegetative growth decrease of some strawberry cultivars observed in the field. Results have confirmed the major role of photoperiod, temperature and the effect of growth potential decrease on vegetative growth. Moreover, the results showed that the decrease of vegetative growth was an early event at the end of summer which depended upon strawberry cultivar. On the other hand, we have measured petiole length under standard climatic conditions in a growth chamber, after natural summer and autumnal exposures. Observations of strawberry plants under these conditions revealed a decrease of their growth potential which also depended upon strawberry cultivar. Results also confirmed the possible action of growth potential decrease on the vegetative growth at the end of summer. Consequently, the observation of strawberry plants under standard conditions can be used, as a test, to assess the exact moment of dormancy induction.

At present, many strawberry plants (Fragaria x ananassa Duch.) are cold-stored in autumn and replanted the next summer to provide good crops the following spring. These plants should be dormant (low growth potential) with abundant nutrient reserves (e.g. carbohydrates) to withstand storage. Because no rapid test was available to assess the induction of autumnal dormancy of strawberry plants, we have evaluated the level of relative nutrient deficiency of growth organs (correlative inhibition, one cause of dormancy) by intracellular pH measurements on Elsanta strawberry plants. To control the validity of this evaluation, carbohydrate accumulation in the storage parenchyma was measured and the dormancy level was estimated by measuring the petiole length of Elsanta plants under controlled conditions. The results showed that petiole length and intracellular pH of buds were low at the beginning of autumn, implying that the measurements of intracellular pH can be a marker of dormancy induction in strawberry plants. Then, in the middle of autumn, when the first changes occurred, growth inhibition remained even as the intracellular pH measurements implied a break of correlative inhibition. This should suggest that another growth inhibition remains after the first chilling.

The mechanisms of saccharose and sorbitol transport in Prunus persica leaves were investigated in plasma membrane vesicles purified by aqueous 2-phase partitioning and equilibrated in pH 7.5 buffer containing K+. The imposition of an artificial proton motive force energized an active uptake of both saccharose and sorbitol. The maximum uptake rate of saccharose was 2.5 times higher than that of sorbitol. Saccharose and sorbitol uptake exhibited saturation kinetics suggesting they were carrier-mediated. Apparent Km for the saccharose and the sorbitol uptake were 0.36 and 0.67 mM, respectively. Active absorption of saccharose was completely inhibited by a non-permeant thiol reagent, PCMBS, contrary to sorbitol absorption. These results suggested that saccharose and sorbitol were transported at least by two different carriers.

It has previously been shown that cotyledonary pricks induced modifications of ion levels (H + and K + ) in hypocotyl cells of Bidens pilosa. These modifications differed according to the light quality : H + levels increased and K + levels decreased in white light (WL), whereas H + levels decreased and K + levels increased in blue light (BL). In this study, in order to determine the mechanism responsible for these ionic modifications, plasma membrane vesicles have been isolated and characterized from hypocotyl cells. The effects of light quality and cotyledonary pricks on plasmalemma ATPase activity (EC 3.6.1.3) were studied. Cotyledonary pricks induced, in WL, rapid (5 min) and transient (restoration in 60 min) inhibition of plasmalemma ATPase activity. Conversely, in BL, a rapid and transient stimulation was observed. These results suggest that, in Bidens pilosa, plasmalemma ATPase is involved in 'short-term' ionic level modifications induced by traumatisms.

Summary Sucrose and sorbitol are the predominant forms of photosynthetically reduced carbon transported in Prunus persica . The mechanisms of sucrose and sorbitol transport were investigated in plasma membrane vesicles isolated from peach tree leaves. The maximum uptake was obtained for sucrose (2.5 times higher than sorbitol uptake). Active absorption of sucrose was completely inhibited by a non-permeant thiol reagent, PCMBS, contrary to sorbitol absorption. These results suggested that sucrose and sorbitol were transported at least by two different carriers. The absorption of exogenous sucrose and sorbitol in the bud and the underlying territories of the bud and their distribution in the single node cuttings were evaluated during the vegetative rest period. The utilization of PCMBS, an inhibitor of sucrose transporter, allowed for the estimation of the active absorption of sucrose in different isolated territories. It was assumed that the sink capacities of tissues depend on their potentialities to absorb carbohydrates by H + /symport. The presence of unlabelled sucrose during PCMBS pretreatment decreased the inhibition exerted by PCMBS: it suggested that the inhibitor was bound specifically on the sucrose transporter. The «active» absorption related to the transporter could therefore be distinguished from the total absorption, including diffusion. The evolution of exogenous sucrose and sorbitol was evaluated during the vegetative rest period, in isolated territories and in single node cuttings. In natural conditions, three periods were disclosed: 1) from September to October, the bud, intrinsically able to grow, was inhibited by the stem; 2) from November to December the bud was in a state of deep dormancy, and it was intrinsically unable to absorb nutrients; and 3) January was characterized by the reacquisition of bud growth potential and bud burst. Concerning sorbitol, a similar evolution was observed but it seemed that sorbitol was principally utilized a few days before budburst.

Summary Fluidity changes between PM from buds and stands, isolated from dormant, non-dormant and nondormant with an additive cold period peach trees, were monitored by steady-state fluorescence polarization using the lipophilic probe 1,6-diphenyl 1,3,5-hexatriene. The break of dormancy, induced by low temperatures, appeared to be correlated with an increase of the PM fluidity from buds, which became higher than from stands. The corresponding apparent energies of activation, calculated from the Arrhenius plots, were modified with the break of dormancy. Indeed, energies of activation were different between the two territories from dormant material but appeared very close in non-dormant material. A superimposed cold treatment on non-dormant peach tree triggered an enhancement of the fluidity differences between PM from buds and stands. However, the apparent energies of activation between the two territories were not modified. These results were discused according to the trophic hypothesis.

Format du poster : A0; absent

Addition of methyl oleate to a Streptomyces hygroscopicus NRRL B-1865 culture modified the metabolic properties of this strain. This addition decreased the pH of the medium, increased the valine uptake of the cells and reduced their consumption of glucose until the beginning of antibiotic biosynthesis, which was delayed. At the same time, an increase in growth (× 1.8) and a marked improvement in antibiotic production (× 20) could be observed. The use of labelled methyl oleate showed that methyl oleate was not a precursor of antibiotics produced by S. hygroscopicus NRRL B-1865. It is suggested that methyl oleate addition may cause some alteration in membrane permeability, inducing an increase in H+ extrusion and stimulating the accumulation of branched amino acids, known to be direct precursors of polyether antibiotics.

International audience; Plasma membrane H+-ATPase (PM H+-ATPase) plays a key role in nutrient transport, stress responses and growth. To evaluate proton motive force differences between apical and basal parts of acrotonic 1-year-old shoots of walnut (Juglans regia L. cv 'Franquette') trees, spatial and seasonal changes in PM H+-ATPase were studied in mature xylem tissues. During both the dormancy and growth resumption periods, and in both the apical and basal parts of the stem, PM H+-ATPase activity showed positive correlations with the amount of immunodetectable protein. In spring, at the time of growth resumption, higher activities and immunoreactivities of PM H+-ATPase were found in the apical part of the stem than in the basal part of the stem. In spring, the decrease in xylem sugar concentration reflected the high sugar uptake rate. Our data suggest that PM H+-ATPase plays a major role in the uptake of carbohydrates from xylem vessels during growth resumption. These results are discussed in the context of the acrotonic tendency of walnut shoots.

Budbreak anomalies in temperate fruit trees grown under mild conditions have often been described. However, only few authors approached the physiological evolution of leaf buds all along the dormancy period according to the temperature pattern. The aim of this study was to characterize the evolution of peach leaf bud dormancy through some physiological and biochemical parameters under temperate winter conditions and under total cold deprivation after the endodormancy onset. Two treatments were applied in peach trees cv. Redhaven: (i) Regular Chilling Amounts — RCA and (ii) Total Chilling Deprivation — TCD. Buds were sampled periodically from different parts of the stem (terminal, medium and basal ones). We recorded the evolution of: carbohydrate concentrations (glucose, fructose, sucrose, sorbitol and starch), respiration rate, water contents and energy metabolism (ATP and ADP ratio). The dynamics of these parameters were compared and correlated with dormancy evolution ("one node cuttings" test) and budbreak patterns in plank:. The endodormancy intensity of terminal buds was significantly lower than those of median and basal buds in early October. Under RCA treatment, this gradient faded and the bud endodormancy release was completed at the same time in all positions along the stem. Thereafter, the "cuttings" test indicated that terminal buds grew slightly faster than median and basal buds, and, consistently, budbreak in planta started with the terminals buds, followed by the medians and then by the basal ones. The carbohydrate contents showed a transitory change only when the buds began to grow after the endodormancy was released under RCA. Respiration, water content and ATP/ADP changed dynamics only under RCA and only after the end of the endodormancy (their respective changes were very parallel). The dynamics of none of the tested parameters could be related with the endodormancy dynamics, but respiration, water content and ATP/ADP could be consistent markers of the actual bud growth before bud break (in this respect, ATP/ADP could not show differences between the terminal and axillary buds while respiration and water content could).

Sucrose has been reported to play multiple roles in the winter biology of temperate woody species. However, no report on the molecular basis of sucrose transport in xylem tissue has yet been made. In the walnut tree, it is demonstrated that during the autumn-winter period, active absorption of sucrose from xylem vessels to parenchyma cells (sucrose influx) is much higher when samplings were taken shortly after a period of freezing temperatures. Here, the question of whether this increased sucrose influx mirrors a regulation of sucrose transporters in xylem tissue was tested. A putative sucrose transporter cDNA (JrSUT1: Juglans regia sucrose transporter 1) was isolated. Over the autumn-winter period, JrSUT1 transcripts and respective proteins were present in xylem parenchyma cells and highly detected when samplings were performed shortly after a freeze-thaw cycle. This up-regulation of JrSUT1 level was confirmed in controlled conditions and was not obtained in bark. Immunolocalization studies showed that JrSUT1 and plasma membrane H+ -ATPase (JrAHA) were colocalized to vessel-associated cells (VACs), which control solute exchanges between parenchyma cells and xylem vessels. We propose that JrSUT1 could be involved in the retrieval of sucrose from xylem vessel. All these data are discussed with respect to the winter biology of the walnut tree.

International audience

In peach trees (Prunus persica L. Batsch cv. Redhaven), sorbitol is a primary photosynthetic product and may play an important role in the budbreak process. Surprisingly, before budbreak (from January to early March), the concentration of sorbitol in the xylem sap decreases, while that of hexoses (glucose and fructose) increases. The aim of this work was to study the control of sorbitol uptake into vegetative buds by hexoses. Sorbitol uptake was selectively inhibited by hexoses at low and physiological concentrations and this effect was both reversible and concentration-dependent. In addition, the active uptake of sorbitol significantly declined in the plasma membrane vesicles-enriched fraction purified from glucose-treated vegetative buds, suggesting that the inhibitory action of glucose was at the membrane level. Finally, among several glucose analogues tested, only hexokinase substrates (2-deoxyglucose and mannose) were able to mimic the glucose effect, which was completely blocked by the hexokinase inhibitor mannoheptulose. These results represent the first steps towards a better understanding of polyol transport control in plants.

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Vegetative buds of peach (Prunus persica L. Batsch.) trees act as strong sinks and their bud break capacity can be profoundly affected by carbohydrate availability during the rest period (November-February). Analysis of xylem sap revealed seasonal changes in concentrations of sorbitol and hexoses (glucose and fructose). Sorbitol concentrations decreased and hexose concentrations increased with increasing bud break capacity. Sucrose concentration in xylem sap increased significantly but remained low. To clarify their respective roles in the early events of bud break, carbohydrate concentrations and uptake rates, and activities of NAD-dependent sorbitol dehydrogenase (SDH), sorbitol oxidase (SOX) and cell wall invertase (CWI) were determined in meristematic tissues, cushion tissues and stem segments. Only CWI activity increased in meristematic tissues shortly before bud break. In buds displaying high bud break capacity (during January and February), concentrations of sorbitol and sucrose in meristematic tissues were almost unchanged, paralleling their low rates of uptake and utilization by meristematic tissues, and indicating that sorbitol and sucrose play a negligible role in the bud break process. Hexose concentrations in meristematic tissues and glucose imported by meristematic tissues correlated positively with bud break capacity, suggesting that hexoses are involved in the early events of bud break. These findings were confirmed by data for buds that were unable to break because they had been collected from trees deprived of cold. We therefore conclude that hexoses are of greater importance than sorbitol or sucrose in the early events of bud break in peach trees.

Vegetative-bud dormancy in peach (Prunus persica L. Batsch) trees is known to be correlated, at least partially, with properties of the underlying bud tissues during winter. Variations in the activity and amount of plasma-membrane H+-ATPase were observed. A full-length cDNA, PPA2 (Prunus persica H+-ATPase 2) and three partial cDNAs (PPA1, PPA3 and PPA4) for the plasma-membrane H+-ATPase from peach trees were isolated by reverse transcription (RT)-coupled rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR). The accumulation of plasma membrane H+-ATPase transcripts was then studied in vegetative buds during dormancy and breaking of dormancy. Competitive RT-PCR analysis revealed that, during dormancy, the plasma membrane H+-ATPase transcripts were higher in the tissues underlying the buds than in the buds themselves. After dormancy release, the level of PPA1, 2, 3 mRNA increased, whereas the level of PPA4 decreased in the buds. When trees were kept in a greenhouse (i.e. sheltered from chilling), no accumulation of PPA mRNA could be detected. These results suggest that there is a differential accumulation of H+-ATPase mRNA between the bud and the underlying bud tissues during dormancy, and that chilling could act as a decisive factor.

Plant dormancy and dormancy breaking depend, at least partially, on close relationships between buds and tissues underlying bud (bud stands). In Prunus persica, the dormancy was related to high nutrient absorption in bud stands linked to high plasmalemma ATPase (EC 3.6.1.3) activity. Two plasmalemma fractions was isolated from peach vegetative buds and bud stands using aqueous phase partitioning and ultracentrifugation. Results of markers enzyme assays indicated that both plasmalemma enriched fractions obtained were highly purified. During the dormancy period plasma membrane ATPase amount and activity were higher in bud stands than in buds. Moreover, assays performed at different temperatures (4, 18, 30 °C) indicated modifications of kinetic parameters (Km, Vm) in both tissues during dormancy release. In buds, from November to February, Km declined at 4°C and increased at 30 °C whereas no changes was measured at 18 °C and Vm increased at all temperature. In bud stands, no changes of Km was measured at 4 °C and 18 °C whereas an increase occurred at 30 °C and Vm decreased at all temperature. According to the results, it can be postulated that dormancy release in peach-tree could be related to modifications of plasma membrane ATPase properties, in buds and bud stands, during winter time.

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The absorption of sucrose and sorbitol by the bud and the stem of Prunus persica was evaluated during the rest period. The utilization of p-chloromercuribenzenesulfonic acid (PCMBS), an inhibitor of sucrose transporter, allowed estimation of the active absorption of sucrose in different isolated organs (bud, bud's underlying tissues and stem). It was assumed that the sink capacities of tissues depended on their potential to absorb carbohydrates by active transport. Endogenous carbohydrate levels in the tissues were also measured. In the bud, the qualitative and quantitative distribution of carbohydrates could be related to the absorption potential of the bud. During dormancy, the bud exhibited a low absorption potential and increased its sucrose concentration by starch hydrolysis. Soluble sugars accumulated during winter. During dormancy release, the bud was able to absorb carbohydrates allowing carbon storage. The data presented led to the proposal of a scheme of events occurring during bud dormancy and its release.

Cited Reference Count: 42 ref.Csirohttp://www.publish.csiro.au/nid/102.htmfrancois.beaujard@clermont.inra.fr; International audience; Tree nutrient research concentrated on endogenous C and N remobilisation in spring has neglected to acknowledge the possibilities of significant effects of N uptake before bud break, especially on the quality of regrowth and N reserve remobilisation. To investigate this subject, experimental studies were performed on young poplars (Populus tremula x Populus alba, clone INRA 717-1B4) grown with a controlled nutrient supply: (i) without N, 'control'; (ii) N supplied throughout the course of the experiment, 'N-supply'; and (iii) N supplied only before bud break, 'N-pulse'. Results confirm the hypothesis that poplar scions can significantly take up nitrate before bud break, amounting to similar to 34% of the total N stored the previous year. After bud break, emerging leaves restart the sap flow, which increased nitrate uptake to support the regrowth. N-pulse and N-supply treatments were found to have significant effects shortly after a growth period, i. e. by increasing N content of all tissues (e.g. 37 and 81% in new shoots respectively), leaf area (18 and 29%) and specific leaf area (20 and 35%). Therefore, results confirm the hypothesis that early N supply plays a significant role in the N status and N remobilisation involved in the spring regrowth of young trees.

Summary Three distinct Mg 2+ -PPases have been detected in the microsomal vesicles from Jerusalem artichoke. Separation of the different membranes on a linear sucrose gradient showed that these activities were associated with tonoplast (d = 1.11-1.12), Golgi (d = 1.14-1.15) and plasma membrane (d = 1.17-1.18). PPase activity located in the plasmalemma enriched fractions seemed to be different from ATPase activity, considering the effects of the well known effectors of the plasmalemma ATPase type.

We studied seasonal variation in xylem sap pH of Juglans regia L. Our main objectives were to (1) test the effect of temperature on seasonal changes in xylem sap pH and (2) study the involvement of plasma membrane H+-ATPase of vessel-associated cells in the control of sap pH. For this purpose, orchard-grown trees were compared with trees grown in a heated (> or = 15 degrees C) greenhouse. During autumn, sap pH was not directly influenced by temperature. A seasonal change in H+-ATPase activity resulting from seasonal variation in the amount of protein was measured in orchard-grown trees, whereas no significant seasonal changes were recorded in greenhouse-grown trees. Our data suggest that H+-ATPase does not regulate xylem sap pH directly by donating protons to the xylem, but by facilitating secondary active H+/sugar transport, among other mechanisms.

In temperate woody species, the vegetative growth period is characterized by active physiological events (e.g., bud break), which require an adequate supply of soluble sugars imported in the xylem sap stream. One-year-old shoots of walnut (Juglans regia L. cv. 'Franquette') trees, which have an acrotonic branching pattern (only apical and distal vegetative buds burst), were used to study the regulation of xylem sugar transporters in relation to bud break. At the end of April (beginning of bud break), a higher xylem sap sucrose concentration and a higher active sucrose uptake by xylem parenchyma cells were found in the apical portion (bearing buds able to burst) than in the basal portion (bearing buds unable to burst) of the sample shoots. At the same time, xylem parenchyma cells of the apical portion of the shoots exhibited greater amounts of both transcripts and proteins of JrSUT1 (Juglans regia putative sucrose transporter 1) than those of the basal stem segment. Conversely, no pronounced difference was found for putative hexose transporters JrHT1 and JrHT2 (Juglans regia hexose transporters 1 and 2). These findings demonstrate the high capacity of bursting vegetative buds to import sucrose. Immunological analysis revealed that sucrose transporters were localized in all parenchyma cells of the xylem, including vessel-associated cells, which are highly specialized in nutrient exchange. Taken together, our results indicate that xylem parenchyma sucrose transporters make a greater contribution than hexose transporters to the imported carbon supply of bursting vegetative buds.