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A comprehensive model of stem and root diameter variation was developed. The stem (or root) was represented using two coaxial cylinders corresponding with the mature xylem and the extensible tissues. The extensible tissues were assumed to behave as a single cell separated from the mature xylem by a virtual membrane. The mature xylem and the extensible tissues are able to dilate with temperature and grow. Moreover, the extensible tissues are able to shrink and swell according to water flow intensity. The model is mainly based on the calculation of water volume flows in the "single cell" that are described using the principles of irreversible thermodynamics. The elastic response to storage volume and plastic extension accompanying growth are described. The model simulates diameter variation due to temperature, solute accumulation, and xylem, water potential. The model was applied to the peach (Prunus persica) stem and to the plum (Prunus domestica x Prunus spinosa) root. The simulation outputs corresponded well with the diameter variation observed. The model predicts that variations of turgor pressure and osmotic potential are smaller than the variations of xylem water potential. It also demonstrates correlations between the xylem water potential, the turgor pressure, the elastic modulus, and the osmotic potential. The relationship between the diameter and the xylem water potential exhibits a substantial hysteresis, as observed in field data. A sensitivity analysis using the model parameters showed that growth and shrinkage were highly sensitive to the initial values of the turgor pressure and to the reflection coefficient of solutes. Shrinkage and growth were sensitive to elastic modulus and wall-yielding threshold pressure, respectively. The model was not sensitive to changes in temperature.

Le comportement de p?chers (Prunus persica (L) Batsch) a ?t? ?tudi? sur des jeunes arbres en pots, sous serre, soumis ? des r?gimes diff?rents d'alimentation hydrique : ? arrosage conduit en fonction de l'amplitude maximale de contraction diurne de la tige (micromorphom?trie); ? arrosage quotidien compensant la transpiration; ? alimentation en eau r?duite (arrosage seulement lorsque les arbres manifestent une fermeture stomatique : lot sec). Les arbres arros?s par la m?thode micromorphom?trique ne se diff?rencient pas de ceux dont on compense quotidiennement la transpiration, pour la croissance des parties v?g?tatives et des fruits, mais ils consomment 20% d'eau en moins. Les racines des arbres en pilotage micromorphom?trique s'?tendent en outre davantage que celles des arbres arros?s quotidiennement. Par rapport aux 2 autres traitements, le traitement sec r?duit fortement la transpiration des arbres, la croissance des pousses de l'ann?e, la croissance secondaire de la tige et la taille des fruits. Il semble provoquer une r?duction du nombre de radicelles et dans les parties a?riennes, une reprise de la croissance de certaines pousses. La discussion porte sur la sensibilit? de la croissance ? la contrainte hydrique selon le type d'organe et sur le principe de la conduite micromorphom?trique de l'irrigation. Response of young peach trees in pots subjected to different levels of water alimentation. II: Behaviour of tree growth and development. Three irrigation schedules were tested on young peach trees (Prunus persica L Batsch) grown in pots under greenhouse conditions : ? micromorphometric irrigation, watering the trees according to their daily stem contractions; ? daily irrigation, maintaining the trees in the constantly humid substrata which compensated for transpiration losses; ? dry; watering the trees only when the stomata were closed. The trees watered according to the micromorphometric method (fig 1) consumed less water compared with those under daily irrigation (table I). However, there was no difference in tree development and fruit growth between the 2 previous treatments (figs 2, 3, 4). There was no difference in the final mean area of 1 leaf between the 3 treatments (table II), although the roots of the trees under the micromorphometry treatment developed better (fig 5, table III). The dry treatment, on the one hand, strongly limited tree transpiration and on the other hand, reduced shoot elongation, stem diameter expansion and fruit size (figs 2, 3, 4). Moreover, the death of certain radicles occurred and the resumption of shoot growth was observed in trees under this treatment. The growth sensibility of different organs of peach trees to water stress and the irrigation scheduling based on the micromorphometric method are discussed (fig 6).

Le comportement de certains processus physiologiques est ?tudi? sur de jeunes p?chers (Prunus persica (L) Batsch), en pots sous serre, soumis ? des r?gimes diff?rents d'alimentation hydrique : ? un arrosage conduit en fonction de l'amplitude maximum de contraction diurne de la tige (micromorphom?trie); ? un arrosage quotidien compensant la transpiration; ? une alimentation en eau r?duite : traitement arros? seulement lorsque les arbres manifestent une fermeture stomatique (lot ?sec?). La transpiration des arbres et la photosynth?se nette des feuilles sont fortement li?es aux conditions d'alimentation hydrique : une r?duction de la transpiration se met en place d?s que le milieu se dess?che; la photosynth?se nette des arbres en pilotage micromorphom?trique se maintient au m?me niveau que celle des arbres soumis au traitement ?arrosage quotidien?; on observe m?me une tendance ? para?tre plus forte en condition de rayonnement solaire faible ou moyennement fort. Il n'y a pas de diff?rence significative pour le potentiel hydrique et la r?sistance stomatique entre les arbres du traitement ?arrosage quotidien? et ceux du pilotage micromorphom?trique. Une limitation de la photosynth?se appara?t en condition de rayonnement fort sur les arbres du lot ?sec? par rapport ? celle des arbres arros?s tous les jours; le potentiel hydrique des arbres du traitement ?sec? est plus bas et la r?sistance stomatique plus ?lev?e. Par ailleurs, les ?volutions, du potentiel hydrique foliaire, de la r?sistance stomatique et de l'amplitude maximum de contraction diurne de la tige sont examin?es en fonction de l'?tat hydrique du milieu. Les liaisons entre ces diff?rents param?tres physiologiques sont analys?es. Response of young peach trees in pots subjected to different levels of water alimentation. I: Effects on transpiration, stomatal resistance, photosynthesis, leaf water potential and micromorphometric variations of stem diameter. Three irrigation schedules were tested on young peach trees Prunus persica (L) Batsch grown in pots (fig 1) under greenhouse conditions: ? "micromorphometric irrigation", watering the trees according to maximum amplitude of daily stem contractions (fig 5), ? "daily irrigation", maintaining the trees in constantly humid substrata and irrigating in compensation for transpiration loss, ? "dry", watering the trees only when they closed their stomata. Both tree transpiration and leaf photosynthesis depend on the levels of water alimentation: transpiration reduction (fig 2) takes place as soon as the substrata dry out and net photosynthesis (table I) appears to be improved in the trees irrigated by the micromorphometric method under the conditions of low and medium light intensity, but limited in the trees subjected to "dry" treatment with a strong light intensity as compared with that observed in the trees which were irrigated daily. However, there are no significant differences in the water status and the stomatal resistance between the trees subjected to "daily irrigation" treatment and those on the "micromorphometry" schedule (figs 2, 3). The leaf water potential in the trees under the "dry" treatment is lower and their stomatal resistances are higher (figs 2, 3) than in trees on the daily irrigation schedule. Furthermore, the changes in the leaf water potential, the stomatal resistance and the daily maximal stem contraction (figs 6, 7) are examined according to substratum moisture and the relations between these different physiological parameters are studied (table II and fig 8).