La secheresse exceptionnelle de 2003 a ete l'occasion de faire le point de nos connaissances sur les mecanismes ecophysiologiques permettant aux arbres de traverser un tel evenement climatique extreme. L'analyse a ete conduite a l'echelle de l'arbre et du peuplement, tandis que l'intensite de la secheresse a ete quantifiee a l'aide d'un calcul de bilan hydrique sur neuf sites forestiers europeens contrastes du reseau CARBOEUROPE. Le role du couvert dans les echanges avec l'atmosphere est rappele puis integre dans l'analyse des reductions de bilan d'eau et de carbone en 2003 dus a la regulation stomatique. Les caracteristiques du complexe sol-racine, important a la fois pour l'acces a la ressource et a l'efficience de son absorption, constituent un des premiers traits d'adaptation a la secheresse. La reponse et les adaptations des especes ont surtout ete analysees en termes de diversite inter-specifique de fonctionnement hydraulique et du couplage entre proprietes hydrauliques et regulation stomatique. Enfin, nous discutons l'hypothese selon la quelle les dysfonctionnements hydrauliques ou les deficits de mise en reserve sont impliques dans les reactions differees de croissance, de developpement, d'induction de deperissement. Par exemple, des mesures de reserves glucidiques dans les troncs de chenes menees en fin d'ete 2003 ont permis de predire l'etat des couronnes des arbres au printemps 2004. Les faibles taux d'amidon etaient associes a une forte mortalite de branches et de jeunes pousses.

/pdf/temperate-forest-trees-and-stands-under-severe-drought-a-2nxodt1dqz.pdf

Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences

WATER RELATIONS AND THE VULNERABLE PIPELINE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 EARLY EFFORTS TO DETECT CAVITATION AND EMBOLISM . . . . . . . . . . . . . . . . . . . . . 21 ACOUSTIC DETECTION OF CAVITATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Audio (Low-Frequency) Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Ultrasonic Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Physics of Sound Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 HYDRAULIC DETECTION OF EMBOLISM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 CAVITATION AND EMBOLISM IN NATURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 MECHANISMS OF EMBOLISM FORMATION. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Water Stress-Induced Embolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Embolism Formation by Winter Freezing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Pathogen-Induced Embolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 EMBOLISM REPAIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 30 VULNERABILITY OF XYLEM TO WATER STRESS-INDUCED EMBOLISM . . . . . . 31 HYDRAULIC ARCHITECTURE AND SUFFICIENCY OF TREES . . . . . . . . . . . . . . . . . . . . . . 33 SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Vulnerability of Xylem to Cavitation and Embolism

Languishing for many years in the shadow of plant inorganic nitrogen (N) nutrition research, studies of organic N uptake have attracted increased attention during the last decade. The capacity of plants to acquire organic N, demonstrated in laboratory and field settings, has thereby been well established. Even so, the ecological significance of organic N uptake for plant N nutrition is still a matter of discussion. Several lines of evidence suggest that plants growing in various ecosystems may access organic N species. Many soils display amino acid concentrations similar to, or higher than, those of inorganic N, mainly as a result of rapid hydrolysis of soil proteins. Transporters mediating amino acid uptake have been identified both in mycorrhizal fungi and in plant roots. Studies of endogenous metabolism of absorbed amino acids suggest that L- but not D-enantiomers are efficiently utilized. Dual labelled amino acids supplied to soil have provided strong evidence for plant uptake of organic N in the field but have failed to provide information on the quantitative importance of this process. Thus, direct evidence that organic N contributes significantly to plant N nutrition is still lacking. Recent progress in our understanding of the mechanisms underlying plant organic N uptake may open new avenues for the exploration of this subject.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2008.02751.x

Uptake of organic nitrogen by plants.

Summary
This review reports on the processes associated with carbon transfer and metabolism in leaves and growing organs and the role of long-distance transport and vascular links in the regulation of carbon partitioning in plants. Partitioning is clearly influenced by both the supply and demand for photosynthate and is moderated by vascular connections and the storage capacity of the leaves and pathway tissues. However there appears to be little more than circumstantial evidence either that short distance transfer of carbon within either the source or the sink, or that long-distance transport in the phloem, are limiting photosynthesis or growth directly. Although individual biochemical and physiological processes relating to photosynthesis and growth may be well understood, the factors primarily responsible for the control of carbon partitioning in plants have not been clearly identified. There is a need for a greater understanding of organ initiation and development (source and sink formation and potential size), the clear identification of whether growth is sink or source limited (including possible sink-controlled photosynthesis) and a detailed assessment of the role of storage in buffering developmental and environmental changes in sink and source activity. Also more information is needed on the role of hormonal and nutritional factors in regulating source and sink activity (organ interactions not directly associated with carbon transfer).

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1990.tb00524.x

Tansley Review No. 27 The control of carbon partitioning in plants

The predominant emphasis on harmful effects of environmental stresses on growth of woody plants has obscured some very beneficial effects of such stresses. Slowly increasing stresses may induce physiological adjustment that protects plants from the growth inhibition and/or injury that follow when environmental stresses are abruptly imposed. In addition, short exposures of woody plants to extreme environmental conditions at critical times in their development often improve growth. Furthermore, maintaining harvested seedlings and plant products at very low temperatures extends their longevity. Drought tolerance: Seedlings previously exposed to water stress often undergo less inhibition of growth and other processes following transplanting than do seedlings not previously exposed to such stress. Controlled wetting and drying cycles often promote early budset, dormancy, and drought tolerance. In many species increased drought tolerance following such cycles is associated with osmotic adjustment that ...

Acclimation and adaptive responses of woody plants to environmental stresses

Xylem embolism in winter and spring as well as the occurrence of positive xylem pressure were monitored in several diffuse-porous and one ring-porous tree species (Fraxinus excelsior). In Acer pseudoplatanus and Betula pendula embolism reversal was associated with positive (above-atmospheric) xylem pressures that frequently occurred during a 2-month period prior to leaf expansion. In Acer high stem pressures were occasionally triggered on sunny days after a night frost. The other species investigated showed no positive xylem pressure during the monitoring period in 1995. Populus balsamifera exhibited a complete embolism reversal in 1994, but, like Fagus sylvatica, recovery was slow and incomplete in 1995. Fraxinus did not refill embolized vessels, but relied entirely on the production of new earlywood conduits in May. Populus × canadensis Moench “robusta” did not recover from embolism during the monitoring period. Under a simulated root pressure of 20 kPa however, excised branches of Populus × canadensis restored maximum hydraulic conductance within 2 days, illustrating the great influence of even small positive pressures on cnductivity recovery in spring. In the absence of positive pressure there was no substantial refilling of embolized vessels within a rehydration period of 9 days.

https://sites.ualberta.ca/~hacke/pdf/Oecologia96.pdf

Xylem dysfunction during winter and recovery of hydraulic conductivity in diffuse-porous and ring-porous trees.

The seasonal pattern in starch, various sugars, protein, and fat, and their interrelationship, has been followed in 3-year-old branch wood of poplar trees (Populus x canadensis Moench ‘robusta’) under natural site conditions. The deposition of starch, protein and fat proceeds at different times. Starch accumulates from May until October, fat mainly during the summer months, and protein when the leaves are yellowing in September and October. The maximum concentrations in the branch wood were 15–18 μg starch, 6–9 μg protein, 4–8 μg fat, 10–15 μg sucrose, and up to 30 μg total sugars per milligram dry weight (DW). During starch deposition periods no increased sucrose level is found in the tissue. The maximum daily starch deposition rate was 0.2–0.4 μg starch/day/mg DW of wood. During starch hydrolysis in late autumn and winter, a dramatic increase in sucrose and its galactosides is measured (up to 15–27 μg/mg DW in total). In early spring, before budbreak, the concentrations of these sugars diminishes sharply. In contrast to this clear-cut starch-to-sugar conversion in autumn no significant starch-to-fat conversion is detected. An elevated content of free glycerol, however, is found in winter. In spring, starch and storage protein are mobilized completely, or almost completely, in poplar twig wood. A noteworthy pool of maltose is found transiently during autumn (up to 8 μg/mg DW) and again in spring. The results demonstrate that the individual storage materials, e.g. starch, protein, and fat, are accumulated fairly independently in the wood storage parenchyma. Tissue sugar levels, in contrast, appear to be closely related to the seasonal variations in starch content, on the one hand, and to the acclimation and deacclimation of the cells, on the other. The interrelations of the storage materials and sugars are discussed.

Storage, mobilization and interrelations of starch, sugars, protein and fat in the ray storage tissue of poplar trees

Variation in vulnerability to xylem cavitation was measured within individual organs of Populus balsamifera L. and Alnus glutinosa (L.) Gaertn. Cavitation was quantified by three different techniques: (a) measuring acoustic emissions, (b) measuring loss of hydraulic conductance while air-dehydrating a branch, and (c) measuring loss of hydraulic conductance as a function of positive air pressure injected into the xylem. All of these techniques gave similar results. In Populus, petioles were more resistant than branches, and branches were more resistant than roots. This corresponded to the pattern of vessel width: maximum vessel diameter in 1- to 2-year-old roots was 140 [mu]m, compared to 65 and 45 [mu]m in rapidly growing 1-year-old shoots and petioles, respectively. Cavitation in Populus petioles started at a threshold water potential of -1.1 MPa. The lowest leaf water potential observed was -0.9 MPa. In Alnus, there was no relationship between vessel diameter and the cavitation response of a plant organ. Although conduits were narrower in petioles than in branches, petioles were more vulnerable to cavitation. Cavitation in petioles was detected when water potential fell below -1.2 MPa. This value equaled midday leaf water potential in late June. As in Populus, roots were the most vulnerable organ. The significance of different cavitation thresholds in individual plant organs is discussed.

Drought-induced Xylem Dysfunction in Petioles, Branches, and Roots of Populus balsamifera L. and Alnus glutinosa (L.) Gaertn.

The vulnerability of xylem vessels to water stressinduced cavitation was studied by measuring hydraulic conductivity and ultrasound acoustic emissions [AEs) in Fagus sylvatica L. f. purpurea (Ait.) Schneid. and Populus balsamifera L.. The occurrence of xylem embolism in summer was investigated in relation to leaf water potential and stomatal conductance. Populus was extremely vulnerable to cavitation, losing functional vessels due to embolism at water potentials lower than —0.7 MPa. Fagus experienced embolism when water potential fell below —1.9 MPa. Midday water potentials often approached these threshold values. When evaporative demand increased rapidly on sunny days, water loss became limited by low stomatal conductance. Thus water potentials fell only slightly below the threshold values inducing cavitation. Despite the differences in vulnerability, both species tolerated a similar embolism rate of about 10% in the summer. There was no embolism reversal during prolonged periods of rain. AEs were predictive of loss in hydraulic conductivity, indicating that AEs were mainly confined to vessels. Finally, vessel length distribution, vessel diameter (tangential axis), vessel density, and vessel wall thickness had been determined for both species investigated. Populus had longer and wider vessels than Fagus, whereas vessel wall thickness was similar in both species.

https://sites.ualberta.ca/~hacke/pdf/JXB_05.pdf

Jörg J. Sauter

Papers

Xylem dysfunction during winter and recovery of hydraulic conductivity in diffuse-porous and ring-porous trees.

Storage, mobilization and interrelations of starch, sugars, protein and fat in the ray storage tissue of poplar trees

Drought-induced Xylem Dysfunction in Petioles, Branches, and Roots of Populus balsamifera L. and Alnus glutinosa (L.) Gaertn.

Vulnerability of xylem to embolism in relation to leaf water potential and stomatal conductance in Fagus sylvatica f. purpurea and Populus balsamifera

Temperature-induced Changes in Starch and Sugars in the Stem of Populus × canadensis «robusta»