Search

Forests, although essential for human survival due to their numerous ecosystem services, have been facing a significant dieback caused by intense droughts since the beginning of the Anthropocene. As global climatic models predict an increment in the frequency and intensity of droughts in the near future, which will consequently increase the risk of tree mortality, it is crucial to better understand the physiological bases of drought-induced tree mortality in order to identify physiological thresholds that will allow us to better predict the impact of climate change on forest ecosystems. While xylem hydraulic failure has been recognised as a ubiquitous factor in tree death under drought conditions, fundamental evidence for the mechanistic link between hydraulic failure, cell mortality and tree mortality has not yet been identified. Therefore, the main objective of this thesis was to elucidate this link while identifying key physiological thresholds beyond which tree survival is greatly compromised. Thus, by focusing first on two species, a conifer (Pseudotsuga menziesii M.) and an angiosperm (Prunus lusitanica L.) the thesis work demonstrated that the levels of hydraulic failure previously considered as lethal for trees (P50 for the conifers and P88 for the angiosperms) were not precise enough to define with certainty the fate of a tree after a drought event. However, although the causal link between xylem hydraulic failure and cell death remained unclear, the interaction between these two phenomena appeared to explain the ability of trees to recover from water stress. Thus, by combining the detection of cavitation events using the optical technique, measurements of relative water content at the tissue level, and measurements of cell lysis on an easily accessible organ: the leaf, we demonstrated that the hydraulic failure of the xylem, by causing a drop in relative water content below a critical level, the RWCcrit, caused cell death. Finally, based on the assumption that not all cells were equally important for the survival of trees and that this survival should certainly be linked to the maintenance of the integrity of cells responsible for plant growth, i.e., the meristematic cells, we sought, by working on a conifer species (Abies concolor (Gordon & Glend) Lindl. Ex Hildebr) and an angiosperm species (Fagus sylvatica L.), to show that the meristematic cells were indeed key cells when evaluating the mechanistic link between hydraulic failure and tree mortality. On the latter part, although statistical analyses revealed again the importance of the level of stem relative water content before re-watering for tree survival, preliminary results obtained by transmission electron microscopy showed that, for the same level of stem relative water content, plants with intact meristematic cells were more likely to survive the drought event than the others as soon as they were re-watered.; Les forêts, bien qu’essentielles à la survie de l’Homme du fait de leurs nombreux services écosystémiques, font face à d’importants dépérissements causés par des sécheresses intenses depuis le début de l’Anthropocène. Comme les modèles climatiques globaux prévoient, pour un futur proche, une augmentation de la fréquence et de l’intensité de ces sécheresses qui, par conséquence, accentuera le risque de mortalité des arbres, il apparait alors crucial de mieux comprendre les bases physiologiques de la mortalité des arbres induite par sécheresse afin d’identifier des seuils physiologiques permettant de mieux prédire l’impact de ces changements climatiques sur les écosystèmes forestiers. Si la défaillance hydraulique du xylème a été reconnue comme un facteur omniprésent de la mort des arbres en conditions de sécheresse, des preuves fondamentales du lien mécaniste reliant la défaillance hydraulique, la mortalité des cellules et celle des arbres n’ont pas encore été identifiées. Ainsi, l’objectif principal de cette thèse était d’élucider ce lien tout en identifiant les seuils physiologiques clés au-delà desquels la survie de l’arbre est grandement compromise. Dès lors, en s’intéressant, tout d’abord, à deux espèces, un conifère (Pseudotsuga menziesii M.) et une angiosperme (Prunus lusitanica L.) le travail de thèse a démontré que les niveaux de défaillance hydraulique considérés jusqu’alors comme létaux pour les plantes (P50 pour les conifères et P88 pour les angiospermes) n’étaient pas assez précis pour définir avec certitude le sort d’un arbre après un événement de sécheresse. Cependant, alors que le lien causal entre défaillance hydraulique du xylème et la mortalité cellulaire restait non élucidé, l’interaction entre ces deux phénomènes semblait expliquer la capacité des arbres à récupérer d’un stress hydrique. Ainsi, en combinant, à la fois, la détection des événements de cavitation à l’aide de la méthode optique, des mesures de contenu en eau relatif au niveau tissulaire, et des mesures de lyse cellulaire au niveau d’un organe facilement accessible : la feuille, nous avons mis en évidence que la défaillance hydraulique du xylème, en entrainant une baisse du contenu en eau des tissus en dessous d’un niveau critique, le RWCcrit, provoquait la mort des cellules. Finalement, en partant du postulat que toutes les cellules n’avaient pas la même importance dans la survie des arbres et que cette survie devait certainement être liée au maintien de l’intégrité des cellules responsables de la croissance de la plante, c’est-à-dire des cellules méristématiques, nous avons cherché, en travaillant sur un conifère (Abies concolor (Gordon & Glend) Lindl. Ex Hildebr) et une angiosperme (Fagus sylvatica L.), à montrer que les cellules méristématiques étaient bien des cellules clés lors de l’évaluation du lien mécaniste entre la défaillance hydraulique et la mortalité de l’arbre. Sur cette dernière partie, bien que les analyses statistiques aient à nouveau révélé l’importance du niveau de contenu en eau relatif des tissus de la tige avant réhydratation pour la survie des arbres, des résultats préliminaires obtenus en microscopie électronique à transmission ont montré qu’à niveaux de contenus en eau égaux, des plantes présentant des cellules méristématiques intègres étaient plus à même de survivre à l’événement de sécheresse que les autres et, ce, dès leur ré-arrosage.

International audience

International audience

International audience

2 páginas.- 3 referencias.- Comunicación oral presentada en el BP2021: XXIV Reunión de la Sociedad Española de Biología de Plantas y XVII Congreso Hispano-Luso de Biología de Plantas, 7 y 8 de julio de 2021. online, Stomata are present on all land plants and are key features for vascular plant water content regulation on Earth. Their primary function, i.e., stomatal closure to control water los s under soil and atmospheric drought, is Ihought to prevent cavitation in the vascular system (Brodribb et al. 2017). However, stomata are found to close much before the xylem cavitates - i.e., the leaf water potential at which stomata close by 50% (IV gs50) is much less negative than the water potential at which the xylem loses 50% of its conductivity (lV_x50) (Martin-St Paul et al. 2017). The mechanism that would allow sto mata to close promptly to a decrease in transpiration in relation to a change in leaf water potential before the decrease in hydraulic conductance is still elusive. Our hypothesis is that the loss of root-soil hydraulic conductivity, more than xylem vulnerability to embolisms, is Ihe primary constraint on transpiration during drought (RodriguezDominguez and Brodribb 2020). Thus, sto mala would close when the water potential around the roots drops more rapidly than the increase in transpiration. We investigated whether this loss of root-soil hydraulic conductivity, probably caused due to root shrinkage and the formation of air-filled gaps, aml/or damage to fine roots, appeared to be an important constraint on transpiration during drought. We conducted physiological and imaging experiments on maize plants undergoing moderate drought. We performed highresolution imaging (micro-CT) of leaves and the root-soil interface and measured in parallel the soil and plant water potentials. Transpiration, stomatal conductance, root hydraulic conductance and soil and plant water potential were also measured during soil drying in a similar set of plants. The formation of air-filled gaps along individual maize roots was visualized and quantified, finding an agreement between the soil water potential at which roots shrank and root hydraulic conductance decreased, and the soil water potential at which sto mata c1osed. These results proved the hypothesis that the loss of contact between roots and soil, and probably other root cortex modifications, triggered stomatal c10sure and transpiration reduction., Microcomputed tomography measurements were conducted at the PSYCHE beamline at SOLEIL Synchrotron (Paris, France). C.M.R-D. was supported by a "Juan de la Cierva - Incorporación" post-doctoral fellowship (Spain) and was granted a Junior Fellowship by the University of Bayreuth Centre of Intemational Excellence "Alexander von Humboldt" for conducting this specific experiment.

3ème prix de la meilleure communication; National audience; Global climatic models predict an increment in the frequency and intensity of drought events which have already shown to have important consequences on tree survival and forest dieback. Results from a first experiment showed that the actual thresholds used for recovery from drought based on percentage loss of conductance (PLC) (i.e., 50% for conifers, 88% for angiosperms) do not provide accurate insights about the tree capacity for surviving extreme drought events. Therefore, it is crucial to identify and understand not only the mechanisms leading to tree mortality under extreme drought conditions but also the physiological thresholds beyond which trees die. This information will help to improve the predictions of mechanistic models predicting tree mortality and thus improve our forests distribution predictions. Differences in stem relative water content (RWCStem) and the level of electrolytes leakage (EL) were directly related to the capacity of the trees to recover from drought. Thus, one of the main causes of drought-induced tree mortality should be the direct cellular consequences of dehydration. Indeed, as a tree approaches its critical water status, it should enter the phase of cell death in which dehydration tolerance becomes important in delaying mortality. However, the link between the RWC and cell mortality has not been properly evaluated yet. Therefore, we exposed to water-stress plants from three different species showing different resistance to drought. As the stress became more severe, we monitored in leaves the level of EL and determined a threshold in RWC below which the leaves suffered an irreversible destruction of their cells. Whether this cellular death is due to cell cavitation or to the collapse and cytorrhysis of the cell is still unclear. Thus, we plan to use cryo-SEM techniques to answer this question

Master; Les modèles climatiques globaux prévoient une augmentation de la fréquence et de l’intensité des sécheresses affectant la survie des arbres. Afin d’améliorer la précision des modèles mécanistiques prédisant la mort des arbres, il apparait crucial de comprendre ces processus en condition de sécheresse extrême et d’identifier les seuils physiologiques clés au-delà desquels les arbres meurent. Ainsi, l’objectif principal de cette étude était d’identifier les traits physiologiques clés, à l’échelle tissulaire, pouvant servir de seuil de mortalité chez les arbres. Pour cela, plusieurs individus de deux espèces différentes ont été exposé à un événement de sécheresse et leur capacité à récupérer a été suivie après leur réhydratation. Les résultats montrent que les pourcentages de perte de conductance hydraulique communément acceptés comme des seuils de mortalité (P50 pour les conifères et P88 pour les angiospermes) ne seraient qu’une vague approximation de la capacité des arbres à survivre. Cependant, aucun des traits physiologiques suivi n’a pu déterminer un seuil identifiant la mort des arbres. La présence de cellules vivantes au niveau de l’écorce et du phloème permet de postuler que les arbres pourraient survivre à un événement de sécheresse si la continuité hydraulique entre les racines et les cellules vivantes était maintenue.

14 páginas.- 8 figuras.- 1 tablas.- 62 referencias.- Supporting information the online version of the published article http://dx.doi.org/10.1111/nph.18578, Xylem hydraulic failure (HF) has been identified as a ubiquitous factor in triggering drought-induced tree mortality through the damage induced by the progressive dehydration of plant living cells. However, fundamental evidence of the mechanistic link connecting xylem HF to cell death has not been identified yet. The main aim of this study was to evaluate, at the leaf level, the relationship between loss of hydraulic function due to cavitation and cell death under drought conditions and discern how this relationship varied across species with contrasting resistances to cavitation. Drought was induced by withholding water from potted seedlings, and their leaves were sampled to measure their relative water content (RWC) and cell mortality. Vulnerability curves to cavitation at the leaf level were constructed for each species. An increment in cavitation events occurrence precedes the onset of cell mortality. A variation in cells tolerance to dehydration was observed along with the resistance to cavitation. Overall, our results indicate that the onset of cellular mortality occurs at lower RWC than the one for cavitation indicating the role of cavitation in triggering cellular death. They also evidenced a critical RWC for cellular death varying across species with different cavitation resistance. © 2022 The Authors. New Phytologist © 2022 New Phytologist Foundation., This research was supported by La Region Auvergne-Rhone-Alpes ‘Pack Ambition International 2020’ through the project‘ ThirsTree’ 20-006175-01, 20-006175-02 and the Agence Nationale de la Recherche, grant/award no. ANR-18-CE20-0005, ‘Hydrauleaks’

Drought-associated woody-plant mortality has been increasing in most regions with multi-decadal records and is projected to increase in the future, impacting terrestrial climate forcing, biodiversity and resource availability. The mechanisms underlying such mortality, however, are debated, owing to complex interactions between the drivers and the processes. In this Review, we synthesize knowledge of drought-related tree mortality under a warming and drying atmosphere with rising atmospheric CO2. Drought-associated mortality results from water and carbon depletion and declines in their fluxes relative to demand by living tissues. These pools and fluxes are interdependent and underlay plant defences against biotic agents. Death via failure to maintain a positive water balance is particularly dependent on soil-to-root conductance, capacitance, vulnerability to hydraulic failure, cuticular water losses and dehydration tolerance, all of which could be exacerbated by reduced carbon supply rates to support cellular survival or the carbon starvation process. The depletion of plant water and carbon pools is accelerated under rising vapour pressure deficit, but increasing CO2 can mitigate these impacts. Advancing knowledge and reducing predictive uncertainties requires the integration of carbon, water and defensive processes, and the use of a range of experimental and modelling approaches., Enhanced drought frequency and magnitude have impacted tree mortality, leading to multiple examples of regional-scale dieback. This Review outlines the mechanisms leading to mortality, including carbon starvation and hydraulic failure.

International audience; Abstract Global climatic models predict an increment in the frequency and intensity of drought events, which have important consequences on forest dieback. However, the mechanisms leading to tree mortality under drought conditions and the physiological thresholds for recovery are not totally understood yet. This study aimed to identify what are the key physiological traits that determine the tree capacity to recover from drought. Individuals of a conifer (Pseudotsuga menziesii M.) and an angiosperm (Prunus lusitanica L.) species were exposed to drought and their ability to recover after rehydration monitored. Results showed that the actual thresholds used for recovery from drought based on percentage loss of conductance (PLC) (i.e. 50% for conifers, 88% for angiosperms) do not provide accurate insights about the tree capacity for surviving extreme drought events. On the contrary, differences in stem relative water content (RWCStem) and the level of electrolytes leakage (EL) were directly related to the capacity of the trees to recover from drought. This was the case for the conifer species, P. menziesii, for which higher RWCStem and lower EL values were related to the recovery capacity. Even if results showed a similar trend for the angiosperm P. lusitanica as for the conifers, differences between the two traits were much more subtle and did not allow an accurate differentiation between trees able to recover and those that were not. RWCStem and EL could work as indicators of tree capacity to recover from drought for conifers but more studies are required to confirm this observation for angiosperms.