Stomata play a significant role in the Earth's water and carbon cycles, by regulating gaseous exchanges between the plant and the atmosphere. Under drought conditions, stomatal control of transpiration has long been thought to be closely coordinated with the decrease in hydraulic capacity (hydraulic failure due to xylem embolism). We tested this hypothesis by coupling a meta-analysis of functional traits related to the stomatal response to drought and embolism resistance with simulations from a soil-plant hydraulic model. We report here a previously unreported phenomenon: the existence of an absolute limit by which stomata closure must occur to avoid rapid death in drought conditions. The water potential causing stomatal closure and the xylem pressure at the onset of embolism formation were equal for only a small number of species, and the difference between these two traits (i.e. safety margins) increased continuously with increasing embolism resistance. Our findings demonstrate the need to revise current views about the functional coordination between stomata and hydraulic traits and provide a mechanistic framework for modeling plant mortality under drought conditions.

/pdf/plant-resistance-to-drought-depends-on-timely-stomatal-4bv3uouj42.pdf

Plant resistance to drought depends on timely stomatal closure.

https://publikationen.bibliothek.kit.edu/1000120986/83353716

A first assessment of the impact of the extreme 2018 summer drought on Central European forests

'Summary'	14	
I.	'Introduction'	14	
II.	'The transpiration supply function'	15	
III.	'The transpiration loss function'	17	
IV.	'General properties of the supply–loss theory'	19	
V.	'Variations on the theme'	19	
VI.	'Predicting responses to climate'	21	
VII.	'Implications for productivity and mortality: the chronic stress hypothesis'	22	
VIII.	'Conclusion'	24	
 	'Acknowledgements'	24	
 	References	24	


Summary
Climate change exposes vegetation to unusual drought, causing declines in productivity and increased mortality. Drought responses are hard to anticipate because canopy transpiration and diffusive conductance (G) respond to drying soil and vapor pressure deficit (D) in complex ways. A growing database of hydraulic traits, combined with a parsimonious theory of tree water transport and its regulation, may improve predictions of at-risk vegetation. The theory uses the physics of flow through soil and xylem to quantify how canopy water supply declines with drought and ceases by hydraulic failure. This transpiration ‘supply function’ is used to predict a water ‘loss function’ by assuming that stomatal regulation exploits transport capacity while avoiding failure. Supply–loss theory incorporates root distribution, hydraulic redistribution, cavitation vulnerability, and cavitation reversal. The theory efficiently defines stomatal responses to D, drying soil, and hydraulic vulnerability. Driving the theory with climate predicts drought-induced loss of plant hydraulic conductance (k), canopy G, carbon assimilation, and productivity. Data lead to the ‘chronic stress hypothesis’ wherein > 60% loss of k increases mortality by multiple mechanisms. Supply–loss theory predicts the climatic conditions that push vegetation over this risk threshold. The theory's simplicity and predictive power encourage testing and application in large-scale modeling.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.13354

What plant hydraulics can tell us about responses to climate-change droughts.

Drought-induced cavitation resistance varies considerably between tree species and forest ecosystems (Maherali et al., 2004; Delzon et al., 2010) and is closely linked to survival under severe drought in both conifers (Brodribb & Cochard, 2009; Brodribb et al., 2010) and angiosperms (Kursar et al., 2009; Anderegg et al., 2012; Barigah et al., 2013; Urli et al., 2013). Choat et al. (2012) recently reported that most trees operate very close to their threshold of cavitation, leaving them potentially vulnerable to drought-induced mortality in a warmer/drier world (Engelbrecht, 2012). Indeed, species growing in dry environments are more resistant to droughtinduced cavitation (more negative water potential at 50% cavitation, P50) but experience a more negative minimumwater potential (Pmin) than those growing in wet environments. The so-called hydraulic safety margin, the difference between the level of water stress experienced by a species in the field (Pmin) and the level of water stress leading to hydraulic failure, is, therefore, remarkably narrow, whatever the forest species and biome considered (Choat et al., 2012). This pattern provides clues to the global droughtinduced mortality currently observed, even in very wet environments, such as tropical forests (Allen et al., 2010). Klein et al. (2014) play down the functional significance of the hydraulic safety margin in the vulnerability of forests to drought, pointing out the important role played by additional mechanisms, such as the ability of trees to repair embolism. While it is obvious that drought-induced forest dieback is a complex process involving a number of biotic and abiotic factors, we would like to draw the attention of scientists to the state of evidence for embolism repair, thereby guiding research on tree drought resistance into the most relevant and fruitful directions

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/nph.12798

Recent advances in tree hydraulics highlight the ecological significance of the hydraulic safety margin

Water stress-induced xylem hydraulic failure is a causal factor of tree mortality in beech and poplar

Understanding drought tolerance mechanisms requires knowledge about the induced weakness that leads to tree death. Bud survival is vital to sustain tree growth across seasons. We hypothesized that the hydraulic connection of the bud to stem xylem structures was critical for its survival. During an artificial drastic water stress, we carried out a census of bud metabolic activity of young Populus nigra L. trees by microcalorimetry. We monitored transcript expression of aquaporins (AQPs; plasma membrane intrinsic proteins (PIPs), X intrinsic proteins (XIPs) and tonoplast membrane intrinsic proteins (TIPs)) and measured local water status within the bud and tissues in the bearer shoot node by nuclear magnetic resonance (NMR) imaging. We found that the bud respiration rate was closely correlated with its water content and decreased concomitantly in buds and their surrounding bearer tissues. At the molecular level, we observed a modulation of AQP pattern expressions (PIP, TIP and XIP subfamilies) linked to water movements in living cells. However, AQP functions remain to be investigated. Both the bud and tree died beyond a threshold water content and respiration rate. Nuclear magnetic resonance images provided relevant local information about the various water reservoirs of the stem, their dynamics and their interconnections. Comparison of pith, xylem and cambium tissues revealed that the hydraulic connection between the bud and saturated parenchyma cells around the pith allowed bud desiccation to be delayed. At the tree death date, NMR images showed that the cambium tissues remained largely hydrated. Overall, the respiration rate (Rco2) and a few AQP isoforms were found to be two suitable, complementary criteria to assess the bud metabolic activity and the ability to survive a severe drought spell. Bud moisture content could be a key factor in determining the capacity of poplar to recover from water stress.

http://herve.cochard.free.fr/pdf/Barigah-et-al_Tree%20Physiol-2013.pdf

Marie Douris

Papers

Water stress-induced xylem hydraulic failure is a causal factor of tree mortality in beech and poplar

Modulation of bud survival in Populus nigra sprouts in response to water stress-induced embolism.