Narrow vessels cavitate first during a simulated drought in Eucalyptus camaldulensis.

Establishing drying-limits for mortality of different tree species and understanding the anatomical and physiological traits involved is crucial to predict forests' responses to climate change. The xylem of Eucalyptus camaldulensis presents a complex of solitary vessels surrounded by different imperforate tracheary elements and parenchyma that influence, in a poorly known way, its hydraulic functioning. We aimed at describing the dynamics of embolism propagation in this type of xylem, seeking any vessel-size pattern, and unraveling the threshold of xylem embolism leading to nonrecovery after drought in E. camaldulensis. We assigned potted saplings to a protracted water-stress for 70 days. We relied on colorimetric and hydraulic methods to test for links between xylem anatomy and embolism propagation in the main stem. On average, the occurrence of embolism was randomly distributed in the stem xylem, but the probability of embolized vessels was higher than predicted by chance in the narrowest vessels of individuals that experienced low to moderate water-stress. The saplings could recover from severe water-stress if their percentage loss of conductance (PLC) was <77%, but not when the PLC was ˃ 85%. We concluded that, contrary to results reported for most species, the narrowest vessels are the most vulnerable to cavitation in E. camaldulensis, suggesting a lack of tradeoff between xylem efficiency and safety (in response to drought) at the tissue level. These results challenge the well-established paradigm of the effect of vessel size on cavitation, which states that the widest conduits are the most vulnerable to both freeze-thaw and drought-induced cavitation.

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

BACKGROUND AND AIMS: Extreme water stress episodes induce tree mortality, but the physiological mechanisms causing tree death are still poorly understood. This study tests the hypothesis that a potted tree's ability to survive extreme monotonic water stress is determined by the cavitation resistance of its xylem tissue. METHODS: Two species were selected with contrasting cavitation resistance (beech and poplar), and potted juvenile trees were exposed to a range of water stresses, causing up to 100 % plant death. KEY RESULTS: The lethal dose of water stress, defined as the xylem pressure inducing 50 % mortality, differed sharply across species (1·75 and 4·5 MPa in poplar and beech, respectively). However, the relationships between tree mortality and the degree of cavitation in the stems were similar, with mortality occurring suddenly when >90 % cavitation had occurred. CONCLUSIONS: Overall, the results suggest that cavitation resistance is a causal factor of tree mortality under extreme drought conditions.

Insights into xylem vulnerability to cavitation in Fagus sylvatica L.: phenotypic and environmental sources of variability.

Xylem vulnerability to cavitation is a key parameter in understanding drought resistance of trees. We determined the xylem water pressure causing 50% loss of hydraulic conductivity (P(50)), a proxy of vulnerability to cavitation, and we evaluated the variability of this trait at tree and population levels for Fagus sylvatica. We checked for the effects of light on vulnerability to cavitation of stem segments together with a time series variation of P(50). Full sunlight-exposed stem segments were less vulnerable to cavitation than shade-exposed ones. We found no clear seasonal change of P(50), suggesting that this trait was designed for a restricted period. P(50) varied for populations settled along a latitudinal gradient, but not for those sampled along an altitudinal gradient. Moreover, mountainside exposure seemed to play a major role in the vulnerability to cavitation of beech populations, as we observed the differences along north-facing sides but not on south-facing sides. Unexpectedly, both north-facing mountainside and northern populations appeared less vulnerable than those grown on the southern mountainside or in the South of France. These results on beech populations were discussed with respect to the results at within-tree level.

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

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.

Genotypic variability and phenotypic plasticity of cavitation resistance in Fagus sylvatica L. across Europe.

Xylem cavitation resistance is a key physiological trait correlated with species tolerance to extreme drought stresses. Little is known about the genetic variability and phenotypic plasticity of this trait in natural tree populations. Here we measured the cavitation resistance of 17 Fagus sylvatica populations representative of the full range of the species in Europe. The trees were grown in three field trials under contrasting climatic conditions. Our findings suggest that the genotypic variability of cavitation resistance is high between genotypes of a given population. By contrast, no significant differences were found for this trait across populations, the mean population cavitation resistance being remarkably constant in each trial. We found a significant site effect and a significant site × population interaction, suggesting that cavitation resistance has a high phenotypic plasticity and that this plasticity is under genetic control. The implications of our findings for beech forest management in a context of climate change are discussed.

Putative role of aquaporins in variable hydraulic conductance of leaves in response to light.

Molecular and physiological studies in walnut (Juglans regia) are combined to establish the putative role of leaf plasma membrane aquaporins in the response of leaf hydraulic conductance (K(leaf)) to irradiance. The effects of light and temperature on K(leaf) are described. Under dark conditions, K(leaf) was low, but increased by 400% upon exposure to light. In contrast to dark conditions, K(leaf) values of light-exposed leaves responded to temperature and 0.1 mm cycloheximide treatments. Furthermore, K(leaf) was not related to stomatal aperture. Data of real-time reverse transcription-polymerase chain reaction showed that K(leaf) dynamics were tightly correlated with the transcript abundance of two walnut aquaporins (JrPIP2,1 and JrPIP2,2). Low K(leaf) in the dark was associated with down-regulation, whereas high K(leaf) in the light was associated with up-regulation of JrPIP2. Light responses of K(leaf) and aquaporin transcripts were reversible and inhibited by cycloheximide, indicating the importance of de novo protein biosynthesis in this process. Our results indicate that walnut leaves can rapidly change their hydraulic conductance and suggest that these changes can be explained by regulation of plasma membrane aquaporins. Model simulation suggests that variable leaf hydraulic conductance in walnut might enhance leaf gas exchanges while buffering leaf water status in response to ambient light fluctuations.