Refilling embolized xylem conduits: is it a matter of phloem unloading?

Long-distance water transport in plants relies on negative pressures established in continuous water columns in xylem conduits. Water under tension is in a metastable state and is prone to cavitation and embolism, which leads to loss of hydraulic conductance, reduced productivity and eventually plant death. Experimental evidence suggests that plants can repair embolized xylem by pushing water from living vessel-associated cells into the gas-filled conduit lumina. Most surprisingly, embolism refilling is known to occur even when the bulk of still functioning xylem is under tension, a finding that is in seemingly contradiction to basic principles of thermodynamics. This review summarizes our current understanding of xylem refilling processes and speculates that embolism repair under tension can be envisioned as a particular case of phloem unloading, as suggested by several events and components of embolism repair, typically involved in phloem unloading mechanisms. Far from being a challenge to irreversible thermodynamics, embolism refilling is emerging as a finely regulated vital process essential for plant functioning under different environmental stresses

The kinetics of rehydration of detached sunflower leaves following substantial water loss.

International audienc

Leafminers help us understand leaf hydraulic design

Leaf hydraulics of Aesculus hippocastanum L. were measured over the growing season and during extensive leaf mining by the larvae of an invasive moth (Cameraria ohridella Deschka et Dimic) that specifically destroy the palisade tissue. Leaves showed seasonal changes in hydraulic resistance (Rlamina) which were related to ontogeny. After leaf expansion was complete, the hydraulic resistance of leaves and the partitioning of resistances between vascular and extra-vascular compartments remained unchanged despite extensive disruption of the palisade by leafminers (up to 50%). This finding suggests that water flow from the petiole to the evaporation sites might not directly involve the palisade cells. The analysis of the temperature dependence of Rlamina in terms of Q10 revealed that at least one transmembrane step was involved in water transport outside the leaf vasculature. Anatomical analysis suggested that this symplastic step may be located at the bundle sheath

The challenge of the Mediterranean climate to plant hydraulics: responses and adaptations.

Mediterranean-type biomes characterized by warm summers with a distinct drought period lasting from 2 up to 10 months occur in several world regions including the Mediterranean basin, S-California, Chile, S-Africa and SW-Australia. All these areas are covered by a peculiar and hyper-diverse vegetation dominated by evergreen trees and shrubs with small and coriaceous leaves. Drought adaptation of Mediterranean plants relies on different mechanisms including deep rooting patterns, avoidance or resistance of cavitation-induced embolism, compensation or repair of embolism-induced hydraulic damage. The complementarity and/or co-occurrence of these physiological traits in different species inhabiting Mediterranean biomes is probably the basis for high plant biodiversity in these fascinating habitats. Ongoing climate changes, leading to enhanced frequency and intensity of drought episodes in Mediterranean biomes, represent a major threat to future conservation of these fragile ecosystems, especially if future harsher climate conditions will overcome the drought resistance limits of Mediterranean plants. Current knowledge about drought resistance mechanisms as well as about processes leading to decline and death of woody plants under extreme climatic conditions is revised and directions for future research are suggested

The kinetics of rehydration of detached sunflower leaves from different initial water deficits.

International audienc

Coping with drought-induced xylem cavitation: coordination of embolism repair and ionic effects in three Mediterranean evergreens.

Embolism repair and ionic effects on xylem hydraulic conductance have been documented in different tree species. However, the diurnal and seasonal patterns of both phenomena and their actual role in plants\u2019 responses to drought-induced xylem cavitation have not been thoroughly investigated. This study provides experimental evidence of the ability of three Mediterranean species to maintain hydraulic function under drought stress by coordinating the refilling of xylem conduits and ion-mediated enhancement of stem hydraulic conductance (Kstem). Vessel grouping indices and starch content in vessel associated parenchyma cells were quantified to verify eventual correlations with ionic effects and refilling, respectively. Experiments were performed on stems of Ceratonia siliqua L., Olea europaea L. and Laurus nobilis L. Seasonal, ion-mediated changes in Kstem (\u394Kstem) and diurnal and/or seasonal embolism repair were recorded for all three species, although with different temporal patterns. Field measurements of leaf specific stem hydraulic conductivity showed that it remained quite constant during the year, despite changes in the levels of embolism. Starch content in vessel-associated parenchyma cells changed on diurnal and seasonal scales in L. nobilis and O. europaea but not in C. siliqua. Values of \u394Kstem were significantly correlated with vessel multiple fraction values (the ratio of grouped vessels to total number of vessels). Our data suggest that the regulation of xylem water transport in Mediterranean plants relies on a close integration between xylem refilling and ionic effects. These functional traits apparently play important roles in plants\u2019 responses to drought-induced xylem cavitation