Conflicting demands on angiosperm xylem: Tradeoffs among storage, transport and biomechanics.
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' 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 (K stem). 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 K stem (ΔK stem) 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 ΔK stem 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' responses to drought-induced xylem cavitation.