Abstract
Gas exchange is constrained by the whole-plant hydraulic conductance (K plant). Leaves account for an important fraction of K plant and may therefore represent a major determinant of plant productivity. Leaf hydraulic conductance (K leaf) decreases with increasing water stress, which is due to xylem embolism in leaf veins and/or the properties of the extra-xylary pathway. Water flow through living tissues is facilitated and regulated by water channel proteins called aquaporins (AQPs). Here we assessed changes in the hydraulic conductance of Populus trichocarpa leaves during a dehydration-rewatering episode. While leaves were highly sensitive to drought, K leaf recovered only 2 hours after plants were rewatered. Recovery of K leaf was absent when excised leaves were bench-dried and subsequently xylem-perfused with a solution containing AQP inhibitors. We examined the expression patterns of 12 highly expressed AQP genes during a dehydration-rehydration episode to identify isoforms that may be involved in leaf hydraulic adjustments. Among the AQPs tested, several genes encoding tonoplast intrinsic proteins (TIPs) showed large increases in expression in rehydrated leaves, suggesting that TIPs contribute to reversing drought-induced reductions in K leaf. TIPs were localized in xylem parenchyma, consistent with a role in facilitating water exchange between xylem vessels and adjacent living cells. Dye uptake experiments suggested that reversible embolism formation in minor leaf veins contributed to the observed changes in K leaf.
Highlights
High gas exchange rates can only be sustained when leaves are kept well hydrated
Leaf hydraulic conductance is highly sensitive to drought To assess how Kleaf declines as a function of Yleaf, we first constructed a vulnerability curve
Leaves were highly vulnerable with 50% and 80% loss of hydraulic conductance occurring at Yleaf = 20.47 MPa and 20.71 MPa, respectively (Figure 1, insert)
Summary
This, in turn, depends on the properties of the xylem pipeline and on the way in which water moves through living cells in roots and leaves [1,2]. Leaf hydraulic conductance is emerging as an important component of whole-plant hydraulic conductance [3,4,5,6,7]. Like in roots and stems, the hydraulic conductance of leaves declines as the water potential becomes more negative. This loss of hydraulic conductance is due to embolism formation in leaf veins [8,9], cell shrinkage [10], collapse of xylem conduits [11], and/or to decline in the permeability of extra-xylary tissues [12]. The hydraulic conductance of these plant organs may be able to quickly recover from the effects of drought [3,15]
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