Leaves are extraordinarily variable in form, longevity, venation architecture, and capacity for photosynthetic gas exchange. Much of this diversity is linked with water transport capacity. The pathways through the leaf constitute a substantial (>or=30%) part of the resistance to water flow through plants, and thus influence rates of transpiration and photosynthesis. Leaf hydraulic conductance (K(leaf)) varies more than 65-fold across species, reflecting differences in the anatomy of the petiole and the venation architecture, as well as pathways beyond the xylem through living tissues to sites of evaporation. K(leaf) is highly dynamic over a range of time scales, showing circadian and developmental trajectories, and responds rapidly, often reversibly, to changes in temperature, irradiance, and water supply. This review addresses how leaf structure and physiology influence K(leaf), and the mechanisms by which K(leaf) contributes to dynamic functional responses at the level of both individual leaves and the whole plant.

The hydraulic conductance of the leaf lamina ( K lamina ) substantially constrains whole-plant water transport, but little is known of its association with leaf structure and function. K lamina was measured for sun and shade leaves of six woody temperate species growing in moist soil, and tested for correlation with the prevailing leaf irradiance, and with 22 other leaf traits. K lamina varied from 7.40 ¥ ¥ ¥ ¥ 10-1 for Vitis labrusca sun leaves. Tree sun leaves had 15-67% higher K lamina than shade leaves. K lamina was co-ordinated with traits associated with high water flux, including leaf irradiance, petiole hydraulic conductance, guard cell length, and stomatal pore area per lamina area. K lamina was also co-ordinated with lamina thickness, water storage capacitance, 1/mesophyll water transfer resistance, and, in five of the six species, with lamina perimeter/area. However, for the six species, K lamina was independent of inter-related leaf traits including leaf dry mass per area, density, modulus of elasticity, osmotic potential, and cuticular conductance. K lamina was thus co-ordinated with structural and functional traits relating to liquid-phase water transport and to maximum rates of gas exchange, but independent of other traits relating to drought tolerance and to aspects of carbon economy.

The question as to what triggers stomatal closure during leaf desiccation remains controversial. This paper examines characteristics of the vascular and photosynthetic functions of the leaf to determine which responds most similarly to stomata during desiccation. Leaf hydraulic conductance (K leaf ) was measured from the relaxation kinetics of leaf water potential (⌿ l ), and a novel application of this technique allowed the response of K leaf to ⌿ l to be determined. These "vulnerability curves" show that K leaf is highly sensitive to ⌿ l and that the response of stomatal conductance to ⌿ l is closely correlated with the response of K leaf to ⌿ l . The turgor loss point of leaves was also correlated with K leaf and stomatal closure, whereas the decline in PSII quantum yield during leaf drying occurred at a lower ⌿ l than stomatal closure. These results indicate that stomatal closure is primarily coordinated with K leaf . However, the close proximity of ⌿ l at initial stomatal closure and initial loss of K leaf suggest that partial loss of K leaf might occur regularly, presumably necessitating repair of embolisms.Stomata appear in the fossil record approximately 400 million years ago (Edwards et al., 1998) at approximately the same time as the evolution of an internal water conducting system in plants. Stomatal evolution is believed to be a response to selective pressure to optimize the ratio of CO 2 uptake to water lost during photosynthesis (Raven, 2002). The evolution of internal conduits for water transport added a level of complexity to optimizing gas exchange during photosynthesis, because of the dependence of water supply capacity upon the water potential in the plant (Sperry et al., 2002). This complexity is evidenced by the variable effects of leaf water potential (⌿ l ) and vapor pressure deficit on stomatal movements among species. Although stomatal aperture responds predictably to guard cell turgor (Franks et al., 1995), the relationships between guard cell turgor and either transpiration (E) or mesophyll turgor are still hypothetical (Buckley and Mott, 2002). Amid mechanistic debate as to the process of stomatal closure, the fundamental question of why stomata close remains unanswered. Given that stomata may predate the evolution of xylem (Edwards et al., 1998; Raven, 2002), it is appropriate to question whether it is vascular or other tissues that provide the trigger for stomatal closure.We focus here on the question of what sets the point of stomatal closure in leaves. That is to say which aspect of a plant's physiology is sufficiently sensitive to decreasing ⌿ l that it requires stomata to be closed and photosynthesis sacrificed to protect from loss of function and damage. A key assumption here is that traits responsible for determining the stomatal response to leaf desiccation are coordinated with physiological characters dictating the sensitivity of the metabolic or transport machinery of the plant to water stress. Candidates for these coordinated traits are likely be located in or near the ...

Increasing concentrations of ions flowing through the xylem of plants produce rapid, substantial, and reversible decreases in hydraulic resistance. Changes in hydraulic resistance in response to solution ion concentration, pH, and nonpolar solvents are consistent with this process being mediated by hydrogels. The effect is localized to intervessel bordered pits, suggesting that microchannels in the pit membranes are altered by the swelling and deswelling of pectins, which are known hydrogels. The existence of an ion-mediated response breaks the long-held paradigm of the xylem as a system of inert pipes and suggests a mechanism by which plants may regulate their internal flow regime.

We investigated the common assumption that severing stems and petioles under water preserves the hydraulic continuity in the xylem conduits opened by the cut when the xylem is under tension. In red maple and white ash, higher percent loss of conductivity (PLC) in the afternoon occurred when the measurement segment was excised under water at native xylem tensions, but not when xylem tensions were relaxed prior to sample excision. Bench drying vulnerability curves in which measurement samples were excised at native versus relaxed tensions showed a dramatic effect of cutting under tension in red maple, a moderate effect in sugar maple, and no effect in paper birch. We also found that air injection of cut branches (red and sugar maple) at pressures of 0.1 and 1.0 MPa resulted in PLC greater than predicted from vulnerability curves for samples cut 2 min after depressurization, with PLC returning to expected levels for samples cut after 75 min. These results suggest that sampling methods can generate PLC patterns indicative of repair under tension by inducing a degree of embolism that is itself a function of xylem tensions or supersaturation of dissolved gases (air injection) at the moment of sample excision. Implications for assessing vulnerability to cavitation and levels of embolism under field conditions are discussed.