The dependence of leaf hydraulic conductance on irradiance during HPFM measurements: any role for stomatal response?

Abstract

This paper examines the dependence of whole leaf hydraulic conductance to liquid water (K(L)) on irradiance when measured with a high pressure flowmeter (HPFM). During HPFM measurements, water is perfused into leaves faster than it evaporates hence water infiltrates leaf air spaces and must pass through stomates in the liquid state. Since stomates open and close under high versus low irradiance, respectively, the possibility exists that K(L) might change with irradiance if stomates close tightly enough to restrict water movement. However, the dependence of K(L) on irradiance could be due to a direct effect of irradiance on the hydraulic properties of other tissues in the leaf. In the present study, K(L) increased with irradiance for 6 of the 11 species tested. Whole leaf conductance to water vapour, g(L), was used as a proxy for stomatal aperture and the time-course of changes in K(L) and g(L) was studied during the transition from low to high irradiance and from high to low irradiance. Experiments showed that in some species K(L) changes were not paralleled by g(L) changes. Measurements were also done after perfusion of leaves with ABA which inhibited the g(L) response to irradiance. These leaves showed the same K(L) response to irradiance as control leaves. These experimental results and theoretical calculations suggest that the irradiance dependence of K(L) is more consistent with an effect on extravascular (and/or vascular) tissues rather than stomatal aperture. Irradiance-mediated stimulation of aquaporins or hydrogel effects in leaf tracheids may be involved.

Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences

“…Diurnal changes in root hydraulic conductance have been ascribed to changes in plasmalemna or tonoplast aquaporins that act as water channels controlling water fluxes between cells [96,129]. At leaf level, irradiance dependence of K L was found to be also driven by aquaporins or hydrogel effect on extravascular (and/or vascular) tissues [145]. Hydraulic resistance in leaves represents an important percentage of tree resistance to water flow and within leaf the main resistance in the liquid phase is extravascular [149].…”

Abscisic Acid Down-Regulates Hydraulic Conductance of Grapevine Leaves in Isohydric Genotypes Only

“…This method first developed by Tyree et al (2005) relies on pushing a solution into a plant part (here, a leaf) at a known delivery pressure. Measurements were performed using the transient method consisting in applying different pressures and recording flow rates to calculate the conductance as the slope of the regression line between flow rate and pressure (Sack et al, 2002).…”

The Grapevine Root-Specific Aquaporin VvPIP2;4N Controls Root Hydraulic Conductance and Leaf Gas Exchange under Well-Watered Conditions But Not under Water Stress

“…Leaf hydraulic resistance is controlled by cell-to-cell water movement at different degrees in different species (Tyree et al, 2005). Increases in leaf conductance induced by light exposure coincide with the enhancement of aquaporin expression in walnut (Juglans regia; Cochard et al, 2007).…”