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

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.