Effects of light intensity and duration on leaf hydraulic conductance and distribution of resistance in shoots of silver birch (Betula pendula)

Variation in leaf hydraulic conductance (K L ) and distribution of resistance in response to light intensity and duration were examined in shoots of silver birch ( Betula pendula Roth). K L was determined on detached shoots using the evaporative flux method (transpiration was measured with a porometer and water potential drop with a pressure chamber). Although K L depended on light duration per se, its dynamics was largely determined by leaf temperature (T L ). Both upper‐crown leaves and branches developed in well‐illuminated environment exhibited higher hydraulic efficiency compared with the lower crown, accounting for vertical trends of apparent soil‐to‐leaf hydraulic conductance in canopy of silver birch revealed in our previous studies. K L varied significantly with light intensity, the highest values for both shade and sun foliage were recorded at photosynthetic photon flux density of 330 μmol m −2 s −1 . Light responses of K L were associated evidently with an irradiance‐mediated effect on extravascular tissues involving regulation of cell membrane aquaporins. Effects of irradiance on K L resulted in changes of Ψ L , bringing about considerable alteration in partitioning of the resistance between leaves and branch (leafless shoot stem): the contribution of leaves to the shoot total resistance decreased from 94% at −1.0 MPa to 75% at −0.2 MPa. Treatment with HgCl 2 decreased hydraulic conductance of both leaves and branches, implying that condition of bordered pit membranes or shoot living tissues may be involved in responses of xylem conductance to Hg 2+ .