Variation in leaf conductance of silver birch: effects of irradiance, vapour pressure deficit, leaf water status and position within a crown

Abstract

Responses of leaf conductance ( g L) to variation in photosynthetic photon flux density ( Q P), leaf-to-air vapour pressure difference (VPD), bulk leaf water potential ( Ψ x) and soil to leaf hydraulic conductance ( G T) were studied in silver birch ( Betula pendula Roth) foliage with respect to leaf position within the canopy. The upper canopy leaves demonstrated 2.0–2.4 times higher ( P < 0.001) daily maxima of g L as compared to the lower-canopy leaves growing in the shadow of upper branches. Functional acclimation of the shade foliage occurred in the form of both a steeper initial slope of the light-response curve and a lower light-saturation point of g L. Leaf conductance decreased if Ψ x fell below certain values after the noon, while the sun foliage experienced greater negative water potentials than the shade foliage. In a diurnal scale the influence of bulk leaf water potential on g L altogether was rather weak. The lower-canopy foliage exhibited more conservative water-use behaviour, having lower maximum stomatal conductance and greater sensitivity to Ψ x bringing about a smaller responsiveness to VPD than the upper canopy foliage. The mean G T was 1.7–1.8 times bigger ( P < 0.001) for the upper canopy, compared to the lower canopy; the shade foliage responded more sensitively to changes in G T; there were steeper water potential gradients between the soil and lower-canopy leaves; g L in the lower-canopy foliage was more strictly controlled by leaf water status—the evidence suggesting that the shade foliage may be hydraulically more constrained. We set up a hypothesis that stomatal conductance at the base of the live crown is limited not only by low light availability but also by plant's inner hydraulic constraints.