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

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 (Psi(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 Psi(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 Psi(x) bringing about a smaller responsiveness to VPD than the upper canopy foliage. The mean G(T) was 1.7-1.9 times bigger (P < 0.001) for the upper canopy, compared to the lower canopy: the shade foliage responded mom 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 five crown is limited not only by low light availability but also by plant's inner hydraulic constraints. (C) 2004 Elsevier B.V. All rights reserved.