Temperature, light and leaf hydraulic conductance of little-leaf linden (Tilia cordata) in a mixed forest canopy

Response of whole-leaf hydraulic conductance (G(L)) in little-leaf linden (Tilia cordata Mill.) to temperature and photosynthetic photon flux (Q(P)) was estimated by the evaporative flux method under natural conditions in a mixed forest canopy. Mean midday G(L) in the lower- and upper-crown foliage was 1. 14 and 3.06 mmol m(-2) S-1 MPa-1, respectively. Over the study period, leaf temperature (T-L) explained about 67% of the variation in G(L), and Q(p) explained about 10%. Leaf water potential and crown position also affected G (L) significantly. About a third of the temperature effect was attributable to changes in the viscosity of water, and two thirds to changes in protoplast permeability (i.e., symplastic conductance). Leaf hydraulic conductance was highly sensitive to changes in Q(P) when Q(P) was less than 200 mu mol m(-2) s(-1), and G(L) sensitivity decreased with increasing irradiance. Sensitivity of G(L) to variation in T-L increased consistently with increasing temperature in the range of 16 to 29 degrees C. There were positive interactions between temperature and light in their effects on G(L): the light response was inore pronounced at higher leaf temperatures. Because of frequent rains during the study period, the trees experienced no soil water deficit, and, within the range experienced, soil water potential had no effect on G(L). Leaf hydraulic conductance exhibited a seasonal pattern that could be explained primarily by temporal variability in mean air temperature and irradiance, in addition to which an age-related trend (P < 0.001) of increasing G(L) frorn the end of June to the beginning of August was observed.