Forced depression of leaf hydraulic conductance in situ:: effects on the leaf gas exchange of forest trees

1. Recent work on the hydraulic conductance of leaves suggests that maximum photosynthetic performance of a leaf is defined largely by its plumbing. Pursuing this idea, we tested how the diurnal course of gas exchange of trees in a dry tropical forest was affected by artificially depressing the hydraulic conductance of leaves (K-leaf). 2. Individual leaves from four tropical tree species were exposed to a brief episode of forced evaporation by blowing warm air over leaves in situ. Despite humid soil and atmospheric conditions, this caused leaf water potential (Psi(leaf)) to fall sufficiently to induce a 50-74% drop in K-leaf. 3. Two of the species sampled proved highly sensitive to artificially depressed K-leaf, leading to a marked and sustained decline in the instantaneous rate of CO2 uptake, stomatal conductance and transpiration. Leaves of these species showed a depression of hydraulic and photosynthetic capacity in response to the 'blow-dry' treatment similar to that observed when major veins in the leaf were severed. 4. By contrast, the other two species sampled were relatively insensitive to K-leaf manipulation; photosynthetic rates were indistinguishable from control (untreated) leaves 4 h after treatment. These insensitive species demonstrate a linear decline of K-leaf with Psi(leaf), while K-leaf in the two sensitive species falls precipitously at a critical water deficit. 5. We propose that a sigmoidal K-leaf vulnerability enables a high diurnal yield of CO2 at the cost of exposing leaves to the possibility of xylem cavitation. Linear K-leaf vulnerability leads to a relatively lower CO2 yield, while providing better protection against cavitation.