Hydraulic constraints on plant gas exchange

Stomatal conductance (g(s)) and transpiration (E) are often positively correlated with the hydraulic conductance of the soil-leaf continuum (k(s-1)). Interaction between g(s) and k(s-1) helps regulate water potential (psi) of leaves. When soil and plant psi decreases during water stress, k(s-1) decreases. A well-documented cause of the decrease in k(s-1) is xylem cavitation. The interaction between k and psi in xylem creates physical limits on the range of psi and E over which gas exchange can occur. Differences in drought tolerance between species correlate with hydraulic limits. Safety margins from complete hydraulic failure are often small enough to require stomatal regulation of psi and E. While stomatal regulation avoids complete hydraulic failure, controlled decreases in plant k can be substantial during drought. Decreasing plant k amplifies the effect of water stress on the leaves and effectively increases the sensitivity of the stomatal response to drought. Increased stomatal sensitivity may promote drought survival. (C) 2000 Elsevier Science B.V. All rights reserved.