Hydraulic constraints on plant gas exchange

Stomatal conductance (gs) and transpiration (E) are often positively correlated with the hydraulic conductance of the soil–leaf continuum (ks–l). Interaction between gs and ks–l helps regulate water potential (Ψ) of leaves. When soil and plant Ψ decreases during water stress, ks–l decreases. A well-documented cause of the decrease in ks–l is xylem cavitation. The interaction between k and Ψ in xylem creates physical limits on the range of Ψ 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 Ψ 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.