Relationships of leaf diffusion resistance of Populus clones to leaf water potential and environment

Leaf diffusion resistance (r ₁) of the upper and lower leaf surfaces of several Populus clones was related to leaf water potential (ψ₁), light intensity, vapor pressure deficit (VPD), and temperature by intrinsicallylinear, logarithmic multiple regression analyses. Regression equations accounted for up to 80% of variation in r ₁ data. Light intensity and VPD varied among clones in importance in influencing r ₁. Pronounced sensitivity of r ₁ of certain clones to VPD was related to drought resistance in their parentage. Increasing r ₁ was significantly positively correlated with ψ₁, in apparent contradiction to prevailing concepts of stomatal response to water status, and this relationship was probably attributable to effects of other environmental variables on ψ₁ and r ₁. Leaf resistance decreased after a storm characterized by winds in excess of 160 km·h^-1. Cuticular disruption and altered stomatal response may have been responsible for the storminduced r ₁ decrease.