Correlations of stomatal conductance with hydraulic, chemical and environmental variables in five urban tree species

Recent findings suggest that stomatal conductance (g(s)) may be as closely linked to plant chemical variables as to hydraulic variables. To test this in an urban field setting, we examined seasonal g(s) in relation to a number of plant and environmental variables in five temperate, deciduous tree species. Stomatal conductance was generally more closely correlated with abscisic. acid concentrations in xylem sap than with shoot water potential, shoot osmotic potential, pH of xylem sap or environmental variables. Seasonal g(s) was mostly poorly correlated with shoot water potential and osmotic potential. Among environmental variables, PPFD accounted for most variability in g(s). We tested a model, developed previously in maize, that describes regulation of g(s) by abscisic acid concentration of xylem sap with leaf water status acting to modify stomatal sensitivity to the abscisic acid signal. This model explained somewhat more variation in g(s) than abscisic acid concentrations alone. Response surface models, especially those incorporating environmental variables, were most successful at explaining g(s). Our findings with urban trees are consistent with the theory of regulation of g(s) by root-sourced abscisic acid. (C) 2001 Elsevier Science BN All rights reserved.