Seasonal patterns of xylem sap pH, xylem abscisic acid concentration, leaf water potential and stomatal conductance of six evergreen and deciduous Australian savanna tree species

Abstract

Deciduous trees of Australia’s northern savannas typically have less-negative leaf water potentials than evergreen species and their stomata are more sensitive to soil drought than those of evergreen species. This paper presents the first investigation of the role of xylem sap pH and abscisic acid content in explaining stomatal behaviour of Australian trees in the field. We measured stomatal conductance, leaf-to-air vapour pressure difference (D) and leaf water potential, xylem abscisic acid (ABA) concentration and xylem sap pH of evergreen, semideciduous and fully deciduous tree species in the field over a 15-month period. Measurements were made during both the wet and the dry seasons. Stomata closed in response to increasing D in both evergreen and deciduous species and were equally sensitive to increasing D or declining leaf water potential. Xylem ABA concentration increased with declining leaf water potential in evergreen and semi-deciduous species, but not deciduous species. Similarly, there was an inverse correlation between stomatal conductance and xylem ABA concentration. Xylem sap pH increased as leaf water potential declined from wet to dry season for evergreen and semi-deciduous species but not for deciduous species. Deciduous species had less-negative water potentials and lower xylem ABA concentrations than evergreen species or semi-deciduous species. We conclude that changes in xylem sap pH and ABA content do occur seasonally in the wet–dry tropics of Australia and that these changes influence stomatal conductance, but only in evergreen and semi-deciduous species. Deciduous species do not appear to modulate either of these chemical signals.