Influence of season, drought and xylem ABA on stomatal responses to leaf-to-air vapour pressure difference of trees of the Australian wet-dry tropics

This paper reports the results of two experiments undertaken to investigate the influence of season and soil drying on stomatal responses to leaf-to-a ir vapour pressure differences. We examined the response of stomatal conduc tance to increasing leaf-to-air vapour pressure difference, in the wet and dry seasons, of five tropical tree species. We also examined leaves of thes e species for anatomical differences to determine whether this could explai n differences in stomatal sensitivity to leaf-to-air vapour pressure differ ences. Finally, we conducted a split-root experiment with one of those spec ies to look for interactions between xylem abscisic acid concentration, pre dawn water potential, leaf area to root mass ratio and stomatal responses t o leaf-to-air vapour pressure differences. Stomatal conductance declined linearly with increasing leaf-to-air vapour p ressure difference in all species. Leaves that expanded in the 'dry' season were more sensitive to leaf-to-air vapour pressure differences than those that had expanded in the 'wet' season. The value of leaf-to-air vapour pres sure difference where 50% of extrapolated maximum stomatal conductance woul d occur was 5.5 kPa for wet season but only 3.4 kPa for dry season leaves. In the wet season, transpiration rate increased with increasing leaf-to-air vapour pressure difference in most example species. However, in the dry se ason, transpiration was constant as leaf-to-air vapour pressure differences increased in most cases. There were significant changes in the proportion of cell wall exposed to air space in leaves, between wet and dry seasons, i n three of four species examined. In the split-root experiment, a very mild water stress increased stomatal s ensitivity to leaf-to-air vapour pressure differences, and stomatal conduct ivity declined linearly with decreasing predawn water potential. However, l evels of ABA in the xylem did not change, and stomatal sensitivity to exoge nous ABA did not change. The ratio of leaf area to root mass declined durin g water stress and was correlated to changes in stomatal sensitivity to lea f-to-air vapour pressure differences.