This paper reports the results of two experiments undertaken to investigate
the influence of season and soil drying on stomatal responses to leaf-to-air
vapour pressure differences. We examined the response of stomatal conductance
to increasing leaf-to-air vapour pressure difference, in the wet and dry
seasons, of five tropical tree species. We also examined leaves of these
species for anatomical differences to determine whether this could explain
differences in stomatal sensitivity to leaf-to-air vapour pressure
differences. Finally, we conducted a split-root experiment with one of those
species to look for interactions between xylem abscisic acid concentration,
predawn water potential, leaf area to root mass ratio and stomatal responses
to leaf-to-air vapour pressure differences.
Stomatal conductance declined linearly with increasing leaf-to-air vapour
pressure 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 pressure difference where 50% of
extrapolated maximum stomatal conductance would 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 season, 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, in three of four species
examined.
In the split-root experiment, a very mild water stress increased stomatal
sensitivity to leaf-to-air vapour pressure differences, and stomatal
conductivity declined linearly with decreasing predawn water potential.
However, levels of ABA in the xylem did not change, and stomatal sensitivity
to exogenous ABA did not change. The ratio of leaf area to root mass declined
during water stress and was correlated to changes in stomatal sensitivity to
leaf-to-air vapour pressure differences.