Relationships between leaf water potential and photosynthetic activity of field-grown grapevines under a mediterranean environment

In order to verify the dependence of net assimilation rate (A) and stomatal conductance (g(s)) on leaf water potential (psi), physiological data from a training system and irrigation experiment conducted during 1991 and 1992 are correlated. In 1991, a very hot and dry season caused a high water stre ss, which induced premature leaf senescence and low photosynthetic rates on the remaining leaves and, consequently, reduced berry sugar accumulation. The irrigation applied in 1992 positively affected grapevine physiology, gr owth and yield without changing wine quality. In stress conditions, A and g (s) measured during the summer on sun leaves at three different periods of the day were highly correlated with predawn leaf water potential (psi(pd)) In non-irrigated plants the correlations between the diurnal values of psi and A or g(s), measured at the same time, were also significant but the coe fficients were lower than those found on the correlation with the psi(pd), In the irrigated vines there were no significant correlations between psi v alues and A or g(s) measured during the photoperiod. Under water stress con ditions, psi(pd) was a very sensitive indicator of the physiological activi ty of sun leaves. During the photoperiod, leaf water status was a poor indi cator of sun leaf photosynthetic activity and its relative importance decre ases with the increase in soil water availability.