Aims: To determine the effects of a

Aims: To determine the effects of applied water amounts at various fractions (0.2, 0.6, 1.0 and 1.4) of grapevine
evapotranspiration on leaf gas exchange of Thompson Seedless grapevines.
Methods and Results: Midday stomatal conductance (gs) decreased linearly as leaf water potential (Yl) and soil
water content decreased. Leaf net CO2 assimilation rate only decreased once midday Yl values were less than
-1.0 MPa and when ~50% of the soil water content at field capacity had been depleted. The mean seasonal midday
A/gs ratio (intrinsic water use efficiency) was greatest for the 0.2 irrigation treatment and decreased as applied water
amounts increased. Diurnal A and gs for vines irrigated at the 0.6 level or greater reached a maximum prior to midday
remained constant thereafter before decreasing late in the afternoon, while those for vines that received less water
decreased subsequent to the first measurement of the day.
Conclusions: A and gs responded differently to vine and soil water statuses under the conditions of this study. There
was no midday depression in either A or gs for vines irrigated at full evapotranspiration.
Significance of the Study: The values of Yl, A and gs reported here would serve as criteria to indicate that vines
were well watered.