Does night-time transpiration contribute to anisohydric behaviour in a Vitis vinifera cultivar?

Abstract

The hypothesis that vines of the Semillon wine grape variety show anisohydric behaviour was tested, i.e. that tissue hydration is unstable under fluctuating environmental conditions. Stomatal conductance and transpiration rates from leaves were measured during the day and at night. Leaf water potential (Ψ_l) in Semillon was negatively correlated to vapour pressure deficit (VPD) both predawn and during the day. Furthermore, Ψ_l fell to significantly lower values than in any of the nine other varieties examined. Night-time values of stomatal conductance (g_n) and transpiration (E_n) in Semillon were up to four times higher than in other varieties; plants enclosed in plastic bags overnight to reduce E_n resulted in better plant–soil equilibration so that predawn Ψ_l in Semillon was the same as in Grenache. These data indicate that the hypothesis is supported, and that night-time transpiration contributes significantly to the low Ψ_l values in Semillon during warm, dry nights. The other contributing factor is daytime stomatal conductance (g_day), which in Semillon leaves was higher than in other varieties, although the decline in g_day with increasing VPD was greater in Semillon than in Shiraz or Grenache. The high values of g_day were associated with high rates of transpiration (E_day) by Semillon through a day when VPD reached 4.5 kPa. When compared to other varieties, Semillon was not unusual in terms of root length density, stomatal density, xylem sap abscisic acid, or leaf electrolyte leakage. Night-time and daytime water loss and insufficient stomatal regulation therefore account for the tendency to anisohydric behaviour shown by Semillon.