Responses of leaf night transpiration to drought stress in Vitis vinifera L.

Night-time transpiration (Enight) is potentially an important factor affecting whole-plant water balance and, thus, water use efficiency. The aims of the present study were: to evaluate night-time changes of stomatal conductance (gnight) and transpiration under different soil water availability conditions for seven grapevine cultivars and to compare leaf-level estimates of night-time water losses with more realistic whole-plant estimates in plants growing outdoors. Two experiments were conducted on seven grapevine cultivars (Vitis vinifera L.) growing in pots maintained at field capacity and drought stress conditions. Night transpiration was evaluated by leaf gas exchange and plant mass measurements. Results showed that Enight and gnight were far above cuticular values, suggesting sustained stomatal aperture during night-time which was reduced under drought stress. Differences between cultivars were found in the extent of drought stress-induced reduction of Enight (from a 36% reduction in Escursac to 82% in Malvasia). However, transpiration losses calculated on the basis of leaf gas exchange measurements often over-estimated the actual whole plant water loss, suggesting the presence of some water inputs to pots/plants during the night, presumably due to dew deposition. Significant stomatal opening persists during the night in irrigated grapevines, leading to substantial water losses by transpiration. Those water losses are significantly reduced under drought stress. It is remarkable that night-time plant water losses can be partially or fully compensated by dew deposition. Therefore, instantaneous leaf gas exchange measurements can result in an over-estimation of the night water losses. The present study shows for the first time that night transpiration due to partially open stomata can be compensated by dew deposition.