THE EFFECT OF DROUGHT AND VAPOR-PRESSURE DEFICIT ON GAS-EXCHANGE OF YOUNG KIWIFRUIT (ACTINIDIA-DELICIOSA VAR DELICIOSA) VINES

To evaluate the effect of drought and vapour pressure deficit (VPD) or stomatal behaviour and gas exchange parameters, young kiwifruit vines (Actimidia deliciosa var. deliciosa cv. Hayward) were exposed to alte rnating periods of drought and drought-relief over two growing seasons . Vines were grown either in the field or in containers. Stomatal cond uctance of fully-expanded leaves rapidly decreased as pre-dawn leaf wa ter potential was reduced below a threshold value of -0.3 MPa. Stomata l conductance reached minimum values of 10-20 mmol m(-1) s(-1), Transp iration rate was similarly sensitive to changes in leaf water status, whereas more severe drought levels were necessary to affect photosynth esis significantly, Net daily carbon gains were estimated at 4.7 and 2 .7 g m(-2) for irrigated and droughted vines, respectively. Gas exchan ge parameters recovered to values of irrigated vines within a few hour s after relief of stress. Rate of recovery depended on the level of st ress reached during the previous drought period. There was a steady de cline in stomatal conductance when VPD was increased from 0.8 to 2.5 k Pa in both irrigated and droughted vines. The VPD at which stomatal co nductance reached 50% of maximum values was 2.1-2.2 kPa for both treat ments. We conclude that stomata were highly sensitive to changes in so il water status and that midday depression of photosynthesis measured in kiwifruit vines was related to water deficits arising in the leaf b ecause of both transpirational losses and to the direct effect of incr easing VPD. (C) 1996 Annals of Botany Company.