Late-season stomatal sensitivity to microclimate is influenced by sink strength and soil moisture stress in 'Braestar' apple trees in South Africa
In the Western Cape region of South Africa, mild weather during the late season is conducive to continued high rates of photosynthesis in apple trees. During the post-harvest period, both stressful environmental conditions and altered tree carbohydrate demand (sink strength) may influence stomatal function and thus gas exchange and tree water requirements. In this study we investigated the interactive effects of the main atmospheric determinants of stomatal conductance (light, leaf temperature, leaf-to-air vapour pressure deficit (VPD)) in response to fruit removal at harvest and soil moisture stress after harvest (cessation of irrigation). An 8-year-old 'Braestar' orchard was used. Preand post-harvest (pre-senescent) photosynthetic light response curves were generated at leaf temperatures of 20-40°C. Stomatal conductance showed increased sensitivity to increasing leaf temperature after harvest, resulting in a reduction in the optimum temperature for photosynthesis from 25-30 to 20-25°C. This ensured the maintenance of high instantaneous leaf water use efficiency (WUE) post-harvest at all temperatures, compared to reductions in WUE but higher photosynthetic rates at high leaf temperature pre-harvest. We suggest that this reflects a change from a carbohydrate-demanding strategy (high sink demand) pre-harvest, to a water-conserving strategy post-harvest when sink demand is reduced. Following harvest, daily irrigation was either shut off or continued for 11 days, and light-saturated gas exchange measured regularly at varying levels of VPD in droughted trees. Photosynthetic rates, stomatal conductances and intercellular CO2 concentration (ci) remained relatively constant for the first 6 days, followed by reductions linked to a period of high ambient atmospheric evaporative demand and subsequent partial recovery. Droughted and non-droughted trees showed similar gas exchange values at the end of the droughting period when measured at 25°C, but gas exchange of droughted trees remained repressed when measured at 30°C due to increased non-stomatal limitation. Throughout the study, only cuvette VPD levels above 3.5 kPa (measured at 30°C) significantly reduced gas exchange, with no interaction with duration of droughting. This study showed that post-harvest stomatal control was more attuned to ambient atmospheric evaporative demand in the orchard than to slowly developing soil moisture stress. We conclude that both atmospheric factors and changing sink demand influence stomatal control of apple leaves post-harvest, and that this should be taken into account when determining irrigation strategies. © 2003 Elsevier Science B.V. All rights reserved.