Water relations and sunburn in apple fruit.

[摘要] ENGLISH SUMMARY: Sunburn in apple is a major problem in South Africa with losses of up to 50% of total yield.The primary objective of this study was to investigate the relationship between tree waterrelations and sunburn development under South African conditions. Cultural practices thatmay affect sunburn development through modification of plant water status and the effect ofmicroclimate on fruit peel anatomy was also studied.Induced water stress increased sunburn necrosis and incidence in 'Granny Smith' and sunburnseverity in 'Cripps' Pink' apple. Although not very pronounced, chlorophyll fluorescencepointed to photochemical changes within the peel under water stress. Water stress influencedascorbic acid levels in peel, indicating the occurrence of oxidative stress. Leaf net carbonassimilation was reduced due to lower stomatal conductance and possibly alsophotoinhibition.Pulsing irrigation reduced the incidence of sunburn in 'Golden Delicious' and 'Granny Smith'compared to an under irrigated control. The reduction in sunburn might be attributed to eitherthe improved plant water status, which reduced tree stress compared to the under irrigatedcontrol, or the comparatively more vegetative growth resulting in shading of fruit. Canopyambient temperature was lower under pulsing irrigation and plant water status was improved.Foliar S˗ABA (active form of abscisic acid) application had no effect on sunburn incidenceand severity under South African conditions over three growing seasons. The main applegrowing regions in South Africa (33-34º S latitude) experience high radiation, whiletemperatures often exceed 40 ºC during the growing season. S-ABA application resulted inlower stomatal conductance with a consequent decrease in gas exchange and reduction in fruitsize. Tree water status in terms of stem water potential was improved due to the reduction intranspiration.Fruit transpiration and water loss was higher in young compared to mature fruit, whichseemingly corresponds with increasing susceptibility of fruit to sunburn during fruit growth.Stomata are present in young fruit and are under regulatory control, which makes them moreeffective for transpiration. In mature fruit, the predominant lenticel type was partially openand not effective for transpiration. Lenticel density and morphology varied considerablywithin and between cultivars, between different canopy positions, and between productionareas. Although fruit transpiration and water loss decreased over the growing season, it did not always follow the same pattern as fruit surface morphology. In addition, the contributionof transpiration and water loss to the fruit heat balance is negligible. Hence, differences infruit surface morphology may not explain differences in cultivar and fruit sunburn sensitivity.In conclusion, this study provides insight into ecophysiological and biochemical changes thatoccur under water stress and that might have a bearing on sunburn development in apple fruit.The study provides further proof of the link between plant water status and sunburndevelopment and shows that good management of plant water status through irrigation isinstrumental to controlling sunburn in a warm production region.

[发布机构] Stellenbosch University