Tree water relations and sunburn in pome fruit

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 water relations and sunburn development under South African conditions. Cultural practices that may affect sunburn development through modification of plant water status and the effect of microclimate on fruit peel anatomy was also studied.

Various trials were conducted and the word limitation of the abstract doesn’t allow the detailed description of all methodology (Please refer to dissertation and publications for detail).

Induced water stress increased sunburn necrosis and incidence in ‘Granny Smith’ and sunburn severity in ‘Cripps’ Pink’ apple. Chlorophyll fluorescence pointed to photochemical changes within the peel under water stress. Water stress influenced ascorbic acid levels in peel, indicating the occurrence of oxidative stress. Leaf net carbon assimilation was reduced due to lower stomatal conductance and possibly also photoinhibition.

Pulsing irrigation involves application of extra water to the orchard floor during a heat wave, with the intention to reduce sunburn. Treatments during 2013-2014 consisted of a control scheduled per the farmer’s current practice using microsprinklers, and pulsing irrigation at 25, 37 and 75 L h-1 per tree in addition to normal irrigation when ambient air temperature ≥30 °C (5 min on and 20 min off). Supplemental, pulsed irrigation reduced sunburn incidence in ‘Golden Delicious’ and ‘Granny Smith’ compared to the control. Unfortunately, the control treatment was under irrigated. The reduction in sunburn might be attributed to either improved plant water status, which reduced tree stress compared to the under irrigated control, or comparatively more vegetative growth resulting in shading of fruit.

Foliar S˗ABA (active form of abscisic acid) application had no effect on sunburn incidence and severity under South African conditions over three growing seasons. S-ABA application resulted in lower stomatal conductance with a consequent decrease in gas exchange and reduction in fruit size. Tree water status in terms of stem water potential was improved due to the reduction in transpiration.

Fruit transpiration and water loss was higher in young compared to mature fruit, which seemingly 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 more effective for transpiration. In mature fruit, the predominant lenticel type was partially open and not effective for transpiration. Lenticel density and morphology varied considerably within and between cultivars, between different canopy positions, and between production areas. 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 contribution of transpiration and water loss to the fruit heat balance is negligible. Hence, differences in fruit surface morphology may not explain differences in cultivar and fruit sunburn sensitivity.

In conclusion, this study provided insight into ecophysiological and biochemical changes that occur 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 sunburn development and shows that good management of plant water status through irrigation is instrumental to controlling sunburn in a warm production region.  Results on the potential benefit of supplemental, pulsed irrigation was inconclusive due to the under irrigation of the control treatment.