ISHS Acta Horticulturae 636: XXVI International Horticultural Congress: Key Processes in the Growth and Cropping of Deciduous Fruit and Nut Trees
SUNBURN ON APPLES - CAUSES AND CONTROL MECHANISMS |
| Authors: | J.N. Wünsche, J. Bowen, I. Ferguson, A. Woolf, T. McGhie |
| Keywords: | Malus x domestica, antioxidants, chlorophyll, heat-shock protein, fruit skin properties |
| Abstract:
On the basis of our results so far, sunburn is probably caused by over-heating of the fruit surface due to excessive levels of incoming solar radiation in combination with high ambient air temperatures. The complexity of the underlying physiological mechanisms of this skin blemish is, however, not fully understood. It has been hypothesised that sunburn damage may be an expression of plant defense mechanisms involved in the response to oxidative stress. The sensitivity of apple cultivars to sunburn may be partly related to the physiochemical properties of the fruit skin. The homogeneity, thickness and composition of the epicuticular wax layer, the amount of hair on the skin surface, and the concentration and quantity of skin pigments could offer some protection against sunburn by increased reflection of visible and particularly infrared light accompanied by decreased fruit temperature. Our research has shown that chlorophyll a and b concentrations in the skin decreased considerably from healthy to suntinted skin tissue, probably resulting from photobleaching of the chlorophyll. Pigments with substantial radical scavenging ability such as ß-carotene and chlorogenic acid concentrations increased substantially with greater sunburn damage. Heat shock proteins (hsps) are believed to confer tolerance to heat by protecting proteins from irreversible denaturation and breakdown. Northern blot analysis of apple tissue RNA from heat exposure treatments has shown that hsp 17 and hsp 70 gene expression were associated with high fruit temperature. It has yet to be identified how soon gene expression occurs and whether hsp transcripts are translated to high protein levels, following high temperature exposure of apple fruit. To avoid sudden exposure of fruit to intense heat and solar radiation, overhead sprinkling and coating the tree with a reflective kaolin particle film were effective orchard practices for reducing incidence and severity of sunburn. More work is needed to identify the sunburn sensitivity of apple at different stages of fruit development and to understand the time response between sun injury and visible sunburn symptoms on a range of cultivars. | |
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