SUNBURN – THE COST OF A HIGH LIGHT ENVIRONMENT

ISHS Acta Horticulturae 557: VII International Symposium on Orchard and Plantation Systems

SUNBURN – THE COST OF A HIGH LIGHT ENVIRONMENT

Authors:	J.N. Wünsche, D.H. Greer, J.W. Palmer, A. Lang, T. McGhie
Keywords:	chlorophyll fluorescence, hydro-cooling, skin pigments, spray application
Abstract: The severity of sunburn on apple fruit varies with cultivar, growing condition and tree management practices, yet the cause of this skin blemish is poorly understood. It has been hypothesised that sunburn damage may be a response to high light/UV exposure and/or high fruit skin temperature. Key processes may include the synthesis of heat-shock proteins and UV-absorbing flavonoids, cuticular reflectance of light, epicuticular wax formation and transpiration. First-season’s research project on ‘Braeburn’ apples focused on the following three areas: Water application treatments to the whole canopy in mid-season reduced mean fruit skin temperature by up to 8°C. Fruit rotation by 180º increased sunburn severity within 8 days of treatment in mid February. 75% defoliated trees and recently replanted trees had significantly more sunburn fruit than non-irrigated and control trees. Chlorophyll fluorescence was used to non-destructively monitor the onset and seasonal development of sunburn on ‘Braeburn’ apples in an orchard environment. Chlorophyll fluorescence was negatively and exponentially correlated to a visual sunburn score. Sunburn lesions were detectable by chlorophyll fluorescence with a markedly reduced signal compared to that obtained from undamaged tissue. Chlorophyll a and b concentrations in the skin decreased considerably from healthy to suntinted skin tissue, probably resulting from photobleaching of the chlorophyll. -carotene and chlorogenic acid concentrations increased substantially with sunburn score and may represent a protection mechanism.
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