Sunburn research summarised

Highlights of projects funded by Hortgro Pome and Hortgro Stone

Hortgro Pome and Hortgro Stone have funded — and continue to fund — extensive research into the development and control of sunburn. The goal is to better understand the causes and mechanisms of sunburn so that control measures can be developed. This article summarises some of the highlights. Consult the research inventory for more on the many scientists involved and their research outputs.

• The development of sunburn is thought to be associated with both heat exposure and UV-B irradiation. Laboratory trials showed that different cultivars respond differently to UV-B, but all are more sensitive to UV-B if they have been growing in the shade. The researchers confirmed that photodamage in acclimated fruit starts at temperatures of around 45°
• Sensitivity to sunburn was tested for Rosy Glow apples grown on a range of dwarfing to vigorous rootstocks. The researchers concluded that the rootstock does not affect the innate sensitivity of the apple peel to high light and temperature. Sunburn prevalence on a tree level is likely to be driven by the exposure of more fruit to potentially damaging levels of light and heat.
• Water stress was found to increase fruit surface temperature and sunburn in Cripps Pink and Granny Smith apples as well as in African Delight and Laetitia plums. Researchers tested various levels of irrigation against normal irrigation for different time periods in a series of trials. Sunburn was both more prevalent and more severe in fruit from trees suffering from water stress. Whereas irrigation at half normal levels increased sunburn compared to the control, irrigation at twice normal levels did not reduce sunburn compared to the control.
• Research on the effect of pulsing irrigation in apples was inconclusive. Pulsing irrigation improved the plant water status and reduced sunburn in Granny Smith and Golden Delicious compared to a control that did not receive pulsing irrigation. However, the control experienced water deficits. Therefore the difference in the occurrence of sunburn could reflect increased sunburn in the control rather than a true decrease in the treatment.
• Researchers measured the colour of Granny Smith apples at different positions in the canopy from fruit set until harvest to see how colour and sunburn relate to light levels and fruit surface temperature. They found that Grannies require good light until around eighty days after full bloom to develop high chlorophyll concentrations and dark green colour. Too much light in the latter half of the season reduced green colour and increased the risk of sunburn and red blush. These results indicate that shade netting — especially draped nets — should significantly improve the colour of green apple cultivars.
• The red colour of apple cultivars such as Topred and Cripps Pink may mask sunburn browning. Studies confirmed that red and blushed cultivars are not protected from sunburn by anthocyanin — the red pigment just hides the sunburn. Analysis of the tissue response of different cultivars revealed that the severity of visible signs of sunburn is not directly correlated to the stress levels of the fruit.
• Sunburnt fruit tend to have higher sugar levels, but higher sugar levels do not predispose fruit to sunburn. Fruit do become more sensitive to sunburn closer to harvest for a large variety of reasons, but high sugar levels are not one of the causes.
• Two years of trials on Braeburn, Fuji, Royal Gala and Cripps Pink apples demonstrated that 20% black shade net was not detrimental to photosynthesis and carbon assimilation rates. The implication is that nets will not have a negative effect on seasonal carbohydrate production and growth. Nets with a high shade percentage may have a negative effect on the extent and intensity of red colour.
• Plums are susceptible to sunburn and internal heat damage. Shade net and optimal irrigation reduced sunburn.

Image supplied by Helen Marais | Stellenbosch University.