Researchers investigate the impact of different irrigation strategies on water stress and sunburn in Cripps Pink apples. By Anna Mouton
The primary cause of sunburn is too much sunlight – no surprise – but other factors also play a role. An experiment based in Stellenbosch looked at the effect of three different irrigation strategies on sunburn in full-bearing Cripps Pink apple trees. The trees were planted in rows that ran northeast to southwest. The orchard had water-retentive soils with a relatively high clay percentage.
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Irrigation levels
Three groups of trees received different irrigation levels for 15 days, starting at approximately 150 days after full bloom. Water was delivered by microjet sprayers on either side of the trees.
Soil water content
Soil water content at a depth of 45 cm was approximately 14% at the start of the experiment. Soil moisture levels dropped for the no-irrigation treatment within the first week. There was a significant difference in soil water content for all treatments at two weeks (Fig 1).
Effect on the trees
The researchers used stem water potential as a measure of tree stress. Under normal conditions, water is pulled up from the roots as a result of negative pressure in the vascular tissues – contained in the stems – of the tree. The negative pressure results from water loss through the leaves. Drier soils increase this negative pressure – think of it as the tree sucking harder.
Increasingly negative stem water potential is therefore an indication that the tree is struggling to obtain water from the soil. In this study, decreased stem water potential was observed with reduced soil moisture content. The researchers concluded that the trees receiving no irrigation were severely water stressed after two weeks (Fig 2).
Effect on fruit surface temperature
Fruit surface temperatures were measured on days 0 and 14 of this trial. Fruit surface temperatures were significantly higher after two weeks in water-stressed trees than in trees receiving normal irrigation.
Other studies have shown that the temperature of the surface of sun-exposed fruit can be as much as 5 – 15°C higher than air temperatures. Fruit surface temperatures of 46°C and higher are associated with sunburn browning, whereas fruit surface temperatures of 52°C and higher are associated with sunburn necrosis.
Daily maximum temperatures over the course of this trial ranged from 23.1°C – 34.8°C. However, temperatures were substantially lower in the second week of the trial compared to the first week. It is therefore likely that fruit attained higher temperatures than were measured on day 14 (Fig 3).
Sunburn
The researchers tagged specific fruit on the western side of each tree at the start of the trial. These fruit had no sunburn on day 0 and were fully exposed to sunlight. Sunburn severity was assessed on days eight and 15, as well as at harvest. Fruit in both the reduced-irrigation treatments had significantly more sunburn than those in the normal-irrigation treatment (Fig 4).
Tagged fruit was also classified according to type of sunburn at harvest. Although the treatments with half irrigation and no irrigation had similar overall percentages of sunburnt fruit, sunburn was more severe in the fruit from trees that were not irrigated. This matters because fruit affected by sunburn necrosis is not suitable for the fresh market (Fig 5).
Data was also collected on the occurrence of sunburn in all fruit harvested from trees in the different treatments. Overall percentages of sunburn were lower than in the tagged fruit, as the tagged fruit had been selected for high risk of sunburn. However, the ratios of different types of sunburn were similar (Fig 6).
References
Makeredza, B., M. Schmeisser, E. Lötze, and W. Steyn. 2013. “Water stress increases sunburn in ‘Cripps’ Pink’ apple.” HortScience 48(4):444-447.
