Heat damage in plums

Heat waves, occurring towards or during the harvesting window of Japanese plum cultivars, hamper production of premium quality plums in the Western Cape Province of South Africa by causing sunburn in the presence of high irradiance. In addition, high respiration rates initiated by high temperatures are thought to deplete internal fruit oxygen and trigger anaerobic respiration with subsequent accumulation of ethanol, resulting in internal damage. Damage that is not apparent at harvest can manifest in cold storage. No information is available on temperature thresholds for thermal damage of the skin and flesh of Japanese plums. In apples, maintaining high stem water potential (SWP) and applying shade netting were reported to alleviate sunburn. Summer pruning is a common practise in Japanese plums, but the timing may affect fruit quality and sunburn incidence. Research in this regard as it pertains to plums is lacking.

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).

African Delight’ plums from exposed, upper canopy positions were larger, advanced in maturity but more susceptible to sunburn. Delaying summer pruning predisposed fruit to sunburn and did not enhance fruit quality. Early summer pruning decreased sunburn, increased fruit size, red colour and total soluble solids (TSS). Abstaining from pruning reduced sunburn but decreased overall fruit quality. Fruit that developed sunburn received >50% photosynthetic photon flux (PPF) of full sun on average while average fruit surface temperature (FST) exceeded 35 °C. Shade net during the hottest part of the season attenuated PPF, and subsequently, decreased FST and sunburn. Deficit irrigation late in the season elevated canopy temperature, FST and sunburn in ‘African Delight’ and ‘Laetitia’ while SWP, flesh firmness, TSS and gas exchange decreased. The increased heat load could be attributed to decreased evaporative cooling as a result of reduced transpiration. Excessive irrigation did not lower FST and sunburn compared to the control. Increases in ethanol at harvest and internal damage after cold storage were higher in more mature fruit treated at 30 °C and 40 °C but tended to decline at 45 °C in ‘Laetitia’ due to curing. In ‘Fortune’, more mature fruit were consistently more susceptible to internal heat damage. No symptoms of internal heat damage were observed in ‘African Delight’ possibly be due to this cultivar’s high skin permeability that prevented accumulation of threshold ethanol levels.

In conclusion, plum producers should adopt early summer pruning practices and incorporate shade nets to reduce sunburn. However, the potential of shade nets and potential negative effects on reproductive development requires further evaluation over the entire growing season. Low SWP increases FST and sunburn possibly due to canopy heating and loss of convectional cooling, explaining why excessive irrigation did not reduce sunburn. High temperature treatments can potentially be used for curing against cold storage enhanced heat damaged if used with methods that circumvent external peel damage.