The external appearance of fruit is a crucial factor in consumer preference (Steyn, 2012). Sunburn, a disorder of the fruit peel caused by high temperature in combination with high irradiance (Glenn et al., 2002; Schrader et al., 2003), negatively affects the appearance of apples in warm production areas such as the Western Cape of South Africa. Depending on irradiance level, temperature and the duration of the photothermal stress (Gindaba and Wand, 2007; Schrader et al., 2003) affected fruit show pigment bleaching, a golden bronze sublethal discoloration, or necrosis of the fruit peel (Racskó et al., 2005; Schrader et al., 2003). Losses resulting from sunburn are therefore attributable to the injury and downgraded appearance of the fruit. Sunburn can decrease class one exportable fruit in South Africa by 20% to 50% in the orchard and packhouse and by a further 10% at the export destination (Bergh et al., 1980).
Although excessive heat and high irradiance are the main inductive factors in sunburn development, indirect factors such as relative humidity, air movement, and various cultural practices such as tree size, canopy structure, pruning, and irrigation may affect the incidence and severity of sunburn (Racskó and Schrader, 2012). Water stress has been reported to exacerbate the development of sunburn (Barber and Sharpe, 1971; Gonda et al., 2006), whereas regular irrigation purportedly decreases the incidence of sunburn, supposedly by facilitating sufficient evaporative cooling in the tree (Van de Ende, 1999). However, little research has been conducted to relate plant water status to sunburn incidence and severity; the link between plant water status and sunburn seems to be based primarily on observations. In a New Zealand study using five three-year-old ‘Braeburn’ trees per treatment, no irrigation did not significantly increase sunburn compared with the control treatment (Wünsche et al., 2001).
Considering the lack of experimental data, we set out to obtain empirical evidence that water stress increases the incidence and severity of sunburn. Hence, the objective of this study was to investigate the effect of water stress on plant water relations and how this affects sunburn development in apples.
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