A Study of the Dynamics of Sunburn Reduction in Apple (Malus domestica) Using Foliar Applications of a Combination of Boron and Calcium

Apple fruit sunburn browning (SBB) is largely a climate-related disorder that reduces fruit quality leading to serious financial losses. The aim of this research was to unravel the mode-of-action by which post-full-bloom foliar boron plus calcium (B+Ca) suppress fruit SBB incidence in apple orchards. Scientific elucidation of such mode-of-action is essential for successful recommendation and adoption of this low-cost approach at farm level. Therefore, in this study, four varying foliar B+Ca formulations were applied post-full-bloom in orchards of a bi-colour (‘Cripps Pink’) and two green (‘Golden Delicious’ and ‘Granny Smith’) apple cultivars. The experimental sites / orchards were located in Elgin and Stellenbosch in Western Cape Province, South Africa, and the three cultivars chosen for this study sharply contrast in SBB susceptibility. A preceding and related study found that peel tissue anatomical differences and mineral concentrations did not to explain the SBB incidence reduction, hence this study focused on investigating the possibility of a biochemical-based mode-of-action. Thus, peel tissue biochemicals (phenolics, chlorophyll, carotenoid and total peroxides) were determined, and compared to SBB incidence at harvest. In 2014/15 season, fruit peel was sampled at four early maturity stages / days after full bloom (DAFB). In 2015/16, samples were collected at four bi-weekly intervals towards fruit harvest maturity window. Results showed that treatments influenced peel biochemical levels in varying degrees and with cases of significant (p < 0.05) interaction effects with fruit maturity stages. Significant SBB suppression occurred for ‘Cripps Pink’ (2015/16) and ‘Golden Delicious’ experiments, where the control (no B+Ca) had highest incidence. In ‘Granny Smith’, the SBB differences within treatments did not differ significantly. No treatment-induced changes in any peel biochemical occurred in patterns that could yield the sought mode-of-action; for instance, all treatments reduced SBB considerably in ‘Cripps Pink’, whereas it was the high B treatments (0.08 and 0.17 g.l-1 ) in ‘Golden Delicious’. This highlighted the influence of cultivar and treatment formulation in respective SBB suppression outcomes and corresponding in peel biochemical levels. With treatment formulation, inclusion of zinc (Zn) significantly lowered photosynthetic pigment attributes, and associated with SBB incidence that was on par with the control (no B+Ca). Overall, this study concluded that different cultivars require different foliar B+Ca formulations to supress SBB, and generally, Zn should not be included. Further results from a multivariate analysis indicated that the sought mode-of-action could be under an additive-metabolite-action phenomenon, involving contributory roles of several peel biochemicals as influenced by particular foliar B+Ca treatments. However, further studies are required to establish the physiological mechanism underlying this postulated mode-of-action