Apple Peel Biochemical Changes after Foliar Application of Combined Boron and Calcium I. Phenolics and Physico-Chemical Attributes

Post-full bloom foliar applications of boron in combination with calcium (B+Ca) suppress fruit sunburn-browning incidence in apples (Malus domestica). However, the mode-of-action is not yet explained and hence, protocols to exploit this seemingly cost-effective peel physiology augmenting approach compared to usual methods (shade-netting, overhead cooling and kaolin application which attenuate excess fruit heat-load), cannot be fully developed or promoted justifiably among growers. Apple fruit sunburn-browning disorders are rampant worldwide, partly because of climate change as well as high cost and inefficacy of the established methods. However, knowledge of B+Ca effect on apple peel physiology may unravel the mode-of-action, paving the way for commercialization of this novel approach. In this study, B+Ca effect on apple peel phenolics and selected fruit physico-chemical attributes was investigated, using studies conducted at two farms for two seasons in Western Cape, South Africa. Four foliar treatments varying in B, Ca and inclusion of zinc (Zn), were applied on ‘Cripps Pink’, ‘Golden Delicious’ and ‘Granny Smith’ apples in 2014/15 (only ‘Cripps Pink’) and 2015/16 (all cultivars) in randomized complete block design experiments with five replications. Fruit peel samples for quantitative determination of total phenolics and total flavonoids analyses were collected at different fruit maturity stages. Significant (p < 0.05) interaction (treatments and fruit maturity stages) effect was observed for total phenolics (‘Golden Delicious’ and ‘Granny Smith’ only) and total flavonoids (all cultivars). Physico-chemical attributes (fruit diameter, size and total soluble solids) were only evaluated in ‘Golden Delicious’ and ‘Granny Smith’, and no significant differences among treatments occurred. Since no clear treatment differences in apple peel phenolic patterns were observed, other appropriate biochemical aspects like photosynthetic pigments should be investigated among such foliar B+Ca treatments.