Abstract
Understanding of the physiological effect of post-full-bloom foliar boron combined with calcium (B+Ca) on apple (Malus domestica) peel tissues is envisaged to give way to the unknown mode-of-action by which these mineral regimens suppress fruit sunburn-browning incidence in orchards. Promotion of this mineral approach among growers, as a certainly cheaper alternative to mitigate fruit sunburn-browning incidence in apple orchards necessitates clear elucidation of its mode-of-action. This study investigated peel photosynthetic pigments and total peroxides (as a measure of oxidative stress) in three apple cultivars, ‘Cripps Pink’, ‘Golden Delicious’ and ‘Granny Smith’ which were treated with four B+Ca treatments varying in levels of B and Ca as well as inclusion of zinc (Zn) in one treatment. Randomized complete block design experiments with five replications were conducted at commercial farms in Western Cape, South Africa. Significant (p 0.05) treatment effect for major pigment aspects and total peroxides occurred in all cultivars, but with strong influence of cultivar and fruit age. For instance, effect of varying B, Ca and possible B+Ca duet-effect on photosynthetic pigments occurred in ‘Cripps Pink’, whereas the Zn-treatment was mainly responsible for significant treatment effects in both ‘Golden Delicious’ and ‘Granny Smith’ apples. Significant treatment effect for total peroxides occurred in ‘Cripps Pink’ and ‘Granny Smith’, yet significant interaction effect occurred with ‘Golden Delicious’, however, these significant results did not yield meaningful peel oxidative stress differences among the treatments. Foliar treatment differences in photochemical efficiency (Fv/Fm) were not significant. The study concludes with firm evidence that foliar B+Ca treatment composition has a significant effect on apple peel photosynthetic pigments depending on cultivar, and Zn is not desirable in the formulation of these treatments.
Highlights
Post-full-bloom foliar applications of boron combined with calcium (B+Ca) suppress fruit sunburn-browning incidence in apple orchards, the underlying mode-of-action is not known [1] [2] [3] [4]
This study investigated peel photosynthetic pigments and total peroxides in three apple cultivars, ‘Cripps Pink’, ‘Golden Delicious’ and ‘Granny Smith’ which were treated with four B+Ca treatments varying in levels of B and Ca as well as inclusion of zinc (Zn) in one treatment
Post-full-bloom foliar boron plus calcium applications affect photosynthetic pigments in apple peels depending on treatment formulation and cultivar idiosyncrasies
Summary
Post-full-bloom foliar applications of boron combined with calcium (B+Ca) suppress fruit sunburn-browning incidence in apple orchards, the underlying mode-of-action is not known [1] [2] [3] [4]. Apple peel phenolic dynamics among foliar B+Ca treatments did not yield clear treatment induced patterns [7], yet these treatments had significantly suppressed class 1 (very mild type) sunburn-browning in two distinct cultivars [3]. This raised need to investigate other apple peel biochemical aspects, still those relevant to apple fruit sunburn-browning development, for instance, photosynthetic pigments [8] [9], and oxidative stress status and/or species [10] [11] [12]. These research findings raise possibility that foliar B+Ca may influence levels of apple peel chlorophylls as a physiological effect, and suppress sunburn-browning if increments or preservations of chlorophyll pigments occur
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