Epicuticular waxes: A natural packaging to deal with sunburn browning in white grapes

Epicuticular waxes on grapevine berry cuticle provide protection of the inner tissues from biotic and abiotic stresses. However, little is known about their role in protecting grape epidermis from sunburn damages. This study investigated the effect of wax disruption in ten sun-exposed white skinned Vitis vinifera L. varieties. Browning symptom appearance was quantified one day and four days after wax disruption. It was also examined the content of the main photosynthetic pigments (chlorophyll a, chlorophyll b and carotenoids) and total phenolics in the skins four days after the treatment. The disruption of epicuticular waxes promoted grape sunburn: the skin browning intensity increased from 39.8 to 67.1 after one day and four days from the treatment, respectively. The loss of green color after wax disruption resulted from the degradation of chlorophyll a, since its content in the treated berries (9.01 μg g−1 of skin) was lower than that in the control berries (17.16 μg g−1) after four days from the treatment. The co-occurrence of wax disruption in a sunlight excess environment caused also the photo-oxidation of carotenoids, which were higher in control berries (19.86 μg g−1) than in the treated berries (14.51 μg g−1). The browning intensity and the difference in the total phenolics of the skins between treated and control berries were significantly correlated, suggesting that the polymerization of phenolics into brown compounds after wax disruption could further enhance browning symptoms. Therefore, epicuticular waxes could be considered as an important natural coating for grapes, providing defence against water loss and pest or pathogen attacks, but also effectively limiting sunburn browning and maintaining quality for both wine grapes and table grapes.