Phytochemical screening and antioxidant activity on berry, skin, pulp and seed from seven red Mediterranean grapevine varieties (Vitis vinifera L.) treated with kaolin foliar sunscreen

In the latest decades, global warming impacts become an emergent issue, and short-term techniques such as kaolin have been proposed to mitigate summer stress effects on Mediterranean viticulture. Nevertheless, the impact of this technology on grape berry composition, particularly on different fractions of whole berry (skin, pulp, and seed) is not yet studied, being these data crucial to the expansion of waste and by-product industry. Thus, the purpose of the present study was to fulfill the identified gap of knowledge by investigating how a pre-veraison kaolin (5% w/v) exogenous application may impact secondary metabolites and antioxidant capacity (ABTS (center dot+) and DPPH methods) at veraison and maturation, in whole berries and their fractions (skin, pulp, and seed), from seven red varieties ('Touriga Franca', 'Touriga Nacional', 'Tinta Francisca', 'Vinhao', 'Grenache', 'Borracal', and 'Cornifesto') under Mediterranean conditions conjugated with global warming. Afterwards, biometric, color and oenological parameters (soluble solids, pH, acidity, and soluble protein) were also quantified in whole berries. Our findings indicate that kaolin increased the berry size (up to 62.2 %) which reduced the skin-to-pulp weight ratio and consequently the anthocyanins contents (reaching - 41.7 %). Additionally, particle film reduced leaf temperature (until - 11.78 %) avoiding thermal stress which seems to decrease the production of proteins in grape berries treated (up to - 27.01 %), indicating a potential reduction of heat shock proteins production. Our results enlighten that it was crucial to analyze the several fractions of grape berries separately, because output resulting from the whole berry might lead to an inaccurate conclusion. In this vein, although often no changes were reported in the whole berry, when analyzing their specific fractions, we found that kaolin enhanced the production of secondary metabolites and antioxidant capacity in the seed (reaching 49,40 and 48.29 %, respectively), leading to a positive influence on berry quality which might be favorable to industrial by-products expansion. The findings also support that kaolin can represent a resilience strategy aiming to improve berry quality under the changing climatic scenario.

Automated summary

The study investigated the impact of kaolin on grape berry composition and quality, showing that kaolin increased berry size, reduced the skin-to-pulp weight ratio, and consequently affected anthocyanin content. Additionally, kaolin decreased protein production in grape berries and enhanced secondary metabolites and antioxidant capacity in the seed, indicating a positive influence on berry quality.