Effects of different regulated deficit irrigation treatments on phenols in grape berries

Abstract:【Objective】Regulated deficit irrigation treatments have significant effects on the content of phenolic metabolites in the peels of grape (Vitis vinifera L.) berries. Studying the effects of various regu- lated deficit irrigation treatments on the metabolism of phenolic substances in grape berry peels can not only provide reference for improvement of grape berries quality, but also provide methods to save water and thereby improve the economic benefits.【Methods】Three-year-old ridge-rooted Reliance (Vitis la- brusca L.× V. vinifera L.) and Zitian Seedless (V. vinifera L.) were selected as the materials. The same pruning and fertilization regimes were applied to all grapevines. Regulated deficit irrigation treatments included 2.5 L irrigation every 4 days (T1) and 5 L every 8 days (T2) from prior to veraison till full ber- ry maturity, and irrigation with 5L every 4 days was set as the control. At ripe stage, berries with uni- form maturity and no mechanical damage were collected for quality analysis. Total soluble solids (TSS) were measured using a PAL-1 refractometer. Total phenol was determined by Folin-Phenol reagent. To- tal anthocyanins were measured using an ultraviolet spectrophotometer. The skin was separated from the pulp and both tissues were immediately frozen in liquid nitrogen, and stored at -80 °C for further use. Phenolic metabolites of the grape skins were measured by high performance liquid chromatogra- phy-mass spectrometry (HPLC-MS).【Results】The results showed that T1 and T2 treatments increased TSS and total phenol content in Zitian Seedless grape, and total anthocyanins and most phenolic metab- olites in the both cultivars. Seventeen anthocyanins, twenty flavonols, sixteen flavanols, three other fla- vonoids, fifteen phenolic acids and four stilbenes were detected in our study. In Reliance berries, the contents of TSS and total phenol in T2 was significantly higher than in T1. Compared with the control in Zitian Seedless, the contents of TSS and total phenol increased significantly in the groups of the two regulated deficit irrigation treatments and there was no significant difference between T1 and T2. In T2, cyanidin and delphinidin were significantly increased in Reliance peels, and their concentrations were higher than in T1. The contents of methylated anthocyanin (peonidin, petunidin and malvidin) were sig- nificantly higher in T1 than in T2, but the contents of anthocyanins without glycosides were higher in T2. We also found that T1 significantly increased the contents of cyanidin and peonidin in Zitian Seed- less, and T2 significantly increased the contents of tri-substituted anthocyanins (delphinidin and malvi- din). Flavonol metabolites in the two cultivars mostly existed in the form of flavonol glycosides. In Reli- ance berries, the contents of all flavonol metabolites but quercetin in T2 were significantly higher than in T1. However, in Zitian Seedless berries, the flavonol metabolites in T1 exceeded that in the other two groups. Among the flavonol metabolites, the myricetin and kaempferol conjugates were higher in T1 than in T2, but isorhamnetin-3-rutinoside, rutin, quercetin-3-glucuronide and quercetrin contents were the opposite. For flavanols, water stress could significantly increase their contents in both cultivars. However, in Reliance berries, the contents of flavanols in T2 were significantly higher than in T1, but there was an opposite trend of increased flavanols under the two treatments in Zitian Seedless berries. In both cultivars, except for proanthocyanidin-B2, most flavan-3-ols and their polymers were signifi- cantly higher in T1 compared with T2. Dihydroquercetin, phloridzin and luteolin were detected in both cultivars. Among them, the contents of dihydroquercetin and phloridzin increased, while that of luteolin decreased under T1 and T2. In Reliance berries, the contents of 2,5-dihydroxybenzoic acid, cis-caftaric acid, ellagic acid, p-coutaric acid, chlorogenic acid, caffeic acid, p-coumaric acid and 3,4-dihydroxyben- zoic acid in T2 were significantly higher than in T1 and the control. In Zitian Seedless berries, the con- tents of 5-O-caffeoylquinic acid, cis-caftaric acid, caffeic acid and p-coumaric acid in T1 were signifi- cantly higher than in T2, while the contents of ellagic acid and syringic acid were in a opposite pattern. In addition, all the stilbenes were increased by the two regulated deficit irrigation treatments.【Conclu- sion】Moderate water stress applied from veraison can not only promote the accumulation of phenolic metabolites and thus improve the quality of grape berry, but also improve water utilization.