Effects of water stress on berry quality and organic acid synthesis gene expression in Beihong grape

[Objective] Water stress refers to the phenomenon in that the normal physiological function of plants is disrupted due to inadequate or excessive soil water. This condition is mainly categorized as waterlogging stress and drought stress. In response to the imperative for developing water-saving agriculture, effective control of soil water content has been employed to impact plant growth, fruit quality, fruit physiological indicators, and fruit gene expression. Beihong grape is a novel variety characterized by cold resistance and disease resistance that was initially selected in 1965. It is well-suited for cultivation in northern regions and exhibits strong cold resistance. The aim of this study was to examine the effects of water stress on grape berry quality and elucidate the pattern of organic acid accumulation in grapes under different soil water content conditions. [Methods] The nine-year-old Eurasian grapevines of Beihong was chosen as the experimental material. T1 (soil relative water content 65%-80%, waterlogging stress), T2 (soil relative water content 40%-55%, drought stress), and CK (soil relative water content 55%-65%, control) were established on the 18th day after flowering. The basic quality (100 grain weight, soluble solids, titrable acids, total phenols, tannins and anthocyanins) and accumulation of organic acids in grape berries were measured. Real-time fluorescence quantitative PCR was utilized to detect the expression of genes related to organic acid synthesis. [Results] Under water stress condition, 100 grain mass of grapes decreased with the decrease of soil water content. On the whole, T2 treatment resulted in a significant decrease in 100 grain mass of grapes, while T1 treatment increased 100 grain mass of grapes. The soluble solids content increased gradually with grape ripening, and T2 treatment was significantly higher than other treatments, while T1 treatment was not conducive to the accumulation of soluble solids. Under T1 condition, the titrable acid content of grapes was lower from the berry expansion stage to the early stage of veraison, and T1 treatment resulted in fruit volume increase and acid concentration dilution. However, from the later stage of veraison to the mature stage, the titrable acid content with T1 was significantly higher than that with other treatments, and drought stress was significantly lower than that with other treatments, indicating that moderate water stress could reduce the acid content of grapes and accelerate berry ripening, thereby improving fruit quality. The contents of total phenol and tannin in berries showed a similar trend. At the initial stage of treatment, the contents of total phenol and tannin were higher under drought stress condition, but decreased with the progress of treatment. This indicated that short-term drought stress was beneficial to the accumulation of total phenol and tannin in berries, while long-term drought stress was not. Anthocyanins in grapes showed a trend of single-peak increase. At 105th day after flowering, the content of anthocyanins in T2 was the highest, while the content of anthocyanins in T1 treatment was always significantly lower than that in CK and T1 treatments. Anthocyanins are compounds formed by the interaction of anthocyanins and glycogroups, and water stress promoted the accumulation of sugars, thus affecting the accumulation of anthocyanins. The contents of malic and citric acids increased first and then decreased, and malic and citric acids reached their highest values on 45th day after flowering. The effects of water stress on malic and citric acids were similar to those of tartaric acid. With the increase of treatment time, the content of malic and citric acids decreased significantly under drought stress condition, and water stress promoted the accumulation of malic and citric acids before the veraison stage. Different degrees of water stress could regulate the malic acid anion channel and the activity of malic acid moving protein on the membrane, thus affecting the transport of malic acid. Water stress affected genes related to organic acid synthesis in grape. The expression levels of VvIDH and VvPEPC were higher from 45 to 65 days after flowering, while the contents of tartaric, malic and citric acids were higher at this stage, which was closely related to the accumulation of organic acids in the early stage of veraison. The expression levels of VvCS and VvMDH in grapes were higher from 85 to 105 days after flowering. Overall, T1 treatment could induce the expression of genes related to organic acid synthesis, while T2 treatment inhibited the expression of genes related to organic acid synthesis at the later stage of treatment. [Conclusion] Soil water stress affected fruit quality and organic acid content. Drought stress treatment with 40% to 55% soil water content after color transformation significantly reduced 100 grain weight and titrable acid content, and drought stress treatment with 40% to 55% soil water content at berry maturity significantly reduced organic acid content. Drought stress can improve the berry quality of Beihong grape to some extent, and affect the expression of genes related to organic acid accumulation and synthesis.