Difference in anthocyanin composition between winter and summer grape berries of‘Cabernet Sauvignon'under two-crop-a-year cultivation

Abstract：【Objective】The grape skin colour varies mainly due to the difference in the composition and the content of anthocyanins. Climatic conditions have significant influence on grape ripening and quality. In Nanning, a two-crop-a-year grape culture system is adopted for different wine grape cultivars. However, the differences in composition and content of anthocyanins between winter and summer grape berries are still unclear. The present study analyzed the differences in anthocyanins in the winter and summer grape berries of‘Cabernet Sauvignon' (Vitis vinifera L.) under two-crop-a-year cultivation system.【Methods】The composition and contents of anthocyanins in berry skin were analyzed using HPLC-MS and the physical and chemical indexes were analyzed during developmental stages. Anthocyanin analysis was done on frozen grapes after removing the pedicels. Skins were taken from the frozen berries, grinded into powder and freeze-dried at -40 ℃. Grape skin powder (0.50 g) was immersed in methanol (10 mL) containing 2% formic acid. This extraction was performed with the aid of ultrasound for 10 min, and the mixture was then shaken in the dark at 25 ℃ for 30 min at a rate of 150 r·min^-1. The homogenate was centri-fuged at 8 000× g for 10 min and the supernatant was collected. The residues were re-extracted four times. All the supernatants were pooled, reduced to dryness using a rotary evaporator and then re-dissolved in 10 mL of solvent mixed with 90% mobile phase A and 10% mobile phase B. Solvent A was 2% (V:V) formic and 6% acetonitrile in water, and solvent B was acetonitrile containing 2% formic acid and 44% water. The resulting suspensions were filtered through 0.22 μm filters prior to HPLC-MS analysis.An Agilent 1100 series LC-MSD trap VL was used for anthocyanin detection. A flow rate of l mL·min^-1 at ambient temperature was used. Proportions of solvent B in the mobile phase varied as follows: 1-18 min, 10% to 25%; 18-20 min, 25%; 20-30 min, 25% to 40%; 30-35 min, 40% to 70% and 35-40 min, 70%to 100%. Injection volume was 30 μL, and the detection wavelength was 525 nm. The column temperature was 50 ℃. MS conditions were: electrospray ionisation (ESI) interface, positive ion model, 30 psi nebulizer pressure, 12 mL·min^-1 dry gas flow rate, 300 ℃ dry gas temperature, and scans at m·z-1100-1500. Anthocyanins were quantified at 525 nm as malvidin-3-O-glucoside using calibration curves obtained within a concentration range between 0.5 and 500 mg·L^-1, with linear correlation coefficients greater than0.999.【Results】The results indicated that the fresh weight of the winter grape was smaller than that of the summer grape, but the winter grape had a higher berry skin weight and soluble solid concentration. 17 anthocyanins were detected in winter grape skins at mature stage, and 16 anthocyanins were detected in summer grape skins. The content of total anthocyanins and most of the anthocyanins in the winter grape skins were significantly higher than those in the summer grape skins. However, the summer grape skins contained higher content of malvidin-3-O- (trans-6-O-coumaroyl) -glucoside. The winter grape had higher contents of 3'5'-substituted and 3'-substituted anthocyanins than the summer grape, and the proportion of 3'5'-substituted anthocyanins in the winter grape was higher than that in the summer grape. The contents and proportions of non-acylated and coumaroylated anthocyanins in the winter grape were higher than those in the summer grape, but the proportion of acetylated anthocyanins in the summer grape skins were higher. The content of non-methylated and methylated anthocyanins were higher in the winter grape, but the proportion of methylated anthocyanins were higher in the summer grape. In Nanning, effective accumulative temperature and sunlight hours during the growing season of the winter grape were higher than those during summer grape growing season, but the average temperature of the 3 months before harvesting, rainfall during berry maturation, and hydrothermic coefficient of the 2 months before harvesting in the winter grapes were all lower than those in the summer grapes. From green fruit stage to full mature stage, the daily maximum temperature and minimum temperature displayed decreasing trends for the winter grape, but they showed increasing trends for the summer grape. Daily maximum temperature during the growing season of the winter grape was always lower than 35 ℃, but there were 29 days with daily maximum temperature ≥35 ℃ during summer grape growing season.【Conclusion】The second half of the year has much a longer sunlight time, less extreme high temperatures (≥35 ℃) and drier climatic conditions than the first half of the year in Nanning, where two-crop-in-a-year grape cultivation system is adopted.Thus, the winter grape berries have a higher level of maturity, with higher contents of total anthocyanins and stable anthocyanins than the summer grape. Generally, the climate condition of Nanning in the second half of the year was more favorable for wine grape production.