Effects of foliar nitrogen regulation on the quality of wine grape 'Cabernet Sauvignon' during veraison

The integrated light and simplified cultivation of water and fertilizer has been widely applied at the eastern foothills of Helan Mountain in western China. However, it is a high demand for the nitrogen nutrition of wine grapes at the current stage. This study aims to demonstrate the regulation of the foliar nitrogen on wine the grape 'Cabernet Sauvignon' during the veraison period, with the location in the Lilan Winery, Yongning County, Ningxia Hui Autonomous Region, China (38°28′N, 105°97′E). The test material was 8-year-old wine grape 'Cabernet Sauvignon'. A single-factor randomized block design was utilized with the number of blocks equal to the number of replicates. A total of six subjects were chosen for the experiment, including ammonium sulfate, calcium ammonium nitrate, urea, phenylalanine, glutamic acid, and control (water), which were reused three times each. Among them, 20 vines were used per replicate subject and 60 vines per treatment for a total of 18 plots. The amount of nitrogen fertilizer in each treatment was converted to 1.5‰ urea and other quality pure nitrogen. The foliage was firstly sprayed with nitrogen fertilizer three times (July 15, July 31, and August 13) during the veraison period. The photosynthetic indicators of wine grapes were then measured after ten days (August 22), including the net photosynthetic rate, transpiration rate, intercellular CO2 concentration, stomatal conductance, leaf area, water use efficiency, and chlorophyll. Once the wine grapes were ripe (September 23), the morphology and yield were determined, including the particle size, spike length, 100-grain mass, and yield plant. At the same time, the quality indicators of wine grapes were also determined in this case, including the soluble solids, titratable acids, tannins, anthocyanins, total phenols, and yeast assimilable nitrogen. A variance analysis was carried out on the measured photosynthetic, morphological, yield, and quality indicators, in order to evaluate the different treatments, photosynthesis, and quality indicators of wine grapes. Three principal components were then obtained to establish a comprehensive evaluation function. The scores were calculated and sorted for the optimal treatment. The result indicated that all five treatments improved the physiology and quality of wine grapes, compared with the control group. The best performance (up to 29.21%) was achieved by spraying ammonium sulfate on the soluble solids in the berries. Furthermore, the greatest synergy was also obtained in the chlorophyll and yeast assimilable nitrogen under the calcium ammonium nitrate, which increased by 20.22% and 41.95%, respectively, compared with the control. The best effect was obtained under the urea treatment for the net photosynthetic rate, particle size, 100-grain mass, titratable acid, and anthocyanin. Phenylalanine presented the best effect on the leaf area, stomatal conductance, tannins, and total phenols. Glutamic acid improved the morphology and yield of wine grapes. Specifically, the spike length and yield plant significantly increased by 13.61% and 12.66%, respectively, compared with the control group. Three principal components contributed 59.52%, 22.13% and 11.77%, respectively, compared with the nitrogen fertilizer treatment. Phenylalanine and urea gained the highest scores for ten indicators, such as the wine grape physiology and quality. The titratable acid was best treated with urea, indicating the best acid value of 0.68% in the production area. Moreover, the anthocyanin content was as high as 2.28 mg/g under the urea treatment, which was higher than the rest of nitrogen treatments. There was the best effect of Phenylalanine on the increasing tannin content, up to 19.88 mg/g, which was 5.63%-24.87% higher than that of the rest nitrogen treatments. By contrast, the total phenolic content was as high as 19.56 mg/g under phenylalanine treatment, which was 8.91%-27.34% higher than the others. In conclusion, foliar spraying phenylalanine and urea during the veraison stage can be expected to improve the physiological characteristics of wine grapes and the quality of berries. The traditional cultivation techniques can also be optimized to promote low nitrogen application efficiency. The finding can provide a strong reference for the wine grape production in mountain areas.