Effect of kaolin on the photosynthetic and fluorescence characteristics and fruit quality of wine grapes

In response to the problems caused by frequent high temperatures during the development of wine grapefruits in the northern piedmont of the Tianshan Mountains in Xinjiang, this study used three red wine grapes (Vitis vinifera), namely ‘Cabernet Sauvignon,’ ‘Merlot,’ and ‘Cabernet Franc,’ as test materials. During the fruit-swelling period, the leaves were sprayed with kaolin (6%), with conventional management as a control (CK). By measuring photosynthetic characteristics, fluorescence characteristics, and fruit quality indicators, a comprehensive analysis was conducted to systematically explore the effect of kaolin on wine grapes in the northern foothills of the Tianshan Mountains in Xinjiang. The results showed that after kaolin treatment, the leaf temperature of the three red wine grapes decreased by 8.9%, 8.2%, and 9.3%, respectively, compared with CK, effectively increasing the content of chlorophyll a. ‘Merlot’ increased by 52% compared to CK. After kaolin treatment, the net photosynthetic rates of all three varieties improved. For the JIP-test parameters, V_j was significantly reduced compared with CK, and the quantum yields of electron transport flux (φ_Eo) and PSⅡ final electron acceptor reduction per photon (φ_Ro) were significantly increased. The φ_Eo parameters of the three varieties increased by 58.1%, 93%, and 61.1% compared with CK, and the φ_Ro parameters increased by 87.5%, 33.3%, and 66.6%, respectively. The absorption of light energy per unit reaction center (ABS/RC) and unit area (ABC/CSm) of the three varieties was significantly increased compared with CK, effectively improving the absorption and utilization of light energy per unit reaction center and unit area. The dissipated energy per unit reaction center (DI_O/RC) and unit area (ABC/CSm) and the parameters of surface heat dissipation energy (DI_O/CSm) were significantly reduced compared with CK, which relatively avoided the harm caused by high-temperature stress, In terms of fruit quality, the total flavonoids, flavonoids, and flavanols in the skins of various grape varieties treated with kaolin significantly increased. Compared with CK, the flavonoid content in the skins of ‘Cabernet Sauvignon’ ‘Merlot’ and ‘Cabernet Franc’ increased by 62.3%, 63.9%, and 54.4%, the flavonoid content increased by 79.1%, 69.1%, and 56.7%, and the flavanols content increased by 48.6%, 66.0%, and 27.2%, respectively. In the seeds, the flavonoid content of ‘Cabernet Franc’ increased by 68.2%, and the total flavonoid content of ‘Cabernet Sauvignon’ increased by 67.1%. This study showed that spraying kaolin on the leaves of Xinjiang wine grapes can effectively reduce the leaf temperature of three red wine grapes, ‘Cabernet Sauvignon’ ‘Merlot’ and ‘Cabernet Franc’ in the northern piedmont of the Tianshan Mountains. This treatment reduces the heat dissipation per unit area of leaves, increases the photosynthetic rate, enhances the absorption and utilization of light energy per unit reaction center and unit area of leaves, and effectively mitigates the damage caused by high-temperature stress in summer. Spraying kaolin on the leaves also effectively increased the total acid and some phenolic substances in the fruits of these three wine grapes, improving their fruit quality.