Colored Shade Nets can Relieve Abnormal Fruit Softening and Premature Leaf Senescence of “Jumeigui” Grapes during Ripening Under Greenhouse Conditions

When grapes reach maturity, they usually experience extremely high-temperature periods in southern China, causing premature leaf senescence, abnormal fruit softening, and fruiting period shortening. Their quality and production efficiency are also severely affected. ‘Jumeigui’ grapes were examined in terms of fruit quality and leaf senescence under shading treatments; green, blue, black, and gray aluminum foil nets were used for shading, and their spectra were measured. At the same density, shade net color significantly affected cooling and shading efficiencies, and gray net had the best light transmission and cooling effect. Shading treatment significantly alleviated abnormal grape softness during hot periods. Total soluble solids (TSS) content and grape coloration were affected under gray, blue, and green shade nets. TSS exceeded 18% under gray, blue, and green nets, meeting the requirements of first-class high-quality fruit. However, peel coloration was not notably affected under gray and blue shade nets, while the non-shading treatment produced clear heat-stress damage, especially on the edges of old leaves. The net photosynthetic rate of the bottom five old leaves under the non-shading treatment was significantly lower than that under the shading treatment, indicating that high light and heat caused premature leaf senescence. In summary, colored shade nets can reduce the temperature and light in the greenhouse, while alleviating premature senescence of perennial grape plants. However, the quality of the grapes treated using black shade nets was poor; superior quality was achieved using gray and blue shade nets. These results can be applied in future cultivation facilities during high-temperature periods.