The Effects of Light Duration and Intensity with Supplemental Light-Emitting Diode Lights on Grape Photosynthesis, Yield, and Fruit Quality

‘Shine muscat’ grapevines must be cultivated in a protected facility to avoid insects and rain during the rainy season; however, this decreases the light intensity falling on the canopy of grapes. The use of light-emitting diodes (LEDs), with their high efficiency in converting electricity to light, is a useful method to supplement light for plant growth. This study was designed to primarily investigate the effect of the light duration and intensity of supplemental LED lights on grape growth. The photosynthetic and chlorophyll fluorescence measurements of leaves were used to evaluate the performance of photosynthesis. Grape yield and fruit quality were also investigated. Seven different light treatments were utilized to determine the proper light duration and intensity of supplemental LED lights. The results show that the supplemental LED light intensity with the photosynthetic photon flux density (PPFD) of 300 μmol/(m2·s) at 18:00–24:00 showed the highest grape yield, sugar–acid ratio, and economic benefit, with improvement values of 45.1%, 51.4%, and 23.6%, respectively, compared to unsupplemented control vines (CK). The difference between the net photosynthetic rate (Pn), the max net photosynthetic rate (Pmax), and the leaf photosynthetic efficiency (α) between the treatments was negligible. Meanwhile, prolonging the light duration at night was more effective in improving the grape yield and fruit quality than increasing the light intensity in the daytime using supplemental LED lights. The results prove that the supplemental LED lights significantly optimized the light environment and improved grape yield and fruit quality.