Background and Aims: In southern China, where the solar radiation is insufficient for optimised grapevine growth, supplemental light sources are being used. The aim of the present study was to assess the practicability of light-emitting diode (LED) light as a supplemental light source to improve grape photosynthesis and berry composition. Methods and Results: The leaf abaxial surface (Ab) of cv. Summer Black grapevines was illuminated by red, blue and compound LED light strips. In most cases, the net photosynthetic rate (Pn) of the light-treated plants was typically higher than that of the untreated Control (CK) on sunny and rainy days in all stages. Blue light promoted the Pn of the leaf adaxial surface (Ad), whereas red light promoted the Pn of Ab, and the effect of compound light was moderate. The light response curve on the Ad showed that blue and compound light can facilitate the use of high light by the grape leaf. The CO 2 response curve indicated that leaves supplemented with blue light have a significantly advantageous CO 2 assimilation. Grapes supplemented with compound and red light had a berry mass higher than that of the Control, whereas grapes supplemented with blue and compound light had improved berry composition. Conclusions: Blue and compound LED irradiation increased leaf photosynthetic capacity and improved fruit composition, whereas red and compound LED irradiation increased berry mass. Significance of the Study: The supplemental LED lights significantly optimised the light environment and improved photosynthesis and the composition of grape in southern China.

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