Effect of different light qualities on photosynthetic characteristics and fruit quality of Shine Muscat grapevines grown under rain-sheltered cultivation

【Objective】Light serves as a pivotal environmental regulator in plant morphogenesis and physiological adaptation. Under grape facility cultivation, particularly with rain-shelter systems, insufficient light frequently constrains photosynthetic efficiency and reproductive development. To address this limitation, spectral-modulated supplementary lighting emerges as a promising agronomic intervention. This study investigated the impacts of varied light qualities spectra on the ecophysiological responses of Shine Muscat (Vitis labrusca ´ V. vinifera), with emphasis on foliar morpho-anatomical adaptations, stem elongation dynamics, photosynthetic performance indices, fruit biochemical profiles and flower bud differentiation. The findings aim to establish a theoretical basis and practical guidance for optimizing supplementary lighting strategies under facility cultivation.【Methods】Five- year- old rain-sheltered Shine Muscat grapevines were used in this study. Four treatments were set: red light (RL), white light (WL), Redblue light 2∶1 (RBL 2∶1) and CK (no- suplementary-light control). LED lamps (36 W, 640 nm) were installed 30 cm above the canopy and operated 12 hours daily (8:00—20:00) from April 11th to September 7th, 2023. Each treatment consisted of 10 vines with three replicates, separated by shading films. Leaf area, basal cane diameter and internode length were measured on 20 randomly selected canes per treatment after two months. Leaf area was calculated using the formula: Area = long axis × short axis × π/4. For chlorophyll fluorescence parameters, leaves were dark-adapted for 20 minutes, and then subjected to 1-second fluorescence induction using 3000 μmol·m- ²·s -1 light. JIP-test analysis was used to calculate variable fluorescence (Vt) and its difference (ΔVt), along with other fluorescence parameters. During the berry expansion stage, photosynthetic parameters were recorded on sunny days between 8:00 and 18:00 using a CIRAS- 3 portable photosynthesis system. Measurements of Pn (net photosynthetic rate), Gs (stomatal conductance), Ci (intercellular CO2 concentration) and Tr (transpiration rate) were taken at six time points (8:00, 10:00, 12:00, 14:00, 16:00 and 18:00). For berry analysis, 20 clusters were sampled for each treatment, and one berry from the top, middle and bottom of each cluster was measured for longitudinal and transverse diameters using a vernier caliper. Total soluble solids (TSS) and titratable acidity (TA) contents were determined using a refractometer, with TA analyzed after 50-fold dilution. Upon completion of the supplemental lighting treatment, dormant buds at physiological maturity were sampled for molecular analysis. qRT- PCR analysis was conducted using SYBR Green chemistry, with VvActin1 (GenBank accession: XM_010655094.2) serving as the internal reference gene for normalization. Gene-specific primers were designed using Primer Premier 5.0 software, with amplification efficiency validated through standard curve analysis. Relative gene expression levels were calculated using the 2-ΔΔCt method. For phenological assessment, the flowering rate was quantified as the ratio of inflorescence-bearing buds to total observed buds during the subsequent spring phenological phase. Bud sampling followed a randomized complete block design, with three biological replicates per treatment.【Results】The results demonstrated that supplemental lighting, particularly the RBL 2∶1 and RL treatments, significantly promoted leaf growth and expansion on grapevines. The 2∶1 treatment also markedly increased internode length at 4th-7th nodes on the new canes. Both the RBL 2∶1 and WL treatments showed significantly higher maximum photochemical efficiency of PSⅡ (Fv/Fm) and maximum fluorescence (Fm) compared to other treatments. The PI abs value with supplemental lighting treatments was significantly higher than that of the CK and shading treatment (ST). The 2∶1 treatment exhibited significantly greater Po, TRO/CSm and ABS/CSm per unit leaf area compared to the control and other groups, whereas energy absorption per reaction center (ABS/RC) showed the opposite trend. In terms of photosynthetic characteristics, the daily variation of Ci, Gs and Tr was optimal with the RLB 2∶1 treatment, followed by the WL treatment, both significantly higher than CK and ST. The RLB 2∶1 group exhibited the fastest berry growth rate, with single berry weight significantly higher than other groups, and all the light treatments improved berry shape. At maturity, the TSS content in berries with the RBL 2∶1 and WL groups reached the highest levels at 19.67% and 19.63%, respectively, while TA content showed no significant differences among treatments. Additionally, supplemental lighting positively influenced bud differentiation in the following year. The RBL 2∶1 treatment notably advanced budbreak timing and increased budbreak rates. Moreover, key genes involved in the photoperiod pathway, like VvGI and VvSOC1, were significantly upregulated with the RBL 2∶1 and WL treatments. 【Conclusion】In summary, supplementary lighting increased the cane growth rate, leaf area, PI abs value and Fv/Fm value of Shine Muscat grapevines, enhancing light use efficiency. Simultaneously, the ber-ry quality, flower bud differentiation capacity and the expression levels of light-responsive genes were also improved. This provides a solid theoretical foundation for the application of supplementary lighting technology under rain-sheltered cultivation conditions.