Effects of UV-B radiation on the photosystem performance and fruit quality of strawberry in greenhouse

[Objective] The study aims to investigate the impacts of supplemental UV-B radiation on photosynthetic efficiency and fruit quality of strawberries under low UV conditions of facility environment, and to provide reference for enhancing the quality and productivity of protected strawberry. The facility mainly regulates the temperature to ensure the normal flowering and fruiting of strawberries. It is difficult to change the poor winter lighting conditions, and due to the influence of the greenhouse film, some of the light intake inside the facility is blocked, resulting in a significant decrease in sugar, flavor, and color of the fruit under facility cultivation. Improving the photosynthetic performance of strawberry leaves, assimilates synthesis and sugar accumulation in fruit through improved light quality regulation facilities may become a possibility. [Methods] In this study, Xuelixiang strawberries cultivated in a solar greenhouse were used as test materials. During a strawberry fruit development cycle UV-B irradiation was supplemented daily (08:30—10:00) at dose gradients of 0.2, 0.4, 0.6, and 0.8 W·m-2 for 1.5 hours, denoted as T1, T2, T3, and T4, respectively. Plants without UV-B supplementation were used as controls, denoted as CK. Relevant environmental indicators inside and outside the greenhouse, physiological indicators of leaf resistance, leaf photosystem performance parameters, fruit quality, and key growth and development indicators were measured. [Results] The daily average intensity of UV-B in the greenhouse during the experiment was about 52% of that in the open air, with a difference of 0.55-1.13 W·m-2 compared to the open air. The difference was the largest before and after noon. Among the resistance related indicators, the levels of malondialdehyde and proline in T4 significantly increased compared to CK, while the other treatments showed no significant changes. The soluble protein content in T2-T4 increased significantly compared to CK. There was no significant difference in membrane leakage between T4 and CK, indicating that all the UV-B treatments did not cause significant damage to the leaves. During the period of UV-B supplement, Δ VJ and Δ VK on the OJIP curve of the leaves were lower than 0, indicating that UV-B radiation accelerated the electron transfer efficiency on the receptor side of PSⅡ reaction center, but during the relatively high temperature period at noon (12:30—14:30), the OJIP curve of T4 underwent severe deformation, with Δ VK and Δ VJ values rising above the benchmark zero line first, followed by T3, T1, and T2 gradually increasing. Among the treatments, T4 had the largest increase and T2 the smallest, indicating that PSⅡ had been suppressed by light, with the greatest degree of suppression in T4 and the smallest suppression in T2. In the afternoon (14:30—16:30), as the temperature decreased, T1, T2, and T3 could recover quickly. At 16:30, they dropped below the zero line, while T4 required a longer time to recover. This also indicates that the UV-B effect is short-term. The actual photochemical quantum yield of PSⅡ in T1, T2, and T3, as well as the effective photochemical efficiency Fv'/Fm', photochemical quenching coefficient qP, and non photochemical quenching coefficient qN of PSⅡ, were significantly higher than those in CK in most periods. Low dose radiation increased Fv/Fm (maximum photochemical efficiency of PSⅡ) and Δ I/Io (PSI activity), indicating that low-dose UV-B radiation effectively improved the light harvesting ability, light energy conversion efficiency, and efficiency of PSⅡ reaction centers. The apparent electron transfers rate (ETR) was significantly lower than that of CK, while the decrease in T3 and T4 was significantly greater than that of T1 and T2, indicating that the higher the UV-B dose, the more electrons were allocated to other pathways. The contents of chloroplast pigments such as chlorophyll a, chlorophyll b, and carotenoids showed a trend of first increase and then decrease with the increase of UV-B dose, and T2 had the highest chlorophyll contents. As the UV-B dose increased, net photosynthetic rate, stomatal conductance, and transpiration rate all first increased and then decreased, while the intercellular carbon dioxide concentration showed an opposite trend. There were significant differences between T1-T3 and CK, indicating that low to moderate doses of UV-B radiation are beneficial for improving leaf photosynthetic capacity. The fresh mass of aboveground parts in T1 and T2 increased by 41.4% and 46.35% compared to CK (the same below), dry mass by 19.4% and 15.28%, fruit mass by 11.23% and 24.9%, soluble solids by 11.85% and 25.7%, and soluble sugars by 23.57% and 64.96%, respectively. The effect of high doses was not significant, indicating that low-dose UV-B radiation can increase the accumulation and optimize the distribution of plant dry matter, which is beneficial for improving fruit quality. [Conclusion] Under the experimental conditions, the efficiency, photosynthetic performance, and fruit quality were improved by UB-V at 0.2 W·m-2 (T1), 0.4 W·m-2 (T2), and 0.6 W·m-2 (T3). The overall effect was best at 0.4 W·m-2 (T2), and no visible damage was observed in all the treatments.