Effects of supplementary light on ripening and sugar accumulation of cherry under protected cultivation

Abstract: 【Objective】Because of the subtropical monsoon climate in most areas of southern China, the protected cultivation is widely applied in cherry production to reduce the impact of the large precipitation in spring. There are still problems such as weak light intensity, short light duration and uneven light distribution in protected cultivation environment. Therefore, improving the light environment of facility cultivation through artificial light supplement technology has important practical significance for regulating the growth and development and improving yield and quality of cherry. In this study, different supplementary light measures were compared and the best scheme for protected cultivation of cherry was selected.【Methods】The Chinese cherry cultivars Zhujiduanbing and Heizhenzhu were used as materials in this study. Four different types of light sources were used: LED lamp (36 W, RB 6︰1), LED lamp (50 W, RB 6︰1), commodity plant growth lamp (12 W, RB 2︰1, G101) and incandescent lamp (36 W), with no artificial light as the control (CK). All the groups were treated with mulching the ground with reflective film. The distance between the lights was 3 m, and the height from the top of the tree was 50 cm. Time of light supplement was set to 05:00—10:00, 16:00—19:30 on sunny days and 5:00—19:30 on rainy days. The contents of endogenous hormones GAS, IAA and ABA, the percentage of soluble solids, and the enzyme activities of peroxidase (POD), sucrose synthase (SS) and sucrose phosphate synthase (SPS) in cherry flesh were measured from the color transition stage to the harvest stage. The expression levels of genes related to ripening, softening, and sugar synthesis in fruits were also measured in the harvest stage.【Results】Each supplementary light treatment caused the dynamic change of endogenous hormones in the fruit earlier than control, and promoted the fruit development and maturation process of the two varieties. The comprehensive effect of LED light treatment was the best, resulting in thechanging process of endogenous hormones GAS and ABA in the fruit earlier than control. Supplementary light treatments also increased the TSS contents of two cherry cultivars at harvest time. LED light treatments had the most significant effect on improving TSS content, which made the TSS of Zhujiduanbing fruit reach 17.63%, and made the TSS of Heizhenzhu fruit reach 14.67%. The increase of sucrose synthase activity in the fruit of each supplementary light treatment was earlier than control, thus promoting the accumulation of sugar in the fruit. Among them, LED supplementary light treatments had the most obvious promotion effect. LED 36 W treatment made the sucrose synthase vitality of Zhujiduanbing fruit reach a peak 3 days before harvest, which was 69.17% higher than the control during the same period, and increased the sucrose synthase activity of Heizhenzhu fruit by 18.75% compared to control during harvest. Overall, LED 36 W treatment had the best effect, and gene expression levels were measured for this treatment. It was found that the expression of the ABA synthase gene NCED1 was significantly higher with LED 36 W treatment than that with control, which regulated the levels of endogenous hormones related to fruit ripening at the molecular level. The relative expression levels of polygalacturonase gene PG1 and xylanase gene XYL1, which can regulate cell wall degradation, were also significantly higher with this treatment than those with the control, indicating that this treatment could promote fruit softening by positively regulating the process of cell wall degradation. The determination of anthocyanin synthesis pathway genes PAL, CHS, ANS and transcription factor MYB10 in the pericarp during harvest showed that the expression levels of CHS, ANS and MYB10 in Zhujiduanbing pericarp treated with LED 36 W were significantly higher than the control. And in Heizhenzhu pericarp, the expression levels of all four genes with LED 36 W treatment were significantly higher than those with the control. LED 36 W treatment activated the synthesis of anthocyanins in cherry pericarp by activating high-level expression of synthesis pathway genes and transcription factors at the molecular level, thereby promoting the coloring of cherry pericarp. The relative expression levels of sucrose synthase genes SS1, SS6 and sucrose phosphate synthase gene SPSA1 in fruit during harvest were also analyzed. The results showed that with the two cherry cultivars, LED 36 W treatment significantly increased the expression level of SS1 gene in fruit compared to the control, indicating that SS1 played a major role in responding to supplementary light treatment. LED 36 W treatment positively regulated the synthesis of sucrose synthase by activating the expression of SS1, thereby promoting sugar accumulation in cherry fruits through sucrose synthase.【Conclusion】All supplementary light treatments promoted the dynamic changes of endogenous hormones during cherry fruit ripening, and increased the level of soluble solids content in the fruit. Among them, LED (36 W, RB 6∶1) supplementary light treatment could advance the endogenous hormone change process of two Chinese cherry cultivars by 3 to 9 days, increase the activity of sucrose synthase in the fruit by more than 18.75% compared to the control during the same period, and significantly improve the expression of genes related to fruit ripening and softening, sugar synthesis, and anthocyanin synthesis at the molecular level, which had the best effect on promoting cherry ripening and improving fruit quality. It is worthy of application and promotion in cherry practical production.