Effects of light quality on Fuji apple fruit quality

Abstract:【Objective】The study explored the effect of light quality on the quality of Fuji apple fruit to found out the best light application scheme.【Methods】Using Yanfu 3 as the material, 96 Fuji apple plants with normal and consistent growth were selected and divided into 8 groups, for treatments with natural sun light (CK), white light (W), blue light (B), red light (R), and ultraviolet light (UVA), and mixed light treatments of BR1 (blue light∶red light=1∶9), BR2 (1∶6), and BR3 (1∶3), and 12 plantbased replicates were set in each group. One protective tree was left between the different treatment groups to eliminate mutual interference between the treatments. From May 20th to mid-to-early November, light supplement was carried out at 05:00—07:00 and 18:00—21:00 every day. The light supplement was directly from above the plant, at the same height, 2.5 meters above the ground. The soluble sugars, titratable acids, soluble solids, vitamin C, fruit hardness, fruit coloration and anthocyanin content of apple fruit were determined, and the expression of five key genes related to color formation, including MdDFR (dihydroflavonol reductase), MdUFGT (flavonoid glycosidyltransferase), MdCHS (chalcone synthetase), MdF3H (flavanone 3- hydroxylase) and transcription factor MYB10, were analyzed.【Results】Different light quality affected the accumulation of soluble sugars in apple fruit. At 30 days, the soluble sugar content in R, UV, BR2 and BR3 treatment groups increased rapidly, and at 40 days, all treatments increased significantly compared with CK, and at 50 days, the accumulation of solu-ble sugars in BR3 treatment group was much higher than that in the other treatments, and soluble sugar content was increased by 7.69%, which indicated that BR3 treatment group had the most significant effect in promoting the accumulation of soluble sugars. In addition, single red light, blue light and white light also had great effect on soluble sugar content of fruits, which increased by 5.51% , 5.10% and 3.76% , respectively. The titratable acid content decreased significantly in all treatment groups at 50 days, and the blue light treatment group had the most significant effect in decreasing titratable acid content, which was decreased by 15.6%. The influence of BR1, BR3 and white light was the second, with decreases of 7.91%, 8.11% and 9.33%, respectively. Titratable acid content in BR2 treatment was reduced by only 1.81% compared to the control, but it was 1.85% higher in red light treatment than that of the control. The soluble solids content in all treatment groups were increased at 30 days compared with 20 days, and increased in all treatments except blue light treatment and ultraviolet light treatment at 40 days. At 50 days, the soluble solids content in red light and BR3 was the highest, which was 1.18 times that of the control group, and in the BR2 treatment it was 1.13 times that of the control group. Compared with the control, the soluble solids content in BR1, UVA, W and B increased by 4.50% , 5.05% , 3.86% and 6.86% , respectively. The results of these experiments showed that different light quality could increase the content of soluble solids in fruit, and red light and BR3 treatments had the most significant effect in increasing the content of soluble solids. At 20 days, vitamin C content in the control group, the red light treatment and BR2 treatment group reached the peak value. At 30 days, vitamin C contents in white, blue, ultraviolet, BR1 and BR3 treatment groups reached the highest value. At 40 days, vitamin C content in each treatment group decreased. At 50 days, vitamin C was the highest in the red light treatment group, which increased by 28.35% compared with the control group, followed by the purple light treatment, which increased by 18.53%. Compared with the control group, Vc content in BR1, BR2 and BR3 increased by 10.60%, 4.24% and 10.49%, respectively. White and blue light treatment had the least effect on vitamin C content, increasing by only 1.11% and 1.56%. The hardness of fruit was the highest when the light was supplied for 20 days and higher in all light treatments than that of the control group. As the ripeness of the fruit gradually increased, the hardness of the fruit gradually decreased. At 50 day, BR1, BR2 and BR3 had a significantly higher hardness, which increased by 14.83%, 15.60% and 16.37% compared with the control group, respectively. Red light and blue light increased fruit hardness by 11.76% and 8.95%, respectively. The content of anthocyanins in the peel of each treatment group increased rapidly, and the increase was the most significant at 40 days. At 50 days, the content of anthocyanins in the BR3 treatment group was the highest, which was 1.6 times that of the control group. BR1 and BR2 had only 1.29 times that of the control group. Supplementation of single red light promoted the accumulation of anthocyanins in the peel, which was only 1.13 times that of the control. The addition of white light and blue light did not have a significant effect in the increase of anthocyanin content in the peel. The expression of key genes related to color formation, MdDFR, MdUFGT, MdCHS, MdF3H and MdMYB10 were all regulated, and their expression in BR3 treatment group was the most significantly up-regulated, being 4.17, 1.94, 5.23, 6.71 and 5.03 times that of the control group, respectively.【Conclusion】Supplementation of different light quality can increase the content of soluble solids in fruit, and the combination of red light and red blue (3∶1) has the best effect. Red light treatment can increase vitamin C, soluble sugars and titratable acids in the fruit; blue light treatment is effective to reduce the content of titratable acid in the fruit. The combined treatment with red and blue (3∶ 1) light is most effective in increasing the content of anthocyanins in the peel and the expression of genes related to color formation. The treatment also increases the content of vitamin C, soluble sugarsand soluble solids in the fruit, reduces the content of titratable acid, and thus has the best effect in improving the quality of the fruit.