Abstract:【Objective】Citrus is the predominant fruit crop in China, with a plantation area of 2,923,000 hm2 and yielding 55,956,000 tons annually as of 2021. The breeding cycle of citrus virus-free container seedlings spans two years, from the initial rootstock sowing to grafting and subsequent nursery cultivation. The protracted breeding cycle leads to a disconnect between citrus seedling supply and demand, resulting in sporadic instances of inadequate availability. Therefore, the citrus seedlings are insufficient to meet the demand of updating and iteration of citrus orchards in China. The extended breeding period also escalates the production costs of citrus virus-free container seedlings, impeding its widespread adoption. Furthermore, limited light exposure is observed during the early spring and autumn/winter seasons in most citrus producing regions. The use of greenhouses for seedling production exacerbates the light deprivation experienced by citrus rootstocks. The implementation of supplemental lighting stands out as a viable strategy to improve the light condition, which may be a key initiative to accelerate the citrus rootstock breeding cycle and promote resilient citrus rootstocks. The mechanism by which the mixed red and blue light spectrum modulates the photosynthesis and photomorphogenesis of Poncirus trifoliata seedlings remains unclear. The objective of this study was to determine the optimal supplemental light conditions to improve the growth and development of Poncirus trifoliata seedlings. These results will provide a theoretical insight for accelerating the cycle of citrus virus-free container seedling breeding and improving seedling quality.【Methods】Poncirus trifoliata seedlings obtained from Dongkou of Hunan province were used as the experimental material. Five light-supplementing treatments were designed, including RB4:1 (red to blue light ratio of 4:1), RB6:1, RB8:1, RB1:2, and no light supplementation (CK). Seedlings with similar growth potential were selected and transplanted in 14 rows, each containing 7 plants, thereby yielding a total of 98 plants for each treatment. The supplementary light was positioned directly above the Poncirus trifoliata seedlings, and its intensity was maintained at 150 μmol·m-2·s-1 by adjusting its distance from the average height of the seedlings. The seeds were sown in the seedbed on August 12, 2021, and then it transplanted into nutrient bags on September 20, 2021. Supplemental light was applied on November 12, 2021, with a cycle of 6:00 AM to 10:00 PM, and ended on June 12, 2022. The supplemental light cycle was completed on June 12, 2022. Plant height and stem thickness were measured monthly between December 2021 and June 2022. SPAD and Chromatic Aberration a* values were measured in February, April, May, and June 2022. Anthocyanin content was measured monthly from March to June 2022. In June 2022, measurements were taken for leaf length, leaf width, leaf area, total root length, average root diameter, root volume, root surface area, number of root tips, LAI value, fresh weight, dry weight, dry weight of underground part, dry weight of above ground part, root-crown ratio, photosynthetic parameters, chlorophyll fluorescence parameters, soluble protein content, soluble sugar content, superoxide dismutase (SOD) activity, peroxidase (POD) activity, catalase (CAT) activity, and malondialdehyde (MDA) content. 【Results】After 210 days of supplemental light treatment, all four treatments resulted in higher plant height, stem thickness, total root length, average root diameter, number of root tips, and dry matter accumulation as compared to the control. The root-crown ratio exhibited a slight decrease. Leaf length and width displayed a declining trend, however, the aspect ratio of the leaf remained unchanged. The number of branches increased whereas the branch height decreased. The photosynthetic parameters were enhanced with RB6 demonstrating the highest stomatal conductance and net photosynthetic rate. The MDA content of RB6:1 was significantly lower than that of other treatments. Additionally, the soluble protein content, soluble sugar content and the antioxidant enzyme activities (SOD, POD, and CAT) of RB6:1 were significantly higher than those of the control, which may indicate the plans with strong resistance. After 120 days of supplemental light treatment, the anthocyanin contents of RB4:1, RB6:1, RB8:1, RB1:2 were significantly higher than that of the control in leaves. Between 90 and 210 days of the supplemental light treatment, the SPAD values of the four treatments as a whole gradually increased, while their Chromatic Aberration a* and anthocyanin contents gradually decreased. This study focused on the stem thickness of the seedlings. RB8:1 showed the fastest growth between 30 and 120 days of supplemental light treatment. In addition, RB6:1 exhibited the highest stem thickness between 120 and 210 days of treatment, reaching 2.46 mm. At the end of 210 days of treatment, RB6:1 exhibited the greatest stem thickness at 5.03 mm. 【Conclusion】In the Changsha region, optimal growth and maximum stem diameter growth of Poncirus trifoliata seedlings can be achieved by using RB8:1 and RB6:1 light supplementation from early November to early March and from early March to early June, respectively.
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