Effects of transplanting at the seedling emergence stage on the growth and cost-benefit of Poncirus trifoliata 2085

【Objective】The citrus industry, a vital component of global horticulture, faces persistent challenges in rootstock production. Poncirus trifoliata, the most widely utilized citrus rootstock, conventionally undergoes high- density sowing in autumn, with transplanting delayed until the following spring. This prolonged high-density growth phase often leads to suboptimal seedling development, particularly inadequate stem diameter, which reduces grafting success. This study aimed to evaluate an innovative cultivation protocol—transplanting at the seedling emergence stage—as a viable alternative. The primary objective was to conduct a holistic comparison against the conventional method, evaluate its effects on morphological development, physiological vitality, and economic viability, to establish an evidence-based protocol for producing high-quality rootstocks.【Methods】The control group (CK) followed standard nursery practices: seeds were sown in beds filled with a commercial nursery substrate. For the experimental treatment, seeds were sown in seedling trays filled with vermiculite. The key dif-ference between treatments was the timing of transplanting seedlings into their final nursery pots. CK seedlings underwent this process 240 days after sowing, in late March to early April, which aligns with common commercial production practices. In contrast, the treated seedlings were transplanted immediately after emergence, once they reached approximately 3 cm in height, and this entire process was completed within a few weeks of sowing in August. Both groups were subsequently maintained under identical, optimal growth conditions, including following a standardized fertigation schedule. A comprehensive dataset was collected periodically until 390 days after sowing. This dataset included quantitative metrics for emergence, growth (height, stem diameter and branching), leaf development (number, size and area), and root system architecture, all of which were analyzed through digital scanning. We assessed plant vitality by measuring biomass partitioning (the fresh and dry weights of shoots and roots), as well as key physiological indicators such as root activity and in situ chlorophyll fluorescence (ΦPSⅡ, Fv'/Fm', NPQ). A detailed activity-based cost analysis was performed, considering material inputs (substrate and vermiculite) and labor time for sowing and transplanting operations, with all data normalized to the cost of producing 10 000 viable seedlings.【Results】This treatment method demonstrated clear superiority across multiple dimensions, with significant improvements observed during the seedling emergence phase. The treatment group initiated emergence 1-2 days earlier and achieved a final stand establishment rate of 76.75%, which was substantially higher than the control's 63.25%. This suggests that the vermiculite medium would provide a more favorable germination microenvironment. Morphologically, the advantages were profound and sustained. From the initial measurement at 90 days until the study concluded at 390 days, treatment seedlings consistently exhibited greater plant height, stem diameter (critical for grafting at both 1.5 cm and 10 cm heights), and branch number. This consistent lead indicated a powerful early start that was maintained over time. Leaf development followed suit, with the treatment producing significantly more, larger leaves. This is evidenced by the significantly greater leaf width and area measured in mature functional leaves at 390 days. The treatment group showed an early advantage in terms of root growth, with significantly greater total root length, surface area, and number of root tips at 90 and 240 days. Although the CK seedlings exhibited compensatory root growth after delayed transplanting, which narrowed the gap by 390 days, the initial advantage of the treatment group in root architecture likely contributed to its sustained aboveground vigor. Biomass accumulation mirrored these trends, with the treatment producing significantly more total biomass (in terms of both fresh and dry weight) at the early and mid-growth stages. By the end of the trial, the treatment group maintained a significant advantage in aboveground fresh mass, while other biomass metrics showed a positive, though not statistically significant, trend. Physiological assessments provided further evidence of enhanced vitality. Root activity was higher in the treatment group. More importantly, chlorophyll fluorescence kinetics measured during the peak photoperiod (09:00—11:00 am) revealed that treated seedlings operated with higher photochemical efficiency (ΦPSⅡ, Fv'/Fm') while dissipating less excess energy as heat (lower NPQ). This indicates a more efficient, less stressed photosynthetic apparatus that is capable of utilizing light more effectively for growth, rather than for photoprotection. Our economic analysis provided a compelling case for adoption. Using vermiculite for sowing and transplanting tiny, easyto- handle seedlings dramatically simplified the process and resulted in substantial labor savings. The largest cost savings came from the transplanting operation itself: handling young seedlings reduced labor time by 37.85% (saving 26.8 seconds per plant). Overall, it was calculated that producing 10 000 seedlings with the tested treatment cost 18 780 yuan, compared with 21 550 yuan for the CK, representing a direct cost saving of 2770 yuan.【Conclusion】This study provides conclusive evidence that trans-planting Poncirus trifoliata at the seedling emergence stage is a far more effective cultivation strategy than the conventional delayed transplanting method. By circumventing the debilitating phase of highdensity competition early in the life cycle, the treatment protocol promotes the development of superior morphology, enhanced physiology and greater economic viability in the rootstocks. The resulting seedlings have thicker stems, a critical quality trait for grafting, and exhibit enhanced overall vitality. Combined with a significant reduction in production costs, this method makes a compelling case for transforming citrus nursery practices.