Selection of light response models and comparative photosynthetic parameters of Malus Jinxiu under different fertilization treatments

【Objective】Malus Jinxiu is a key component of the characteristic forestry and fruit industry in Xing’an League, with considerable economic and ecological value. Due to its rich nutritional profile and unique medicinal properties, it is well-received by consumers and highly favored in the market. In addition to contributing to local income generation, the cultivation of M. Jinxiu also plays a vital role in maintaining regional biodiversity and promoting sustainable land use. However, despite the growing demand for M. Jinxiu, studies on its light response mechanisms remain relatively limited, particularly regarding the effects of different fertilization treatments on its photosynthetic response and overall physiological performance. Understanding how varying nutrient inputs influence photosynthetic efficiency is essential for improving yield quality and ensuring optimal growth conditions. This study, therefore, aimed to identify the most suitable light response curve model for M. Jinxiu and to conduct a comparative analysis of its photosynthetic parameters under various fertilization treatments. By evaluating photosynthetic gas exchange characteristics and model fitting accuracy, the study sought to provide a scientific basis for precise nutrient management. The results would offer theoretical guidance and data support for optimizing cultivation strategies, enhancing plant productivity, and improving resource use efficiency in the context of ecological agriculture.【Methods】The experimental materials consisted of sixyear-old dwarf M. Jinxiu trees grafted onto Malus baccata (L.) Borkh. rootstocks with GM256 as the interstock, grown in Group 10 of Ping’an Village, Horqin Right Front Banner, Xing’an League. And the trees had undergone three years of cultivation and acclimatization since their introduction, making them well-suited for studying long-term physiological responses under field conditions. To simulate realistic orchard management, a randomized block design was employed based on local fertilization practices. Four fertilization treatments were applied: T1 (0.67 kg Shilomei macronutrient water-soluble fertilizer + 2.66 kg organic water- soluble fertilizer for root protection and fruit expansion), T2 (1 kg + 4 kg), T3 (1.33 kg + 5.34 kg), and a control group (CK) that received no fertilization. Each treatment was replicated to ensure statistical reliability. A Li-6800 portable photosynthesis system was used to measure the diurnal variation in net photosynthetic rate and the photosynthetic light response curves under each fertilization regime. Key photosynthetic gas exchange parameters, including net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and intercellular CO2 concentration (Ci), were recorded and analyzed. To better understand how different fertilization levels affect photosynthetic performance, the light response data were fitted using four mathematical models: the rectangular hyperbola, non-rectangular hyperbola, exponential, and modified rectangular hyperbola. The performance of each model was evaluated based on fitting accuracy indicators such as the coefficient of determination (R2 ), root mean square error (RMSE), and mean absolute error (MAE). Additionally, the accuracy of fitted photosynthetic parameters—such as maximum net photosynthetic rate (Pn max), light saturation point (LSP), dark respiration rate (Rd), light compensation point (LCP), and apparent quantum efficiency (α)—was compared across models and treatments. This comprehensive analysis intended to provide insights into the photosynthetic characteristics and light response adaptability of M. Jinxiu under varying nutrient conditions, offering a scientific basis for the refinement of fertilization strategies and sustainable orchard management practices.【Results】The diurnal variation of net photosynthetic rate (Pn) in M. Jinxiu under all fertilization treatments exhibited a parabolic trend, characterized by a rapid increase from 8:00 to 10:00, a gradual decline from 10:00 to 16:00, and a sharp drop after 16:00. A certain degree of synchronization was observed in the diurnal patterns across different fertilization treatments. The photosynthetic gas exchange parameters of the fertilized treatments displayed a bimodal curve pattern, whereas the control treatment (CK) showed a distinct unimodal curve. Increasing fertilization levels significantly enhanced the net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs), with the T3 treatment showing the most pronounced improvements in all three parameters. After fitting the light response curves of the four fertilization treatments, it was found that all treatments had coefficients of determination (R2 ) greater than 0.99. Among the models tested, the non-rectangular hyperbola model achieved the highest fitting accuracy, while the rectangular hyperbola model had the lowest. The modified rectangular hyperbola model demonstrated superior parameter fitting for dark respiration rate (Rd), light compensation point (LCP), and maximum net photosynthetic rate (Pn max), with root mean square error (RMSE) and mean absolute error (MAE) values closest to zero. The T3 treatment exhibited the highest light saturation point (LSP), reaching 1 884.874 2 μmol · m- 2 · s - 1 , and the largest Pn max at 16.686 8 μmol·m-2 ·s -1 . With increasing fertilization levels, key photosynthetic parameters such as initial quantum efficiency (α), Pn max, LSP, and Rd increased progressively, while LCP decreased. These results suggest that increased fertilization would enhance light use efficiency and reveal the response mechanism of M. Jinxiu photosynthetic parameters to fertilization, providing theoretical support for understanding its ecological adaptability.【Conclusion】Based on the fitting performance of the four light response models, the modified rectangular hyperbola model demonstrated the best overall fit. Among the fertilization treatments, T3-consisting of 1.33 kg of Shilomei macronutrient water-soluble fertilizer and 5.34 kg of organic water-soluble fertilizer for root protection, root development, and fruit expansion resulted in superior photosynthetic parameters compared with other treatments, indicating that it would be a relatively optimal fertilization scheme. However, the results suggest that this treatment still would fall short of meeting the optimal nutritional requirements of M. Jinxiu. Therefore, future studies should refine the fertilization gradient to further assess its impact on plant physiological growth.