Effects of exogenous spraying with 5-ALA on photosynthetic characteristics and fruit quality in Kuerlexiangli pear

【Objective】This study aims to explore the regulatory effects of foliar spraying the pear trees with 5-aminolevulinic acid (5-ALA) on the photosynthetic physiological characteristics of leaves and fruit quality of Kuerlexiangli pear, and further screen out the optimal spraying concentration and frequency that can significantly improve its photosynthetic characteristics and fruit quality, so as to provide scientific technical support for the high- quality and efficient cultivation of Kuerlexiangli pear. 【Methods】Taking the main cultivar Kuerlexiangli pear in Xinjiang as the sample, a two-factor randomized block experiment was adopted to apply combined treatments at different concentrations (0 mg·L-1 , 50 mg · L- 1 , 100 mg · L- 1 , 150 mg · L- 1 , 200 mg · L- 1 ) and different frequencies (1, 2, 3 times) during the fruit swell stage. Ten days after the third spraying, the photosynthetic characteristic indexes of leaves were systematically determined. Specifically, A portable chlorophyll meter was employed to determine the relative chlorophyll content in leaves, and the net photosynthetic rate, transpiration rate, intercellular CO2 concentration, and stomatal conductance were determined with a photosynthesis system. Dynamic monitoring was conducted to clarify the influence of 5-ALA on the photosynthetic physiological activi- ties of leaves. After fruit ripening, the external and internal fruit qualities were comprehensively determined. To analyze the comprehensive impact of 5- ALA on fruit quality, external quality parameters (such as single fruit weight, longitudinal diameter, transverse diameter, and fruit shape index) and internal quality indicators (such as fruit firmness, soluble solids content, stone cell content, vitamin C content, soluble sugar content, and titratable acid content) were determined.【Results】The results showed that exogenous spraying with 5- ALA significantly improved the photosynthetic characteristics of leaves. The relative chlorophyll contents showed a significant increase on the 20th and 30th days after treatment, with the T11 treatment achieving an increase of 5.68% . This suggested that this treatment was capable of effectively enhancing the synthesis of chlorophyll in leaves, laying a material foundation for the efficient progress of photosynthesis. The net photosynthetic rate showed the most significant difference on the 10th day after treatment, with the T11 treatment having the most prominent promotion effect (23.25%), followed by the T7 treatment (18.36%). However, the promotion effect of each treatment gradually weakened on the 20th and 30th days, suggesting that spraying with 5-ALA during the fruit swell stage can effectively improve fruit quality but has no long- term effect. The transpiration rate showed significant differences throughout the determination period (10th, 20th, and 30th days). On the 10th day, the T12 treatment had the best effect, with an increase of 24.07%; on the 20th day, the T11 treatment performed best, with an increase of 17.15%; on the 30th day, the T12 treatment still maintained an increase of 15.15%, indicating that 5-ALA can affect the water metabolism and material transport for plants by regulating transpiration. The intercellular CO2 concentration in the T11 treatment was the lowest (300.97 ppm) on the 10th day, and a lower intercellular CO2 concentration implied stronger carbon assimilation capacity. On the 20th day, there was no significant difference among the treatments, but the T11 treatment was relatively lower. On the 30th day, the T1 and T11 treatments decreased by 12.44% and 9.36% compared with the corresponding controls CK1 and CK3, respectively, further indicating that the carbon utilization efficiency by plants under specific treatments was improved. However, there was no significant difference in stomatal conductance among treatments, suggesting that the influence of 5-ALA on the photosynthetic characteristics of leaves during the fruit swell stage was not mainly achieved by regulating stomatal conductance. In terms of fruit quality, exogenous spraying with 5- ALA had a significant impact on multiple quality indexes. Among them, the external quality indexes such as single fruit weight, longitudinal diameter, and transverse diameter were significantly improved. Compared with CK3, the T11 treatment increased by 9.75%, 6.93%, and 6.61%, respectively, indicating that this treatment can effectively promote fruit growth and development and increase fruit size. In terms of internal quality, the contents of soluble solids, vitamin C, and soluble sugar increased by 8.49% , 28.51% , and 21.39% , respectively, while the stone cell content decreased by 18.66% . These changes significantly improved the taste and nutritional value of the fruit. However, 5-ALA had no significant effect on fruit shape index, firmness, or titratable acid, suggesting that 5-ALA may promote the ripening of Kuerlexiangli pear fruits and enable them to be marketed earlier. Finally, through principal component analysis, the T11 treatment (150 mg ·L-1 5-ALA spray for 3 times) was selected as the optimal treatment scheme by comprehensively considering various fruit quality indexes. Correlation analysis results revealed a significant positive correlation between leaf photosynthetic traits and fruit quality, which confirmed that improving leaf photosynthetic characteristics is an important way to enhance fruit quality.【Conclusion】In summary, foliar spraying with 150 mg · L- 1 5-ALA for three times during the fruit swell stage of Kuerlexiangli pear can significantly enhance leaf photosynthetic characteristics and effectively improve the external and internal fruit quality. It is a cultivation technique worthy of popularization and application in production.