Abstract: 【Objective】 In areas with gentle terrain at medium-to-low altitude is more suitable for kiwifruit industrial development than in mountainous regions. However, these areas often experience prolonged periods of high summer temperatures, resulting in an increase in the rate of fallen leaves and sunburned fruit, and a decrease in fruit quality. Fruit bagging and setting up shading nets above trees can reduce the rates of fallen leaves and sunburnt fruits caused by high temperature in summer. Exogenous 5-aminolevulinic acid (ALA) can improve the photosynthetic capacity, fruit quality, yield and stress resistance of plants. The present experiment was undertaken to explore the effects of exogenous ALA on leaf physiological characteristics and fruit quality of different kiwifruit cultivars. 【Methods】 The experiment was conducted in the Kiwi Resource Garden of Horticultural Research Institute of Hunan Academy of Agricultural Sciences at an altitude of 46 meters. The plants of three Actinidia chinensis cultivars (Chuhong, Fengyue, and Fengshuo) and two strains (S1 , an Actinidia chinensis selection from progeny of Cuiyu seedlings, and S2, an Actinidia deliciosa selection from progeny of Miliang No. 1 seedlings) were used as experimental materials with the age of 6 a to 7 a. Exogenous ALA was sprayed 3 times from early June to early August at the concentration of 10 mg·L-1, and clean water was sprayed as a control (CK). The leaves of Fengyue, Fengshuo and Chuhong with different treatments were picked on August 10, and the chlorophyll content and antioxidant enzyme activity were determined. The fruits of each cultivar were collected when the soluble solid content (SSC) was up to 7.5%, and individual fruit sizes and weights were measured. After ripening, the soluble solid content (SSC) and the dry matter content of fruits of all cultivars were determined. 【Results】 Exogenous ALA significantly increased the content of chlorophyll a, chlorophyll b and carotenoid in Chuhong and Fengshuo leaves. The three indicators in Chuhong leaves were increased the most by 55.98%, 53.57%, and 30.51% respectively, compared to the control. Those in Fengshuo leaves were increased by 20.61%, 17.38%, and 15.07% respectively. While the difference between the treatment and the control of Fengyue was not significant. The soluble sugar content in leaves of Chuhong was significantly higher than that of the control, with an increase of 11.20%. Compared with the controls, the SS in leaves of Fengyue and Fengshuo did not show significant differences. The POD activities in leaves of the three cultivars were higher than those of the controls, and the differences of Fengyue and Fengshuo were extremely significant (p < 0.01) compared to the controls. There was no significant difference in POD activity in Chuhong leaves compared with the control. The SOD activities in the leaves of Chuhong, Fengyue, and Fengshuo were significantly different from those of the controls, increasing by 8.52%, 8.26%, and 13.06% respectively. And the MDA content in the leaves of each cultivar was extremely reduced compared to the controls, decreasing by 16.06%, 10.03%, and 24.24% respectively. The heat tolerance of these three cultivars was in the order of Fengshuo > Chuhong > Fengyue. Additionally, the effects of ALA treatment on POD activity, SOD activity, and MDA content in Fengshuo leaves were the most significant. The results showed that exogenous ALA had more significant effect on improving the key indicators of heat resistance in cultivars with stronger heat tolerance compared to those with weaker heat tolerance. The average single fruit weight of each cultivar was heavier than that of the control with S1 showing the most significant increase at 15.16%. The average fruit weight of each of the other four cultivars belonging to the large-sized fruit type differed significantly from that of the control, except for Chuhong, which belonged to the small-sized fruit type. The results also showed that the effect of exogenous ALA on increasing the average single fruit weight of cultivars with bigger fruit was more significant than that with smaller fruit. The fruit shape index increased for the four cultivars compared to the control, whereas it decreased for Chuhong. The fruit shape indices of S1 and S2 were significantly different from the controls. The SSC of Chuhong fruit was above 17.0%, classifying it as a high-sugar cultivar, while the SSC of the other four cultivars fruits, Fengyue, Fengshuo, S1 and S2, ranged from 13.5% to 15.5%, classifying them as low-sugar cultivars. All cultivars showed significant increases in SSC and dry matter content of the fruit, with S2 showing the largest rate of increase, with a 9.49% increase in SSC and a 14.68% increase in dry matter content. The increase rate of SSC of Fengshuo fruit was lower than that of the others, only increased by 3.90%. And the increase rate of dry matter content of Chuhong fruit was lower than that of the others, with an increase of only 3.53%. The increase rate of SSC of each cultivar was in the order of S2 > S1 > Fengyue > Chuhong > Fengshuo, and the increase rate of dry matter content was in the order of S2 > Fengyue > Fengshuo > S1 > Chuhong. The results also showed that the effect of exogenous ALA on the dry matter content of cultivars with lower SSC was more significant than those with higher SSC. 【Conclusion】 Exogenous ALA can effectively regulate the physiological characteristics of kiwifruit leaves, enhance heat tolerance, and improve fruit quality.
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