Abstract: 【Objective】 Sepal shedding of Kuerlexiangli pear is a key factor in determining fruit quality and affecting its economic value. Nitrogen is an essential element for the growth of fruit trees, and has a profound influence on the growth and biochemical processes of fruit trees. By regulating photosynthesis, respiration and some enzyme activities, nitrogen plays a key role in the growth and development of fruit trees, such as cell division, organ differentiation and tree structure. However, there are limited studies on how the timing of N application affects sepal shedding in Kuerlexiangli pear. In this experiment, we used Kuerlexiangli pear trees as test materials to address the question of whether the sepals of Kuerlexiangli pear are shed. By studying the effect of nitrogen application time on the sepal shedding of 'Kuerlexiangli pear', so as to screen out the appropriate fertilisation period to improve the rate of Kuerlexiangli pear desalination, and to provide a basis for improving the rate of fruit desalination and quality of Kuerlexiangli pear.【Methods】At different growth stages of dormancy 1 (1 March), dormancy 2 (15 March), flower bud expansion (26 March), large bud stage (1 April), and full bloom stage (10 April), four Kuerlexiangli pear plants with the same growth were set up in each treatment, and nitrogen fertiliser treatments were carried out at each growth stage, and the nitrogen fertiliser (urea 0.24 kg/plant) treatments would be applied at different periods. Fruit set rate was assessed on 1 April, and calyx abscission rate was assessed on 10 May. In view of the fact that sepal abscission in young fruits of Kuerlexiangli pear occurs at the time of full blossom and sepals are completely abscissa 10 days after full blossom. Calyx samples were collected from T5-treated persistent and calcified fruits before N application on the day of full bloom (10 April) and 10 days after full bloom (20 April), at both time points. The collected samples were used for the determination of lignin content and related enzyme activities. 【Results】 The fruit setting rate and calyx removal rate between the nitrogen application period and the control showed significant differences. The fruit setting rate and calyx removal rate of different nitrogen application periods were also significantly different from the control. The highest fruit setting rate of 35.30% was observed when nitrogen fertilizer was applied during the flower bud expansion period (March 26), which was an increase of up to 6.95% compared to the control. In contrast, the highest calyx removal rate of 50.50% was recorded in the unfertilized control group. There was a positive correlation between the total nitrogen content in the calyx tube and the fruit setting rate of the calyx, while a negative correlation was observed between the total nitrogen content in the calyx tube and soil, and the calyx removal rate. The total nitrogen content in the calyx tube showed a trend of increasing and then decreasing. The nitrogen application period had a significant impact on the lignin content and the activity of related enzymes in the calyx tube. The highest lignin content was found when nitrogen was applied during the flower bud expansion period (March 26), with 115.64 mg·g-1 in the persistent calyx during full bloom, 112.49 mg·g-1 in the calyx after shedding; 125.73 mg·g-1 in the persistent calyx 10 days after full bloom, and 103.69 mg·g-1 in the calyx after shedding. The lignin content and the activities of PAL and PPO enzymes in the calyx tube at full bloom and 10 days after full bloom showed a trend of increasing, then decreasing, and then rising and falling again across different nitrogen application periods. 【Conclusion】Applying nitrogen fertilizer during the period of flower bud expansion is more beneficial for the fruit setting rate of Kuerlexiangli pear, but not for its calyx removal rate. Not applying nitrogen fertilizer is more beneficial for the calyx removal rate of Kuerlexiangli pear. The lignin content and related enzyme activities of the calyx sepals and detached sepals during the peak flowering period and 10 days after the peak flowering period showed a trend of first increasing and then decreasing. The lignin content, PAL activity, PPO activity, and POD activity of the calyx tube of Kuerlexiangli pear during the peak flowering period and 10 days after the peak flowering period were positively correlated with fruit setting rate; The lignin content, PAL activity, PPO activity, and POD activity in the calyx tube of Kuerlexiangli pear during the peak flowering period and 10 days after the peak flowering period are negatively correlated with the rate of calyx detachment. These enzymes play a key role in the biosynthesis of lignin and thus affect calyx detachment. Applying nitrogen during the period of flower bud expansion can increase the lignin content in the calyx tube, which is beneficial for water transport and thus improves the fruit setting rate of Kuerlexiangli pear, but is not conducive to the shedding of sepals.
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