Effects of nitrogen application time on fruit setting rate and lignin accumulation during sepal development of Kuerlexiangli pear

[Objective] Sepal shedding of Kuerlexiangli pear (Pyrus brestschneideri Rehd.) is a key factor in determining fruit quality and 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. 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 investigate the effects of nitrogen application times on the sepal shedding of Kuerlexiangli pear, so as to screen out the appropriate fertilization period for increasing the sepal shedding of Kuerlexiangli pear. [Methods] Four Kuerlexiangli pear plants with the same growth were set up in each treatment, and nitrogen fertilizer application (urea 0.24 kg per plant) was carried out at different stages, that is, dormancy 1 (1 March), dormancy 2 (15 March), flower bud expansion (26 March), large bud stage (1 April), and full blooming stage (10 April). The 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. The calyx samples of 5 treatments were collected before N application on the day of full bloom (10 April) and 10 days after full bloom (20 April). The collected samples were used for the determination of the lignin content and related enzyme activities. [Results] The fruit setting rate and calyx shedding rate between the nitrogen application periods and the control showed significant differences. The fruit setting rate and the calyx shedding rate of the different nitrogen application periods were also significantly different from those of the control. The highest fruit setting rate (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 with the control. In contrast, the highest calyx shedding rate (50.50%) was recorded in the unfertilized control group. A positive correlation was observed between the total nitrogen content in the calyx tube and the fruit setting rate of sepals. The total nitrogen content in the calyx tube exhibited an initial increase followed by a subsequent decrease. 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 blooming, 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 blooming and 10 days after full blooming 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 was more beneficial for the fruit setting rate of Kuerlexiangli pear, but not for its calyx shedding rate. No-nitrogen fertilizer treatment was 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 rates and were negatively correlated with the rate of calyx detachment. These enzymes wouild play a key role in the biosynthesis of lignin and thus affect calyx detachment. Applying nitrogen during the period of flower bud expansion could increase the lignin content in the calyx tube, which would be beneficial for water transport and thus improve the fruit setting rate of Kuerlexiangli pear, but would not be conducive to the shedding of sepals.