Effects of spring frost on young fruit development and fruit quality in 13 pear varieties（strains）

Abstract:【Objective】The large-scale frost damage in the north of China in spring has a great impact on apple, pear and other fruit production in China. Knowing the characteristics and occurrence of this hazard is important for measures to prevent it effectively. Breeding varieties with strong resistance to spring frost is the most effective way to deal with frost damage. Extreme frosts can be utilized to screen varieties (strains) resist to frost and evaluate the resistance of different pear varieties.【Methods】After the spring frost from April 22 to 23, 2020, young fruits of different pear varieties were divided into four grades according to their damage degrees. The damage grades of young fruit at tree heights of 1.5 m,2.0 m and 2.5 m in 13 pear varieties（strains）were investigated and the freezing injury indexes were calculated. Two weeks later, the fruit set based on cluster and flower were investigated. After harvest, fruit were divided into four grades according to the degree of damage on fruit surface to investigate the effect of spring frost on fruit external appearance. The weight, hardness and soluble solid content of fruit were measured to investigate the effect of frost on edible quality.【Results】The lowest temperature on April 22 was -4.4 ℃, and the duration of sub-zero temperature reached 6.5 h; the lowest temperature on April 23 was -3.7 ℃, and the duration of sub-zero temperature reached 7.5 h. Symptoms of freezing damages on young pear fruit included whole fruit blacking, fruit wilting, development of blister-like white spots on fruit surface and cracks, and flesh and seed blacking or browning. There were significant differences in frost resistance of young pear fruit among different varieties. Fruit from higher tree cano-py had a lower frost damage index. The freezing injury indexes of Jinguang, SJ051105, Huangguan,Meiyu and Guanyu were lower than that of the other varieties, and their freezing injury indexes at 1.5 m were 34.00%, 45.67%, 48.67%, 50.33% and 50.67%, respectively. The young fruits of these five variet-ies had stronger resistance to spring frost. The freezing injury indexes of 041901, 041890 and Qiulu were 75.00%, 79.00% and 88.33% at 1.5 m, respectively, which showed weak resistance to spring frost.The young fruits with light damage-maintained growth. The fruit set in Jinguang, SJ051105, Guanyu,Huangguan and Meiyu was higher; the average cluster fruit set exceeded 81%; and the average flower fruit set was higher than 32% . The average fruit set based on cluster in Qiuguang, Qiulu, 041025,041890 and 041901 was lower than 45%; the average fruit set based on flower was lower than 18% and the young fruit setting rate was frail. The freezing injury index of young fruit and average fruit set rates based on cluster were significantly positively correlated at different heights. At the same height, there was a significant negative correlation between freezing injury index and cluster fruit set rate; the same was with the freezing injury index and fruit set based on flower. There was a significant positive correla-tion between fruit set based on cluster and flower. Spots and cracks developed on young fruit exposed to spring frost. The proportions of normal fruit in Jinguang and SJ051105 reached 95.2% and 93.21%, respectively. They were the only two varieties with almost no defective fruit. Except for SJ051105 and Meiyu, single fruit weight decreased in all varieties, and fruit firmness in Xuehua and 041025 in-creased. Fruit quality in the other varieties did not change significantly. The results showed that spring frost damage had little effect on the quality of the fruit, which recovered the normal growth and develop-ment. Therefore, it is necessary to keep as many fruits as possible after the occurrence of frost damage.【Conclusion】The damage degree of young pear fruit is related to the temperature at the time of frost, and the temperature around the tree increased with tree height and the damage of young fruit at higher canopy position is lighter. Spring frost injury has a great effect on the appearance of the fruit which maintain normal growth and development, but has little effect on their eating quality. Therefore, it is rec- ommended to keep more fruit after the occurrence of frost injury. Jinguang, SJ051105, Guanyu, Huang-guan and Meiyu have a better frost resistance than the other investigated varieties. Fruits of some variet-ies maintain normal growth after freezing injury, but seed development is strongly affected.