Abstract: To clarify how different contents of nitrogen effect the fruit performance after the short-term high temperature during the blossom and fruit period, taking tomato ‘Kaisa’ as the experimental material, the temperature-controlled experiment was conducted from March to August, 2022 with the artificial climate chamber(PGC-FLEX, Canada). The fruit setting characteristics, endogenous hormone content of young fruit, fruit appearance quality and the yield of fruit were studied to comprehend the mechanism how different nitrogen levels effect tomato during the blossom and fruit period, and then find out the optimal content of nitrogen which can effectively alleviate the high temperature damage, improve the fruit quality and yield and give guidance for agricultural activities. Five nitrogen levels i.e.N0 (0.00gplant⁻¹), N1 (3.55gplant⁻¹), N2 (5.32gplant⁻¹), N3 (7.09gplant⁻¹) and N4 (8.87gplant⁻¹), and 3 temperature treatments i.e. T1(the max. temperature/ min. temperature is 28℃/18℃), T2(35℃/25℃) and T3(40℃/30℃) were set in this study. Fifteen treatments were performed with the combination of temperature and nitrogen. The relative humidity was set at 50% to 80%, and the light intensity was 1000μmolm⁻²s⁻¹ from 6：00 to 18：00 in the daytime, and 0μmolm⁻²·s⁻¹ during the rest of the day. The results showed that: (1) with no nitrogen applied, the tomato stepped into the rapid fruiting period in advance after the high temperature stress, while the duration of rapid growth period was significantly shortened. With different nitrogen levels applied, the advent of the rapid fruiting period after the high temperature stress was significantly delayed, while the duration of rapid growth period shortened with the increase of temperature. (2) After exerting 5-day-long high temperature, the content of ZT and IAA in tomato young fruit decreased, while the content of GA increased. Appropriate nitrogen level could increase the contents of endogenous hormones (3) With no nitrogen applied, the quality of tomato fruit decreased significantly after a short period of high temperature, as well as the average yield per plant. Appropriate nitrogen levels may promote the growth of young fruit, polish up the fruiting quality and boost yield when the tomato encountered the short-term temperature stress during the blossom and fruit period. However, excess nitrogen would take a toll instead. This study provides the optimal nitrogen level for each temperature: 3.55-5.32gplant⁻¹ for T1 and T2, 3.55-7.09gplant⁻¹ for T3.

Key words: High temperature stress, Nitrogen, Tomato, Fruit setting characteristics, Endogenous hormone, Fruit appearance quality