Abstract: 【Objective】 The NO-3-N threshold was an interpretation standard for N status and an important basis for vegetable seedling nutrient site-specific management, influenced by the cultivar, fertilizer, temperature, light intensity and other factors. Therefore, the NO-3-N content change degree in tissues of tomato seedlings(Lycopersicon esculentum Mill.) under different varieties, fertilization and environmental conditions was studied, in order to obtain a NO-3-N threshold in tissues of tomato plug seedlings. 【Methods】 Firstly, 17 tomato cultivars were used as materials to analysis the NO-3-N content in different tissues by salicylic acid-sulfuric spectrometry. Then, Jiahong No.4 and Zhongza No.105 were chosen from the 17 cultivars to start the second plug tray experiment. The treatments include: three NPK rates(N 26, 210 and 840 mg/L, P4, 31 and 248 mg/L, and K 29, 234 and 1875 mg/L); three temperatures (32℃/22℃, 28℃/18℃ and 20℃/10℃, day/night); two light intensity conditions(no shaded and 50% shaded); sampling times (2, 10 and 24 h after watering). The NO-3-N contents in different tissues were measured. 【Results】 The NO-3-N contents vary significantly among different cultivars and tissues. The NO-3-N thresholds among different tissues of the same cultivar are from 0.79 to 4.42 g/L, and those in a same tissue of different cultivars are from 2.84 to 4.42 g/L. Compared to the NO-3-N content in the seedlings supplied with normal N, the NO-3-N content in the seedlings supplied with excessive N is increased by 1.86-fold, while in the seedlings fertilized with low N, the NO-3-N content is reduced by 97.3%. Furthermore, the NO-3-N contents in the seedlings are decreased under the low levels of P or K, or the lowtemperature or low light intensity. However, in the seedlings fertilized with excessive P or K, or exposed to high temperature, or sampled at different times after watering, the NO-3-N contents display distinct changing patterns in different tissues.【Conclusions】 The NO-3-N contents in tissues of tomato seedlings were fluctuated under different varieties, fertilization and environmental conditions. The basic values of the NO-3-N contents of the 17 cultivars are corrected with these environmental factors. The NO-3-N thresholds in tissues of tomato plug seedlings are 1.75-2.72 g/L in hypocotyls, 2.07-4.04 g/L in stems, 2.18-4.83 g/L in the first leaf petioles, 0.62-1.52 g/L in the first leaves, 2.31-5.10 g/L in the second leaf petioles, 0.73-1.50 g/L in the second leaves, 2.79-4.09 g/L in the third leaf petioles, 0.40-1.53 g/L in the third leaves, 2.44-4.20 g/L in the fourth leaf petioles, and 0.40-2.13 g/L in the fourth leaves.