Abstract: Taking the moderate heat-tolerant cucumber cultivar ‘Jinchun 4’ as test material, a quartz sand culture experiment was conducted in a growth chamber to investigate the effects of exogenous spermidine (Spd) on the nitrogen metabolism of cucumber seedlings under high temperature (42 ℃) stress. Short term high temperature stress, especially within 4 hours, decreased the plant nitrate N content while increased the plant ammonium N content, whereas pretreated with exogenous Spd increased the plant nitrate N and ammonium N contents and nitrate reductase (NR) activity. Longer term high temperature stress increased the root nitrate N content but blocked the nitrate N translocation to shoot, deactivated root NR activity, and increased the ammonium N content in root and leaf significantly. Foliar spraying Spd under high temperature stress promoted the nitrate N translocation from root to shoot, and increased the NR activity in root and leaf. From the viewpoint of long-term effect, exogenous Spd could have the effects of preventing the excessive accumulation of ammonium N in cucumber seedlings, and of promoting the normalization of the  nitrogen metabolism of cucumber seedlings under high temperature stress.

Key words: Natural secondary forest, Forest species, Structure, Analytic hierarchy process