Abstract: By the method of hydroponic culture, this paper studied the alleviation effects of LaCl₃ on the photosynthetic characteristics of cucumber seedlings
 under nitrate (140 mmol NO₃-·L^-1) stress, with the related mechanisms discussed. Under nitrate stress, the seedlings leaf chlorophyll and carotenoids contents decreased significantly, and the leaf Mg²⁺ATPase, Ca²⁺ATPase activities also decreased. On the 7th day of nitrate stress, the decrease of seedlings photosynthetic rate was mainly due to stomatal limitation; but on the 12th day of nitrate stress, the decrease was mainly due to no-stomatal limitation. Supplement with LaCl₃ could make the cucumber seedlings keep relatively higher leaf Mg^2+-ATPase and Ca^2+-ATPase activities and chlorophyll and carotenoids contents, and applying 20 μmol·L^-1 of LaCl₃ could increase the carotenoids content significantly. LaCl₃ could also improve the leaf gas exchange, and alleviate the decrease of leaf F_v/F_m, Ф_PSII, AQY, CE, and q_p under nitrate stress, which helped the lea
ves making good use of light energy and maintaining higher CO₂ assimilation capacity. An additional 20 μmol·L^-1 of LaCl₃ could alleviate the nitrate
 stress on the photosynthesis of cucumber seedlings efficiently, but an additional 200 μmol·L^-1 of LaCl₃ only had the alleviation effect at the initia
l period of nitrate stress. Our results could benefit to the improvement of greenhouse soil.

Key words: LaCl_3, nitrate stress, photophosphorylase activity, photosynthetic rate, chlorophyll fluorescence parameter, Pararcyplera microptera meridionalis, cold hardiness, supercooling point, lethal temperature, lethal time.