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In China,nitrogen fertilizer application rates in intensive agricultural systems have increased dramatically in recent years,especially in protected vegetable production systems.This excessive use of nitrogen fertilizer has resulted in soil secondary salinization,which has become a significant environmental stress for crops such as cucumber,in the protected farmland of China.So it is necessary to illuminate how crops respond to nitrate stress.The objective of this work was to examine the effects of increased nitrate concentration[14(CK)and 140 mmol L-1(T)]on NO3-concentration,and in vitro and in vivo nitrate reductase activities in the roots and leaves of cucumber(Cucumis sativus L.cv.Xintaimici)seedlings with hydroponic culture.The results showed that the NO3-concentration in the roots and leaves of T seedlings significantly increased over treatment course,and at 12 d increased by 1.08 and 1.72 times with respect to CK seedlings,respectively;in vitro nitrate reductase activity of T was increased dramatically to 1.74 times of CK in the roots at 2 d and 1.56 times of CK in the leaves at 6 d,and then decreased.At 12 d,in vitro activity was still 24.3% higher in the roots and only 9.9% lower in the leaves than CK.Compared with in vitro nitrate reductase activity,in vivo activity responded differently to the increase of treatment time.At the beginning,in vivo nitrate reductase activity in the roots and leaves of T had no significant difference from CK,whereas with the increase of treatment duration,the activity decreased.At 12 d,in vivo activity in the roots and leaves of T lowered by 20.1 and 52.8% with respect to CK,respectively.This evidence suggests that posttranslational activation of nitrate reductase in cucumber seedlings may be seriously inhibited by nitrate stress.