Relationship between leaf SPAD values and the nitrate content and nitrate reductase activity in cucumber at different nitrogen rates.

【Objective】 The study was to confirm the best period,best location and critical nitrogen concentration of plant diagnosis in cucumber.【Method】 A hydroponic experiment was conducted to investigate the changing characters of the leaf SPAD values,nitrate reductase activity and the leafstalks nitrate nitrogen content at different growth stages of cucumber under the conditions of different nitrogen supplying rates respectively.【Result】 The results showed that the leaf SPAD values increased significantly with the increase of N rate,and they all could be used as indices for plant nitrogen nutrition diagnosis.The leaf SPAD values at different positions of leaf responded differently to the increase of N rate.The leaf SPAD values,the nitrate reductase activity and the leafstalks nitrate nitrogen content have changed significantly at the different growth stages and there was a significant relationship in seedling stage and florescence.【Conclusion】 Therefore,the 3L's SPAD values at seedling stage and in florescence and the 7L's in fruiting were more sensitive,which were suggested to be the best leaf position to diagnose the nitrogen nutrition status of the cucumber plants;The best period and critical nitrogen concentration of cucumber plant diagnosis were florescence and 210 mg/kg.