Abstract:There is significant difference in nitrate reductase (NR) activity between clones of Chinese fir with different growth rate.Generally,high NR activity was found in the fast growing clone.Trees with new shoot mean growth of 59 and 76 cm had NR activity values of 0.08 and 0.236 μ mol NO2- per g fresh weight per hour respectively.The level of NR activity varied with the growing period.The maximum activity was observed when the tree is quick growing (Aug.).The NR activity in the leaves on 1 year old branch was higher than that in the leaves on two years old branch.The NR activity in mature leaves on current lateral branches from stem of current year growth higher than that from stem of last year growth and the year before last,which in mature leaves from the middle of the branch of current growth significantly higher than that in young leaves from the top and that in cutinized leaves from the base of the same branch.If the branch with the leaves on were induced with potassium nitrate solution and light,NR activity in leaves increased apparently.Higher NR activity was obtained when treated with 50 m mol/L KNO3 and illumination of 2000 lux for 12 hours.According to the results obtained in these exprements,it is suggested that NR activity could be used as a biochemical indicator of growth rate of Chinese fir.