Abstract:

Abstract: The effects of three nitrogen(N) levels （5，15，25mmol/L）hydroponic culture on plasmalemma permeability, polyphenoloxidase(PPO) activity ,the content of ammonium nitrogen and malonialdehyde (MDA) in Chinese cabbage disease-susceptible strains 03B9 and P8-2 and disease -resistant strains C24 and P8-1, and the relationship with petiole spot were studied. The results showed that high nitrogen level can increase the number of petiole spots. The content of ammonium nitrogen in disease -resistant varieties have fallen and the 14th day ammonium nitrogen of the treatment N25 is only 49.95% of the second day in C24.The effects of nitrogen levels are more significant in 03B9 and P8-2 than C24 and P8-1,so petiole spot may caused by the broken plasmalemma .