The Effects of Nitrogen on Photosynthesis and Activities of Cellular Protection Enzymes in Cotton Cultivars with Different Senescing Properties

Using the cultivar 33B with senescing property and Fengkang6 with non-senescing property to be the experimental materials,the effects of nitrogen application levels on photosynthetic characteristics and activities of cellular protection enzymes were studied.The results indicated that the photosynthetic rates(Pn)in Fengkang6 were all higher than in 33B at two measurement points under two nitrogen levels.With the delaying of treatments,the contents of chlorophyll a,chlorophyll b,carotenoid and soluble protein of 33B and Fengkang6 were all decreased in CK and low-N treatment.But under low-N condition,there were relative high values of above parameters in Fengkang6 compared to those in 33B during late-phase of treatment.The soluble sugar contents in tested cultivars showed an adverse tendency with the soluble protein contents.Under low-N condition,Fengkang6 had higher SOD activities than 33B during late-phase of treatment.Under CK and low-N condition,the MDA contents in Fengkang6 at all assays were lower,and the dry weight per plant and leaf area per plant were higher than those in 33b.Thus,the relative low over-oxidation degree of the cytoplasm membrane was the important physiological basis for Fengkang6 to be senescence delay under different nitrogen application levels.In cotton production,application of nitrogen with suitable amount and at suitable time will be have an important role in delaying senescence,improving photosynthesis and increasing yield by alleviating the cytoplasm over-oxidation and decreasing the ratio of carbon assimilates to nitrogen compounds.