Effects of nitrogen on cotton senescence and the corresponding physiological mechanisms.

【Objective】 The effects of N content in soil on the physiological and biochemical indexes,amount of dry-matter accumula,tissue water content,boll load per unit leaf in cotton were studied in the field.【Method】 The physiological characters of the leaves on stem,amount of dry-matter accumulation and tissue water content of the above-ground cotton plants,boll load per unit leaf was studied under different N levels(low level,0 kg·hm-2,middle level,189.50 kg·hm-2,high level,395.0 kg·hm-2).【Result】 The turning point from bloom to senescence is at the later full boll stage.The results showed that there was a negative correlation between nitrogen and the cotton premature senescence.Compared to mid-nitrogen and high-nitrogen treatments,under the condition of low nitrogen,the content of Chl.and dissolvable protein,photosynthetic rate,the amount of dry-matter accumulation and the activities of SOD and POD all decreased.But the content of MDA and boll load per unit leaf increased.It was concluded that cotton had a faster senescence under low nitrogen treatment.However,the efficiency of nitrogen decreased with the increase of nitrogen(from middle N level to high N level).【Conclusion】N content in soil affects the cotton senescence remarkably.Low-nitrogen decreased the physiological characteristics of leaves,and protmoted the senescence;mid-nitrogen and high-nitrogen delay aging.