Effects of Nitrogen on the Antioxidant Enzyme Activities and Endogenous Hormone Contents of Cotton Leaf under Drought Stress and after Soil Re-Watering during the Flowering and Boll-Forming Stage

Abstract

The flowering and boll-forming stage is the key period for cotton yield. Short-term drought stress during this stage significantly inhibits cotton development and reduces final productivity. Nitrogen application plays an important role in alleviating the adverse effects of dry soil on plant development. The objective of this study was to investigate the effects of nitrogen on plant resistance to soil drought in terms of the changes of antioxidant enzyme activities and endogenous hormone contents in cotton leaf under short-term drought stress(withholding water for eight days then re-watering) . Cotton plants were grown in pots with three N application levels(0,3.73,and 7.46 g N pot-1,equivalent to 0,240,and 480 kg N ha-1,respectively) . Leaf water potential and soil relative water content decreased with increasing N supply at the end of soil drought stress period. At the same time,droughtstress significantly increased SOD and POD activities,MDA and ABA contents in cotton leaf,but reduced CAT activity,soluble protein,ZR,and GA contents,as well as ZR/ABA,IAA/ABA,and GA/ABA. Under soil drought,CAT activity,soluble protein,ZR,IAA,and GA contents,ZR/ABA,IAA/ABA,GA/ABA,and net photosynthetic rate(Pn) in 240 kg N ha-1 treatment were the highest among the three nitrogen treatments,but MDA content and ABA content in 240 kg N ha-1 treatment was the lowest. These results suggested that 240 kg N ha-1 was the optimal nitrogen application rate under short-term soil drought in this experiments,but deficient(0 kg N ha-1) and excessive(480 kg N ha-1) nitrogen supply were of disadvantage to the antioxidant enzyme activi-ties,endogenous hormone contents and photosynthesis. Nitrogen affected antioxidant enzyme activities and endogenous hormone contents not only under water stress but also under re-watering after drought stress. At the 10th day after soil re-watering,there was no difference in SOD and CAT activities,MDA content and Pn between treatments with soil drought and well-watering. ABA content in drought treatment was significantly lower than that in well-watering treatment,but the trends of ZR,IAA,and GA con-tents,ZR/ABA,IAA/ABA,and GA/ABA were opposite. However,more N application was available to increase Pn through pro-moting antioxidant enzyme activities,decreasing lipid peroxidation and adjusting the balance of endogenous hormones(ZR/ABA,IAA/ABA,GA/ABA) in drought treatment after stress alleviation. So,the application of N should be reduced to decrease the ef-fects of stress when drought occurred,but after stress alleviation,farmers could apply additional N fertilizer to compensate for the loss of cotton growth.