Abstract: 【Objective】In order to explore the effects of exogenous nitric oxide on cotton seedlings under nitrogen stress, the morphology characteristics of leaves and roots and cotton growth were analyzed, which would provide a theoretical basis for regulation of exogenous nitric oxide in cotton.【Method】The hydroponic experiment was conducted in a growth chamber. Nongdamian 8 was selected as the test material in this study. The experiment consisted of seven different treatments. The control group (CK) was growing under Hoagland's solution. Six treatment groups were grown under different SNP concentrations with the basis of Hoagland's solution without nitrogen. Concentrations of SNP, respectively, were 0 μmol·L^-1(T0), 50 μmol·L^-1(T1), 100 μmol·L^-1(T2), 200 μmol·L^-1(T3), 500 μmol·L^-1 (T4), and 1 000 μmol·L^-1(T5). The leaf area, root morphology characteristics, water consumption and dry weight of cotton seedlings under nitrogen stress were measured. 【Result】The growth of cotton seedlings above-ground and under-ground were inhibited by nitrogen stress. The increase of leaf number and leaf area was inhibited. The root length, root surface area and root volume of fine root (0.05-0.20 mm) and middle root (0.2-0.45 mm) decreased and the water consumption and dry weight also decreased. Different concentrations of exogenous nitric oxide had different effects on nitrogen-stressed cotton seedlings’ above-ground and under-ground growth. The damage of nitrogen stress was alleviated by low concentration exogenous nitric oxide (SNP concentration: 50-100 μmol·L^-1). The upper and down leaf area significant increased by low concentration exogenous nitric oxide. Fine root and middle root growth were affected significantly, whose root length, root surface area and root volume were increased by low concentration exogenous nitric oxide. Water consumption and dry weight also increased by low concentration exogenous nitric oxide. The alleviation effect reduced with the further increase of SNP concentration when the concentration was more than 100 μmol·L^-1. The leaf number, upper and down leaf area, root surface area and root volume of fine root and middle root, water consumption and dry weight all decreased. It was concluded that different concentrations of exogenous nitric oxide could ameliorate the negative effects of nitrogen stress by improving the growth of shoot and root. SNP at 100 μmol·L^-1 was the best in alleviation the nitrogen stress on cotton seedlings, while the high concentration of SNP enhanced the damage of nitrogen stress. 【Conclusion】 Nitrogen stress decreased the growth of cotton seedlings. Nitrogen deficiency was mitigated, the growth of above-ground and under-ground was increased, the tolerance to nitrogen stress was also increased by lower concentration of NO (SNP concentration: 50-100 μmol·L^-1). The 100 μmol·L^-1 SNP has the best mitigative effect on nitrogen stress.

Key words: cotton, nitrogen stress, nitric oxide, morphology, growth