Physiological and biochemical mechanism of ¹⁵N-urea foliage spraying in increasing the nitrogen uptake and utilization of cotton seedlings

【Objectives】Cotton roots grow slow at the seedling stage, so the absorption of nitrogen by roots can not meet the needs of growth and development of cotton plants, deadlock or weak seedlings are often happened as a result. Foliar nitrogen application can replenish nitrogen nutrition in time and solve problem of periodic innutrition at the seedling stage. In order to clarify the optimum urea concentration of foliar spraying and understand its mechanism of promoting cotton growth and development, the 15N isotope tracer technique was used to investigate effects of foliage spraying urea on nitrogen uptake and utilization, and physiological and biochemical characteristics of cotton.【Methods】A field experiment with randomized complete-block design were conducted with cotton cultivar CCRI79 as an experimental material. Four foliage spraying concentration of 15N-urea solution: 0%, 0.5%, 1% and 2% were sprayed at the seedling stage of cotton seedlings. The uptake and utilization, and the physiological and biochemical characteristics of cotton were investigated. 【Results】1) The foliage spraying of 15N-urea can significantly improve the 15N content of cotton plants. The 15N uptake by cotton leaves is increased gradually from 2 to 96 h after spraying, then declined. 2) The sprayed 15N-urea can be absorbed quickly by cotton plants, and the average N absorption rates with time are consistent in all the treatments. The main uptake of the sprayed N is occured within 12 h.. The absorption rates sharply rise to the highest from 4-6 h after spraying, then decline sharply from 6-8 h, keep fast from 8-12 h and slowdown after 12 h. The average nitrogen absorption rates from 0-12 h is 0.23-0.29 mg/(gh). 3) The foliar supplement of N nutrition can significantly promote the uptake and up transport of the nitrogen absorbed by roots under the treatments of 0.5% and 1% urea concentrations. 4) Under the urea spraying concentrations from 0% to 1%, the nitrate reductase activity, glutamine synthetase activity and chlorophyll content are increased with the increase of spraying concentrations. The physiological and biochemical indexes are the highest under the treatment of 1% concentration. 【Conclusions】The foliar spraying of urea can improve the N nutrition of cotton seedlings significantly by both the direct absorption of foliar sprayed N and accelerated N uptake and up transport by root. The uptakes of sprayed 15N by cotton plants are mainly occurred within the 12 hours after spraying, with the average absorption rates of 0.231-0.293 mg/(gh), depending on the spraying concentrations. The contents of 15N in cotton plants keep increasing until the 96 hours after spraying. The foliar supplement of N nutrition promote photosynthesis by increasing the activities of nitrate reductase and glutamine synthetase, increasing the chlorophyll content, and improve leaf area of cotton plants. The suitable spray concentration is 1% of urea at the seedling stage of cotton.