Effects of Continuous Fixed-Point and Quantitative Nitrogen Application on Canopy Light Distribution and Yield of Cotton in Blossing and Boll-Forming Stages

【Objective】 To reveal the regularity of canopy light distribution and photosynthetic diurnal variation in continuous fixed-point quantitative nitrogen application cotton during boll period and its influence on yield. 【Method】 Three fertilization treatments were carried out continuously without nitrogen application 0(N₀), continuous medium nitrogen 270 (N₂₇₀), continuous high nitrogen 450 (N₄₅₀) kg/hm² to analyze effects of continuous nitrogen application on cotton agronomic traits, canopy structure, canopy spatial distribution during flower-bell stage, diurnal variation of canopy during flower-bell stage, spatial distribution of cotton boll and yield. 【Result】 Cotton plant height, fruit branch number, single boll weight and boll number per plant of continuous nitrogen application treatment were higher than those of non nitrogen treatment, and there were significant effects, but there was no significant difference between nitrogen application treatments. At 10:00-19:00 of flowering and boll stage, PAR interception rate of cotton canopy in different treatments was V-shaped with the center of row spacing as the valley bottom. When the PAR interception rates of cotton population were 0.75-0.9, the light distribution sites of no nitrogen treatment were at the height of 1-4 fruit branches, and the PAR transmittance was still 0.25-0.1, and N₄₅₀ treatment was located above the height of 7 fruit branches, so that the light resources under the 7 fruit branches were little, resulting in serious boll dropping; the PAR light interception of N₂₇₀ treatment at 7 fruit branches and above was still 0.5-0.9, and in the first fruit branch at 0.9-1, cotton population showed a good light environment. The diurnal variation of photosynthesis, transpiration rate (TR) and intercellular carbon dioxide concentration (CI) of cotton in flowering and boll stage were higher than those in non nitrogen treatment, but there was no significant difference among nitrogen application treatments, and stomatal limiting value (LS) was just the opposite. The results showed that the increase of nitrogen application could obviously slow down the phenomenon of midday depression of photosynthesis, but the degree of slowing down of photosynthetic midday depression of cotton with high nitrogen application rate decreased, and after reaching the second peak, the decreasing trend of net photosynthetic rate (PN) was almost consistent with that of N₂₇₀ treatment. Leaf water use efficiency (WUE) without nitrogen treatment after 16:00 decreased rapidly with time, and was gradually lower than that of nitrogen treatment. The highest yield of seed cotton in N₂₇₀ was 4 835.67 kg/hm², which was 7.25% and 5.44% higher than those of N₀ and N₄₅₀, respectively. 【Conclusion】 Continuous nitrogen application at 270 kg/hm² can obtain better cotton population canopy structure, which is beneficial to the light distribution, improve the efficiency of light energy utilization and obtain higher yield.