Effects of shading and nitrogen rate on photosynthetic characteristics of flag leaves and yield of winter wheat

During the later stage of wheat development, low-light stress caused by cloudy and rainy weather conditions severely influences wheat yield in some wheat (Triticum aestivum L.) production areas in North China. In crop production, in order to get high yield, too much nitrogen fertilizer application has resulted in waste of resources and pollution of the environment. Several studies on the effects of shading or nitrogen application rate on wheat growth have been reported, but little studies have been on the interactive influence of shading and nitrogen rate on photosynthetic characteristics and chlorophyll fluorescence of wheat. Thus, a field experiment was conducted to determine the effects of shading and nitrogen application rate on photosynthetic characteristics of flag leaves and yield of winter wheat during jointing and maturity stages. Wheat plants were planted under diffident treatments including two light levels S0 (no shading) and S1 (50% full radiation from jointing to mature stages, produced by black sun-shade net of 50% light transmittance) and three N fertilizer rates N0 (0 kghm^-2), N1 (120 kghm^-2) and N2 (240 kghm^-2). The chlorophyll content, net photosynthetic rate (P_n) and chlorophyll fluorescence parameters of wheat at flower stage (April 23), middle (May 11) and later (May 20) grain-filling stages were investigated, and yield and its components were determined. The results indicated that shading significantly increased chlorophyll content, PSⅡ fluorescence photochemical quenching coefficient (qP) and actual photochemical quantum yield (Φ_PSⅡ) of flag leaves of wheat. It, however, significantly decreased chlorophyll a/b ratio and fluorescence of non-photochemical quenching coefficient (qN). Thus shading treatment increased photochemical efficiency and reduced heat dissipation of flag leaves, which was helpful for better utilization of light energy. Due to energy shortage, P_n under S1 was lower than that under S0 at the period from flowering stage to mid-grain-filling stage. Because of higher chlorophyll content and efficiency of light energy conversion in S1, P_n was higher than that under S0 at late-grain filling stage. With increase in N application rate, P_n, chlorophyll content, qP and Φ_PSⅡ increased significantly, which enhanced the full use of light energy and improved photosynthetic rate. Shading significantly decreased spike number, kernel number per spike and 1000-grain weight, which severely negated the increase in production due to nitrogen fertilizer application. The comprehensive effect of shading and nitrogen significantly influenced chlorophyll content, kernel number per spike and grain yield, but had no significant effect on other indicators. In conclusion, more nitrogen application alleviated the adverse effects of weak light stress on photosynthesis. Compared with the control (N0), P_n for nitrogen treatments (N1 and N2) increased by 11.5%27.4%, especially, N2 (240 kghm^-2) treatment having the best effect among all treatments. At diffident nitrogen levels, although shading treatment improved light energy utilization of wheat, it significantly reduced plant photosynthesis and yield components, and finally led to significant reduction of yield.