Effects of topdressing nitrogen fertilizer on photosynthetic characteristics and seed yields of sudangrass

The photosynthetic characteristics and seed yields of sudangrass response to different topdressing nitrogen fertilizer including six different nitrogen application rates of 0, 150, 300, 450, 600 and 750 kg·ha-1 and the fixed proportion (3∶4∶3) were studied by single factor randomized trials using Sorghum sudanense ‘Xinsu No.2’. The results showed that photosynthesis rate (Pn), stomatal conductance (GS), transpiration rate (Tr) significantly increased with increasing of nitrogen fertilization application rate in a certain range. The values of Pn, Tr, GS in heading stage were significantly greater than that in grain filling stage and heading stage. Excessive nitrogen fertilizer could inhibit the photosynthetic during growth period. The results confirmed that SPAD value can fully reflect the chlorophyll contents. The SPAD value significantly increased with increasing of nitrogen fertilization application rate if the nitrogen fertilizer rate was less than 450 kg·ha-1. Nitrogen fertilizer could effectively improved thousand seeds weight, grains per spike, and also significantly increased sudangrass seed yield in a certain range. However, the seed yield significantly decreased with the increasing of fertilizer. The seed yield was maximized (3 815 kg·ha-1) at the level of 600 kg·ha-1 nitrogen application. In combination with photosynthetic characteristics and seed yield of sudangrass, the best nitrogen application rate was 600 kg·ha-1.