Studying The Effects Of Topdressed N Fertilizer On Seed Yields And N Absorption & Utilization Of Sudangrass

Sudangrass, the most widely cultivated annual grass around the world, due to its high nutritional value, good palatability, advanced regenerate ability, high production and high adaptability to the natural environment. Seed production was a prerequisite for further expand cultivation of sudangrass. Nitrogen(N) fertilizer has played an important role in the seed yield promotion. In recent years, the irrational application of nitrogen fertilizer have been lower the fertilizer use efficiency(FUE) and the higher production cost. These circumstances affected not only the seed yields and quality, but also cause serious environmental problems. In this studied, the effects of yields component factors, photosynthesis, carbon nitrogen metabolism and enzyme activities under different nitrogen fertilizer(urea) application rates under the fixed proportion were analyzed. The main results showed as follows:1. Nitrogen fertilizer topdressing could effectively promote the grass growth ratio, enlarge the leaf area of single plant, increase the seed weight and number of first grains per spike of sudangrass, also increased seed yields significantly. As fertilizer dosage incresing, excessive fertilization cause vigorous vegetative growth, through the continuous increasing of fertilizer topdressing, yield increasing ratio decreased significantly and shattering were became seriously..2. During jointing stage and heading stage, stem diameter and stem diameter growth rate have been increased followed by fertilizer application increasing, when the fertilizer amount exceeds 450 kg/ha, stem diameter growth was inhibited. Topdressing nitrogen fertilizer, which increased the leaf area per plant of sudangrass, and when the amount of nitrogen fertilizer is more than 600 kg/ha, the speed of leaf growth was slow down significantly, leaf area per plant was decreased compared with CK3. The leaf photosynthesis rate, stomatal conductance, transpiration rate increased with the increasing of nitrogen fertilization topdressing significantly. But the xcessive nitrogen fertilizer input could inhibit the photosynthetic reaction of sudangrass in the whole growth period. SPAD value could reflected the chlorophyll contents and the SPAD value was increased significantly with the increasing of nitrogen fertilization rate which under 450 kg/ha. Meantime, when the nitrogen fertilizer input higher than 600 kg/ha, the photosynthesis rate was inhibited.4. The highest leaves SPAD value was showed at 15:00 in nitrogen fertilizer inputs treatments of 300/450/600 and 750 kg/ha, in blank control and 150 kg/ha fertilizer input treatment, the highest SPAD value were showed at about 17:00. in other treatments, the SPAD value on the high peak were at 13:00, the highest transpiration rate curve had a bimodal distribution and the peak were showed at 15:00, at 9:00 the stomatal conductance up to the highest.5. With the increasing of nitrogen fertilizer topdressing, the total sugar and total nitrogen content in leaf and stem of sudangrass were increased significantly under 600 kg/ha input of fertilizer. Nitrogen fertilizer could improve carbon and nitrogen metabolism level, while the nitrogen level was beyond 600 kg/ha. Topdressing nitrogen fertilizer had obvious promoting effect on the total nitrogen content of leaves during the jointing stage and postulation period; keep input the nitrogen fertilizer, the total nitrogen content of leaves showed a decrease. The nitrogen topdressing had a significant positive influence in the metabolism of carbon and nitrogen, carbon metabolism process is delayed compared with the nitrogen metabolism.6. The increasing of nitrogen fertilizer application could improve the activity of NR and GS in the leaf. The highest NR and GS activity were the N5 and N3 treatment in heading stage and grain-filling stage.When nitrogen fertilizer application was less than 450 kg/ ha, the value of GS of grain-filling stage was higherthan heading stage.Nitrogen fertilizer can effectively enhance the activities of sucrose phosphate synthetase, when the amount of fertilizer is 450kg/ha, the enzyme activity reached the maximum, as the amount of fertilizer increasing, activity of these enzymes had been decreased.7. The accumulation of carbon and nitrogen metabolism enzymes of sugar and nitrogen in plants play a catalytic and regulatory role in heading stage. upregulated SPS and GS activity could made the carbon and nitrogen metabolism of plants more vigorous, promote vegetative growth and flower differentiation, improve seed setting rate, increase crop yields. Nitrogen content of stem and leaf at jointing and heading stage has significant influence on the formation of seed yield, especially the nitrogen content of leaves is dominant in this relationship. At grain filling stage, a significantly positive correlationship had been showed between nitrogen content of leaves and grain department, among which, the grain department is the more prominent role.8. Nitrogen physiological efficiency, nitrogen fertilizer efficiency and NUE have been changed with the increment of N application rate, and the N contribution rate and the production value were relative higher under nitrogen input at 600 kg/ha. when the nitrogen fertilizer grading topdressing was up to 600 kg/ha, the actual yields could be highest at 3815 kg/ha, it was the most efficiency nitrogen fertilizer topdressing model.