Effect Of Nitrogen Management On Spikelet Differentiation,Grain Filling And Grain Yield And Its Mechanisms For Large-Panicle Type Rice

In order to maximize rice yield to meet the growing food demand of mankind,excessive nitrogen is frequently and widely applied during rice cultivation.Inappropriate nitrogen application not only caused waste of resources and environmental problems,but also induced or aggravated poor grain filling which affected the release of rice yield potential.Therefore,it is vital important to explore the effect of nitrogen application on rice grain filling,and the relationship between grain filling and yield.Field experiments were conducted in farmers'fields in Lanjie village,Huaqiao Town,Wuxue County,Hubei Province,central China during the rice-growing season in 2013 and in 2014.The experiment set twelve nitrogen treatments including four different nitrogen rate(0,90,130 and 170 kg hm^-2)applied as basal-midtillering topdressing?0-0,90-0,90-40 and 90-80?and three topdressing(0?40 and 80 kg hm^-2)applied at panicle initiation?PI?.The total amount of nitrogen application ranged from 0 to 250 kg hm^-2.Two indica rice cultivars with large panicle,Yangliangyou 6?YLY6?and Lvdaoq 7?LDQ7?were used as the experimental materials.The effects of nitrogen on rice yield,sink capacity,grain filling in different position of the panicle,and nitrogen and carbon metabolism in fertilized spikelets were studied.The main results are as follows:1. In the range of 0-210 kg hm^-2 of nitrogen amount,grain yield of the two rice varieties increased with the increase of nitrogen amount.Grain yield were highest in the treatments of 90-80-40 and 90-40-80 for LDQ7 and YLY6,respectively.Grain yield stopped increasing once nitrogen among surpassed 210 kg hm^-2 because of the stagnation of sink capacity and the declined filled spikelet percentage.Panicle number did not increase when the amount of basal-midtillering fertilizer exceeded 130 kg hm^-2,while nitrogen applied as PI topdressing had no effect on spikelet number per panicle.Therefore,the sink capacity did not further increase once the nitrogen amount increased to a certain level.LDQ7 and YLY6 could obtain the highest grain yield at N rate of 210 kg hm^-2,and 90-80-40 and 90-80-40 were the best N management for LDQ7 and YLY6,respectively.2. The effects of nitrogen on the differentiation and surving of spikelet varied with varieties.In the range of 0-130 kg hm^-2 of nitrogen amount,the number of differentiated and survival spikelets of LDQ7 decreased with the increase of nitrogen.On the contrary,they increased with the increase of nitrogen amount for YLY6 when nitrogen amount ranged from 0 to 170 kg hm^-2.However,the number of survival spikelets did not further decrease and increase when nitrogen amount exceeded 130 and 170 kg hm^-2for LDQ7 and YLY6,respectively.Nitrogen applied as basal-midtillering topdressing showed no effect on the survival spikelet number once it exceeded 130.Survival spikelet number was not affected by PI topdressing.Zero N and 170-210 kg N hm^-2were benefit for the differentiation and surving of spikelet for LDQ7 and YLY6,respectively.3. The lower fertilized spikelet weight of the bottom part of the panicle than the top part was ascribe to its smaller spikelet size,lower grain filling rate and higher proportion of filling duration in the early filling stage.Compare to the top and middle parts of the panicle,the weight of fertilized spikelet and grain filling of the bottom part of the panicle were easier affected by nitrogen.The grain plumpness of the bottom part of the panicle decreased with the increase of nitrogen application amount.It decreased by 14%and 22%for LDQ7 and YLY6,respectively when nitrogen application amount increased from 0 to250 kg hm^-2.When the amount of nitrogen applied as basal-midtillering topdressing or as basal-PI topdressing increased to 170 kg hm^-2,the increase of PI topdressing or midtillering topdressing would significantly decrease the grain plumpness of the fertilized spikelet from the bottom part of the panicle.The decreased weight of fertilized spikelet for the bottom part of the panicle induced by high nitrogen application was caused by the high percentage of partially filled spikelet,which increased by 15 and 28 percentage points for LDQ7 and YLY6,respectively,when nitrogen application amount increased from 0 to 250 kg hm^-2.In general,the increase of nitrogen application would aggravate the poor grain filling in the bottom part of the panicle.The improvement of grain filling could further increase the grain yield by 30%and 34%for LDQ7 and YLY6,respectively.4. At early filling stage?0-3 d after heading?,the changes in nitrogen?N?and carbon?C?concentration and C/N ratio with time for fertilized spikelet varied with panicle positons.Nitrogen and carbon concentration for fertilized spikelet from the top and middle parts of the panicle increased rapidly,while C/N ratio decreased rapidly.However,it was opposite for spikelets from the bottom part of the panicle.As a result,nitrogen and carbon concentration of the fertilized spikelets from the top,middle and bottom parts of the panicle successively decreased and C/N ratio successively increased on the third day after heading.Similar to the change of fertilized spikelet weight,nitrogen and carbon accumulation in fertilized spikelet also showed"S"growth curve.Compared with the top part,it showed later initiation and lower filling rate in carbon and nitrogen accumulation for the fertilized spikelet from the bottom part of the panicle.The results indicate that the poor grain filling of the bottom part of the panicle might be induced by its low nitrogen and carbon concentration and high C/N ratio on the third day after heading.Although nitrogen applied as PI topdressing increased nitrogen concentration and decreased C/N ratio for the fertilized spikelets from the three parts of the panicle,the filling status of the spikelets from the bottom part of the panicle was not improved,which may ascribe the reason that nitrogen application did not change the difference in nitrogen concentration and C/N ratio of the fertilized spikelets among different parts of panicle at the key filling stage?3 days after heading?.Eliminating the difference in nitrogen concentration and C/N ratio between top and middle parts of panicle during initial filling stage?3 days after heading?may be one of the ways to solve the problem of poor grain filling in the bottom part of panicle.