Effects Of Panicle And Grain Nitrogen Fertilizer Rates On Grain Yield In Rice Varieties With Different Panicle Types And Their Mechanisms

Nitrogen(N)fertilizer is an important factor affecting rice yield.The large amount of nitrogen fertilizer input and low nitrogen use efficiency are the outstanding problems in the current rice production in China.Excessive nitrogen fertilizer input will not only cause environmental pollution and waste of resources,but may also lead to poor grain filling rate,and thus affecting the potential of increasing rice yield.Panicle fertilizer and grain fertilizer are common fertilization strategies in rice production.Panicle fertilizer mainly affects the differentiation and degeneration of rice spikelets and affects grain yield,while the application of grain fertilizer might influence the grain yield by regulating grain filling rate and grain weight of rice.It is of great significance for improving grain yield to clarify the effects and mechanisms of spikelet fertilizer application on spikelet development and grain filling of rice varieties with different panicle types.In this study,rice varieties with different panicle types(using the number of spikelets per panicle as the standard,small panicle type:Nanjing 9108,Huaidao 5,with about 120 spikelets per panicle;medium panicle type:Yangliangyou 6,Changyou 5,the number of spikelets per panicle is about 200;the large panicle type:Yongyou 1540,the number of spikelets per panicle is about 280)were selected as the test material.The effects and mechanisms of 5 nitrogen application rates of panicle fertilizer(0,54,108,162,and 216 kg hm-2)and 4 nitrogen application rates of grain fertilizer(0,27,54,81 kg hm-2)on the yield of the above rice varieties were studied,respectively.The main findings are as follows.(1)When N was applied as panicle fertilizer in the rate of 0～216 kg hm-2,grain yields of Nanjing 9108(small panicle type),Yangliangyou 6(medium panicle type)and Yongyou 1540(large panicle type)showed a trend of first rising and then falling with the increase of N application rates.Compared with no panicle fertilizer,the application of panicle fertilizer increased the yield of the small panicle type rice variety—Nanjing 9108 by 12.1%-25.3%,while increased the yield of Yangliangyou 6(medium panicle type)and Yongyou 1540(large panicle type)by 8.33～15.7%and 4.96～11.9%,respectively.Above results indicated that the promotion of panicle fertilizer on rice yield of small panicle type rice varieties was significantly higher than that on rice varieties with medium and large panicle types.The significant increase in the number of grains per panicle was the main reason for the large increase in the grain yield of Nanjing 9108.With the increase of panicle fertilizer rates,the decrease in grain filling rate and 1000-grain weight of rice varieties with all panicle types became more significant.According to the curve equation of N application rates of panicle fertilizer and yields,the optimal N application rates of panicle fertilizer for Nanjing 9108,Yangliangyou 6 and Yongyou 1540 were calculated as 168.3 kg hm’2,138.1 kg hm’2 and 120.0 kg hm-2,resectively.Above results showed that the appropriate N application rates of panicle fertilizer should be adjusted according to panicle types of rice variety.Varieties with small panicle types should appropriately increase the amount of panicle fertilizer,while varieties with larger panicle types can appropriately reduce the amount of panicle fertilizer to increase grain yields.(2)Compsared with no panicle fertilizer application,panicle fertilizer increased the number of primary and secondary spikelets differentiation,degeneration,and existing numbers of Nanjing 9108,Yangliangyou 6 and Yongyou 1540 in different degrees.The increase in secondary spikelets was more obvious.Panicle fertilizer also increased the fresh weight of young panicles and the accumulation rate of aboveground dry matter at each panicle differentiation stage of the above three varieties,as well as the increase rate of the fresh weight of young panicles and aboveground dry matter accumulation from the period of spikelet differentiation to the pollen mother cell meiosis stage.The leaf area index,N contents in stem sheath and leaves,non-structural carbohydrate accumulation in stem sheath,and Z+ZR nucleoside content in young panicles and roots of the three rice varieties also increased with the increase of panicle fertilizer rates.The results of correlation analysis showed that the improvement of the above indexes was beneficial to improve existing number of spikelets in different degrees,thereby increasing the number of grains per panicle and promoting increased yield.(3)When N application rates of grain fertilizer was within the range of 0-81 kg hm-2,with the increase of N application rates,the yield of Huaidao 5(small panicle type),Changyou 5(medium panicle type)and Yongyou 1540(large panicle type)showed a trend of first increasing and then decreasing.The variation of rice yield was mainly caused by grain filling rate and 1000-grain weight.Compared with no grain fertilizer,Yongyou 1540(large panicle type)had a greater increase in yield than Huaidao 5(small panicle type)and Changyou 5(medium panicle type),reaching 3.8～8.5%,under the same grain fertilizer application.According to the curve equation of yield and grain fertilizer application,the optimal N application rates of grain fertilizer for Huaidao 5,Changyou 5 and Yongyou 1540 rice varieties was calculated as 42.7 kg hm-2,52.5 kg hm-2 and 66.5 kg hm-2,indicating that the appropriate amount of grain fertilizer should also be adjusted according to the size of the panicle type of the variety.Varieties with smaller panicle types should appropriately reduce the amount of grain fertilizer,while varieties with larger panicle types can appropriately increase the amount of grain fertilizer to improve the yield of rice varieties with different panicle types.(4)The aboveground dry matter accumulation,SPAD value and leaf surface index of Huaidao 5,Changyou 5 and Yongyou 1540 increased with the increase of grain fertilizer application.The appropriate amount of grain fertilizer also promoted the increase of leaf net photosynthetic rate,root oxidation activity,seed/leaf and seed weight/leaf,and the change trend was basically consistent with the performance of grain filling rate and 1000-grain weight.The application of grain fertilizer reduced the maximum grain filling rate and average grain filling rate in all treatments,which delayed the time to reach the maximum grain filling,but significantly prolonged the active grain filling period,which resulted in an increase in the final grain weight.The trends of the content of abscisic acid and the key enzyme activities of sucrose-starch metabolism(sucrose synthase,ADP-glucose pyrophosphorylase,starch branching enzyme,starch synthetase)in rice grains during grain filling were basically the same as the trends of grain filling rates.Compared with the treatment without grain fertilizer,grain fertilizer significantly increased the abscisic acid content and the activities of key enzymes involved in sucrose-starch metabolism in grains of Huaidao 5 at 20～30 days after heading and those of Changyou 5 and Yongyou 1540 at 30 days after heading.