Effects Of Nitrogen Density Interaction On Physiological Characteristics,Yield And Nitrogen Use Efficiency Of Double Cropping Hybrid Rice

At present,the excessive use of nitrogen fertilizer in rice production can lead to the problems such as low nitrogen use efficiency,decline in efficiency and environmental pollution.In view of these problems,two-factor experiments with different nitrogen application rates and different transplanting densities in early and late rice have been carried out in the National Grain High Yield Project Hunan Yiyang Experimental Demonstration Base,using Zhuliangyou 819 and Taiyou 398 as materials.Among them,the nitrogen application rates of 3 levels（N1,N2,N3）are 0,120 and 150 kg/hm²for the early rice,0,150and 225 kg/hm²for the late rice,respectively.The transplanting densities at 3 levels（M1,M2 and M3）are 13.3cm×16.7cm,13.3cm×20.0cm and 16.7cm×20.0cm for the early rice and16.7cm×20cm,16.7cm×23.3cm and 16.7cm×26.7cm for the late rice,respectively.In this paper,the effects of nitrogen application rate and transplanting density on rice agronomic traits and yield,yield components,photosynthetic characteristics,nitrogen-related enzyme activities and nitrogen use efficiency have been studied.The main results are as follows:1.The yield of the early and late rice was significantly affected by the interaction between nitrogen application rate and transplanting density.According to the correlation,it had been found that the effective panicle and grain number per panicle of the early rice had extremely significant positive correlation with the yield,effective panicle and 1000-grain weight of the late rice had extremely significant positive correlation with the yield.Actual yield:4.20～7.89t/hm²for the early rice and 2.08～5.00t/hm²for the late rice（affected by long time low temperature during the grain filling）.In terms of the nitrogen application rate,N3 and N2 had been found to be significantly higher than N1,and there was no significant difference between N3 and N2.In terms of the transplanting density,M1 and M2 had been found to be slightly higher than M3,but the difference was not significant.Under the interaction of the nitrogen application rate and transplanting density,N3M1、N3M2 and N2M2 treatment not only had higher effective panicles and grains per panicle,but also had higher theoretical and practical yields under the early and late rice.2.The agronomic traits of the early and late rice were significantly affected by the interaction between the nitrogen application rate and transplanting density.Under the interaction of the nitrogen application rate and transplanting density,the tiller number of the early and late rice had been found to be the highest during the whole growth period under N3M1 treatment,the highest value was 526.5 piece/m²for the early rice and 612.0 piece/m²for the late rice,and the tiller number of the early and late rice had been found to be the lowest under N1M3 treatment.Dry matter:the dry matter has been found to reach its peak at maturity stage.The highest value was 13.96t/hm²under the N3M1 treatment for the early rice,and there was no significant difference between N3M1 treatment and N2M1、N2M2treatment.The highest value was 12.68t/hm²under the N3M1 treatment for the late rice.3.The interaction of nitrogen application rate and transplanting density had a significant effect on Photosynthetic Characteristics of early and late rice,but had no significant effect on stomatal conductance of early rice.According to correlation,it had been found that the net photosynthetic rate,transpiration rate and stomatal conductance were positively correlated with the early and late rice yield.SPAD:the milk ripening stage of the early and late rice had been found to be the highest under the N3M2 treatment,and there was significant difference between nitrogen application and no nitrogen application,but no significant difference between density treatment.Leaf area index（LAI）:the leaf area index had been found to reach its peak at booting stage.The highest value of LAI was 4.82 under the N3M1 treatment for the early rice,and there was no significant difference between its treatment and N2M1、N2M2treatment.The highest value of LAI was 4.81 under the N3M2treatment for the late rice,and there was no significant difference between its treatment and N3M1 treatment.The net photosynthetic rate had been found to reach the peak at booting stage,and the highest nitrogen application rate was 34.02co₂m^-2s^-1under the N2M1treatment,followed by N2M2 for the early rice,and the highest nitrogen application rate was26.71co₂m^-2s^-1under the N3M1,followed by N2M1 for the late rice.In terms of the nitrogen application rate,N2 was significantly higher than N3 in early and late rice as a whole,but there was no significant difference in densities among the different treatments.The transpiration rate and stomatal conductance have been found to reach the peak in the early growth stage of the early and late rice.In terms of the nitrogen application rate,the transpiration rate of N2 and N3 was significantly higher than that of N1,and there was no significant difference between N2 and N3 at the late growth stage of early and late rice.The nitrogen application rate had no significant effect on stomatal conductance of the early rice,but N2 and N3 were significantly higher than N1 for the late rice,but there was no significant difference between N2 and N3.In terms of the transplanting density treatment,the main density was slightly higher than M3,but the difference was not significant.Under the interaction between the nitrogen application rate and transplanting density,the net photosynthetic rate,transpiration rate and stomatal conductance of the early rice N2M2、N2M3 and late rice N2M1、N2M2 treatments were not only higher,but also highly correlated with the yield.4.The NR and GS enzyme activities in the early and late rice leaves were significantly affected by the interaction between the nitrogen application rate and transplanting density.According to correlation,it had been found that NR and GS enzyme activities of the early and late rice were positively correlated with the yield or extremely significant positively correlated.NR and GS enzyme activities:In terms of the nitrogen application rate,NR and GS enzyme activities for the early rice were significantly higher than N1 in N2 and N3,and were significantly higher or closed to N1 for the late rice.NR and GS enzyme activities for the early and late rice in all growth stages were higher or close to N1 in N2.In terms of the transplanting density treatment,NR enzyme activity was higher than or close to M3 in all growth stages of the early and late rice,and GS enzyme activity was higher than M3 in all growth stages of the early and late rice.Under the interaction between the nitrogen application rate and transplanting density,NR and GS enzyme activities were not only higher in early rice N2M1、N2M2 and late rice N2M1、N2M2、N2M3,but also more closely correlated with the yield.5.The nitrogen use efficiency of the early and late rice was significantly affected by the interaction between the nitrogen application rate and transplanting density.According to the correlation,it had been found that the correlation coefficient between the yield and total nitrogen accumulation in early rice was the highest,followed by agronomic utilization,and total nitrogen accumulation in late rice was the highest,followed by physiological utilization.In terms of nitrogen application rate,although the harvest index of the early rice was significantly higher than that of N2,the nitrogen use efficiency of other rice was significantly higher than or closed to that of N3.Although the total nitrogen accumulation of late rice was significantly higher than that of N2,nitrogen use efficiency of other rice was significantly higher than or closed to that of N3.In terms of the transplanting density treatment,compared with the conventional density,the nitrogen use efficiency of the early and late rice was higher in M2 than M3.Under the interaction between the nitrogen application rate and transplanting density,Nitrogen uptake efficiency:the highest nitrogen recovery rate was 60.73%in early rice N2M2 treatment and 63.20%in late rice N2M3treatment.Total nitrogen accumulation:N3M1 treatment was the highest in early and late rice,182.20kg/hm²and 182.08kg/hm²respectively.Agronomic utilization rate:N2M2treatment was the highest in early and late rice,which were 21.45kg·kg and 11.21kg·kg respectively.Physiological utilization rate:the highest was 61.91kg·kg in N3M3 treatment for early rice and 29.00kg·kg in N2M2 treatment for late rice.Partial fertilizer productivity:N2M2 treatment was the highest in early and late rice,which were 63.79kg·kg and29.00kg·kg respectively.The nitrogen use efficiency of early rice was higher than that of late rice as a whole,indicating that the yield performance of late rice was poor due to the long-term low temperature during grain filling,so that the nitrogen use efficiency was low.To sum up,N3M3 and N2M2 treatments were not only better for the nitrogen use in the early and late rice,but also more closely correlated with the yield.In conclusion,compared with the normal nitrogen,nitrogen reduction treatment had been found to not significantly reduce the agronomic traits,photosynthetic characteristics,nitrogen-related enzyme activities,and nitrogen use efficiency for the early and late rice.Regarding the yield and yield components,N2M2 treatment in both early and late rice not only showed better yield performance,but also has better yield performance even when the late rice was affected by the long-term low temperature due to the grain filling.And N2M2treatment shows good performance in nitrogen related enzyme activity and nitrogen use efficiency in early and late rice.Among them,the N2M2 treatment was more economical and ecological than the treatments under the higher nitrogen.