Effects Of Nitrogen Application Levels On Post-flower Carbon Metabolism And Yield Formation Of Rice Cultivars With Different Nitrogen Efficiency

Nitrogen is a key nutrient that affects rice yield,and the rational use of nitrogen fertilizer not only increases rice yield but also promotes sustainable agricultural development.Currently,there are many reports on the effect of nitrogen application levels on carbon metabolism and yield formation throughout the entire growth period of rice.However,there is relatively little research on different nitrogen use efficiency rice varieties after flowering.In this experiment,the high nitrogen use efficiency variety Songjing21 and the low nitrogen use efficiency variety Nongfeng1705 were used as test materials.Five nitrogen fertilizer application levels were set（0kg/hm²,75 kg/hm²,150 kg/hm²,225 kg/hm²,and 300 kg/hm²,referred to as N0,N1,N2,N3,and N4,respectively）to study the effects of nitrogen application levels on functional leaf chlorophyll content,ribulose-1,5-bisphosphate carboxylase（Ru BP carboxylase）and phosphoenolpyruvate carboxylase（PEP carboxylase）activities in rice varieties with different nitrogen use efficiency,leaf area,photosynthetic parameters,non-structural carbohydrates in stem and leaf and strong and weak grain,key enzyme activities of sucrose metabolism and starch synthesis,dry matter accumulation,transport and distribution,grain filling characteristics of strong and weak grains,yield and its composition factors.The study aims to clarify the regulatory effects of nitrogen application levels on carbon metabolism in different nitrogen use efficiency rice varieties,as well as the effects of carbon metabolism on yield formation under different nitrogen application levels,providing theoretical basis and technical support for the rational use of nitrogen fertilizer and high-yield cultivation techniques in Heilongjiang Province.The main results of this experiment are as follows:（1）The functional leaf chlorophyll content,Ru BP carboxylase activity,PEP carboxylase activity,leaf area index（LAI）,net photosynthetic rate（Pn）,transpiration rate（Tr）,intercellular CO₂concentration（Ci）,and stomatal conductance（Gs）of both Songjing21 and Nongfeng1705increased with increasing nitrogen application levels.The N4 treatment was significantly higher than the N0,N1,N2,and N3 treatments.The average LAI of each nitrogen treatment of Songjing21 was significantly higher than that of N0 treatment.The functional leaf chlorophyll content,Ru BP carboxylase activity,PEP carboxylase activity,Pn,Tr,Ci,and Gs of Songjing21were significantly higher than those of Nongfeng1705.（2）The soluble sugar content,sucrose content,and functional leaf sucrose phosphate synthase activity（SPS）and sucrose synthase activity（SS）of the stem sheaths and leaves of both varieties showed an increasing and then decreasing trend after flowering,reaching a peak after 20 days.They all increased with increasing nitrogen application levels,and the N4 treatment was significantly higher than the N0,N1,N2,and N3 treatments.Under the same nitrogen application treatment,the peak values of soluble sugar content,sucrose content,and functional leaf SPS activity and SS activity of Songjing21 stem sheaths and leaves were significantly higher than those of Nongfeng1705.（3）The soluble sugar and sucrose content of the two varieties of strong-grain decrease gradually after flowering,while those of the medium-grain and weak-grain increase first and then decrease,reaching their peak values at 10 and 15 days after flowering,respectively.The SPS activity and SS activity of the two varieties of strong-grain,medium-grain,and weak-grain all exhibit an increasing and then decreasing trend after flowering,with the peak values of the strong-grain appearing at 15 days after flowering,and those of the medium-grain and weak-grain appearing at 20 days after flowering.The soluble sugar and sucrose content,SPS activity,and SS activity of the two varieties of strong-grain,medium-grain,and weak-grain all exhibit an increasing and then decreasing trend with increasing nitrogen application levels,with the treatments showing the following order:N3>N4>N2>N1>N0,and the N3 treatment being significantly higher than the other treatments.Under the same nitrogen application treatment,the peak values of the soluble sugar,sucrose content,SPS activity,and SS activity of the strong-grain,medium-grain,and weak-grain of Songjing21 are all higher than those of Nongfeng1705,while the soluble sugar and sucrose content 40 days after flowering are lower than those of Nongfeng1705.（4）The content of amylose and amylopectin in both strong,medium,and weak grains of the two varieties showed an initial increase followed by a decrease with increasing nitrogen application levels.The N3 treatment had the highest content,significantly higher than the N0,N1,N2 treatments,and not significantly different from the N4 treatment at 40 days after flowering.The maximum accumulation rate of amylose and amylopectin in the strong grains occurred from 5to 10 days after flowering,while in the medium and weak grains it was delayed by 5 days compared to the strong grains.The content of amylose in the strong,medium,and weak grains of the Songjing21 variety was lower than that of Nongfeng1705 at 40 days after flowering,while the content of amylopectin was higher than that of Nongfeng1705.The activities of starch-related enzymes in the two varieties of strong,medium,and weak grains showed an initial increase followed by a decrease after flowering.The activity of adenosine diphosphate pyrophosphorylase activity（AGPase）and soluble starch synthase（SSS）in the strong grains reached the highest level at 10 days after flowering,while the activity of granule-bound starch synthase（GBSS）and starch branching enzyme（SBE）reached the highest level at 15 days after flowering.The time for the medium and weak grains to reach the peak enzyme activity was delayed by 5 days compared to the strong grains.The activities of starch-related enzymes in the strong,medium,and weak grains of both varieties showed an initial increase followed by a decrease with increasing nitrogen application levels,the treatments showed N3>N4>N2>N1>N0,and the N3 treatment had significantly higher activities than the other treatments.Under the same nitrogen application level,the peak activities of AGPase,SSS,and SBE in the strong,medium,and weak grains of the Songjing21 variety were significantly higher than those of Nongfeng1705,while the peak activity of GBSS was significantly lower than that of Nongfeng1705.（5）The dry matter accumulation of stem sheaths and leaves in both varieties increased with the increase of nitrogen application level,and N4 treatment was significantly higher than N0,N1,N2,and N3 treatments.The stem sheath distribution rate and leaf distribution rate of N4 treatment were significantly higher than those of N3 treatment 20-40 days after flowering.The panicle allocation rate in N3 treatment was significantly higher than that in N4 treatment after anthesis.The stem sheath material output,material output rate,and stem sheath NSC transport rate in N3treatment of both varieties were significantly higher than those in N4 treatment,and the stem sheath NSC transport amount in N4 treatment was not significantly different from that in N3treatment.The stem sheath NSC transport rate in each treatment of Songjing21 was significantly higher than that in Nongfeng1705.（6）With the increase of nitrogen application level,the maximum grouting rate（GRmax）and average grouting rate（GRmean）of dominant grain decreased,while those of weak grains increased.The time to reach the maximum grouting rate（Tmax）and active grouting period（D）of superior and inferior grains increased with the increase of nitrogen application level.The GRmax and GRmean of strong and weak grains in Songjing21 were lower than those of Nongfeng1705 in each treatment,while Tmax and D were higher than those of Nongfeng1705.（7）The spike length,number of primary and secondary branches,number of grains on primary and secondary branches,grains per spike,and thousand-grain weight of both varieties showed an increasing trend followed by a decreasing trend as nitrogen application increased,with the order of N3>N4>N2>N1>N0.The number of effective spikes,degraded branches,degraded grains,and empty grains increased with nitrogen application,with the highest values observed in the N4 treatment.Meanwhile,the grain filling rate decreased with nitrogen application,and the N4treatment was significantly lower than the other treatments.Both varieties showed an increasing yield trend followed by a decreasing trend as nitrogen application increased,with the order of N3>N4>N2>N1>N0,and the N3 treatment was significantly higher than the other treatments,with no significant difference between the N2 and N4 treatments.Excessive nitrogen application led to a low grain filling rate,fewer grains per spike,and more degraded branches,degraded grains,and empty grains,resulting in a decrease in yield.The number of effective spikes and grains per spike were higher,and the number of degraded branches and degraded grains was lower in the different treatments of Songjing21 compared to Nongfeng1705,resulting in a higher yield.