Effects Of Different Nitrogen Application Rates On Accumulation And Translocation Of Rice Stem Non-structural Carbohydrates And Its Relationship With Vascular Bundles

The stored non-structural carbohydrates(NSC)in rice stems(culms and sheaths)are one of assimilate sources for grain yield formation.The stem large(LVB)and small vascular bundles(SVB)are the structure for stem NSC translocation.However,the physiological mechanism and contributions of LVB and SVB to NSC accumulation and translocation are not well understood.Therefore,field experiments were conducted using twelve recombinant inbred lines(R022,R032,R046,R094,R108,R118,R130,R146,R191,R222,R230,and R232)derived from a cross of Zhenshan 97 and Minghui 63 under different nitrogen application rates.This study mainly investigated(1)the relationships of number and cross sectional area(CSA)of stem LVB and SVB with NSC translocation under different nitrogen application rates,(2)the contributions of stem LVB and SVB to NSC accumulation and translocation and underlying mechanisms under different nitrogen application rates,(3)the effects of different nitrogen application rates on activities of enzymes for sucrose-starch conversion.The objectives of the study are to elucidate the differences in contribution of stem LVB and SVB to NSC accumulation and translocation and underlying physiological mechanism,and to investigate the effects of different nitrogen application rates on NSC accumulation and translocation.The study may provide theoretical basis for green and high yield cultivation and breeding in rice.The main results are as follows:1.The responses of number and CSA of vascular bundle to different nitrogen application rates were inconsistent among the twelve lines.Compared with low nitrogen application(LN),high nitrogen application(HN)increased the number and CSA of LVB per culm in R046,R094,R108,R130,R191,R222,and R230,and had no significant effects on number and CSA of SVB per culm in the twelve lines.Stem NSC translocation were higher under LN than those under HN in the twelve lines.The number and CSA of LVB and SVB per culm were significantly and positively correlated with stem NSC translocation,spikelet number per panicle,and grain yield per panicle.Compared with LN,HN significantly increased the number and CSA of LVB and SVB per square meter.The number and CSA of LVB and SVB per square meter were significantly and positively correlated with stem NSC translocation,grain filling percentage,1000-grain weight,and grain yield,especially under HN.These results show that the optimization of nitrogen application rate enhanced formation of vascular bundles,stem NSC accumulation and translocation,and grain yields.2.Under both LN and HN,R046 and R146 showed higher stem NSC accumulation than R032 and R191 before heading.From 14 d before heading to heading date,the starch density in parenchyma cells surrounding SVB was higher than that surrounding LVB,suggesting that SVB had higher contribution to NSC accumulation than LVB.Compared with LVB under LN and HN,SVB had higher expressions of sucrose transporter(SUT)and cell wall invertase(CWI),and higher plasmodesma densities between sieve element(SE)and companion cell(CC),between CC and parenchyma cell in the phloem(PP),and between PP and PP;additionally,expressions of sucrose synthase(Su S)and adenosine diphosphate glucose pyrophosphorylase(AGP)in parenchyma cells surrounding SVB were higher than those surrounding LVB.These results indicate that SVB had higher contribution to NSC accumulation than LVB,and the high contribution might be partially attributed to high ability of the apoplastic and symplastic unloading,sucrose hydrolysis,and starch synthesis.3.R046 and R146 had higher stem NSC translocation than R032 and R191 after heading under LN and HN.From heading date to 14 d after heading,the decrease in starch density in stem parenchyma cells surrounding SVB was greater than that surrounding LVB under LN and HN,suggesting that SVB had higher contribution to NSC translocation than LVB.Compared with LVB,SVB had higher SUT expression,and higher plasmodesma densities between SE and CC,between CC and PP,and between PP and PP under LN and HN;additionally,expressions of α-amylase and Su S in parenchyma cells surrounding SVB were significantly higher than those surrounding LVB.These results suggest that higher ability of the apoplastic and symplastic loading,starch hydrolysis,and sucrose synthesis might partially explain the higher contribution of SVB to NSC translocation.4.Compared with HN,LN and moderate nitrogen application(MN)increased stem NSC concentration and stem AGP activity before heading,and AGP activity was significantly and positively correlated with NSC concentration,the results suggest that the increased AGP activity was one of reasons for the increased NSC concentration under LN and MN.Compared with R032 and R191,R046 and R146 had higher stem NSC accumulation,higher activities of leaf sucrose synthase in the synthetic direct(Su Ss)and sucrose phosphate synthase(SPS),and higher stem AGP activity before heading under the three nitrogen applications,indicating that the increased NSC accumulation in R046 and R146 might be due to their high ability of leaf sucrose export and stem starch synthesis.5.Stem NSC translocation under LN and MN were significantly higher than that under HN.Compared with HN,LN and MN decreased activities of Su Ss and SPS in leaves,and increased activities of α-amylase,β-amylase,Su Ss,and SPS in stems after heading.Stem NSC translocation was significantly and positively correlated with activities of α-amylase,β-amylase,Su Ss,and SPS.These results suggest that LN and MN decreased leaf sucrose export,and increased stem sucrose export after heading,and further enhanced stem NSC translocation.Compared with R032 and R191,R046 and R146 had higher stem NSC translocation after heading under the three nitrogen applications,which might be attributed to higher activities of enzymes for starch to sucrose conversion in stems and for sucrose to starch conversion in grains.