Effect Of Water-nitrogen Interaction On The Grain Filling Of Inferior Spikelets In Rice

In rice,especially for the cultivars with large panicles,the late-flowering inferior spikelets that locate on proximal branches fill slowly and have small grain weight,which not only limit yield potential,but also decrease grain quality.Improvement in grain filling of inferior spikelets would have great significance in realizing high grain yield and good quality in rice.Nitrogen(N)fertilizer and water are two major factors affecting crop growth and yield.Although there are many reports on the effects of N application and water management on grain filling in rice,the information about water-N interaction on the grain filling of rice inferior spikelets is scarce.Therefore,this study investigated water-N interaction on grain filling.An indica/japonica hybrid Yongyou 2640 and a japonica inbred Huaidao 5 were used and were grown in pots.Three N rates and three irrigation regimes were designed.Three N rates were ON(0 g N Pot’1),medium amount of N(MN,2 g N Pot-1),and high amount of N(HN,4 g N Pot-1).Three irrigation regimes consisted of conventional irrigation(CI,soil water potential was kept at 0 kPa),alternate wetting and moderate drying irrigation(WMD,rewatered when soil water potential reached at-15 kPa),and alternate wetting and severe drying irrigation(WSD,rewatered when soil water potential reached at-30 kPa).Aboveground biomass,root dry weight,root-shoot ratio,grain filling rate,hormonal levels,activities of the enzymes involved in starvh synthesis and the expression of the genes encoding these enzymes in grains,and grain yield were determined.The main results are follows.1.Effect of water-N interaction on grain yield and N use effciencyAt the same N rate,The WMD showed the highest grain yield,the CI is next,and the WSD is the lowest among the three irrigation regimes.In the same irrigation regime,grain yield increased with the increase in N rates.In the WMD regime,however,grain yield showed no significant difference between MN and HN.The water-N interaction on grain yield was significant.At the same N rate and in the same irrigation regime,grain yield was higher for Yongyou 2640 than for Huaidao 5.Among the all treatment combinations,WMD+MN produced the highest partial factor productivity of N fertilizer and agronomic N use efficiency.The results suggest that adoption of medium amount of N and alternate wetting and moderate drying irrigation could achieve high grain yield and high N use efficiency.2.Effect of water-N interaction on grain fillingYongyou 2640 exhibited asynchronous,while Huaidao 5 showed synchronous,grain filling between superior and inferior spikelets.The difference in grain filling rates between superior and inferior spikelets was increased with the increase in N rates.Compared with the CI,the WMD increased,whereas WSD decreased,grain filling rate of inferior spikelets.The combination of WMD+MN increased grain filling rate and grain weight of inferior spikelets.3.Effect of water-N interaction on hormonal levels in grainsDuring the early grain filling period,contents of indole-3-acetic acid(IAA)and zeatin + zeatin riboside(Z+ZR)were higher,but the content of abscisic acid(ABA)was lower,in superior than in inferior spikelets.During the mid and late grain filling periods,the levels of the three hormones were higher in inferior spikelets than in superior spikelets.At the same N rate,the WMD significantly increased,while the WSD decreased,hormonal levels in grains when compared with the CI.Correlation analysis showed that the grain filling rate significantly and positively correlated with the contents of Z+ZR,ABA and IAA in grains,indicating that increases in hormonal levels in grains contribute to the increase in grain filling rate and grain weight of inferior spikelets in the WMD regime.4.Effect of water-N interaction on enzymatic activities in grainsDuring the whole process of grain filling and in both superior and inferior spikelets,changes in activities of sucrose synthase(SuSase),ADP glucose pyrophosphorylase(AGPase)and starch branching enzyme(Q-enzyme)showed a single peak curve.The activities of these enzymes were increased with the increase in N rates.The WMD significantly increased,whereas the WSD significantly decreased,enzymatic activities when compared with the CI.5.Effect of water-N interaction on the expreesion of genes encoding the enzymes involved in starch synthesis in grainsExpreesion of the genes encoding the enzymes involved in starch synthesis was greater in superior than in inferior spikelets at the early grain filling stage,and the results were reversed at mid and late grain filling stages.The expression of these genes was enhanced with the increase in N rates.The WMD significantly increased,whereas the WSD significantly decreased,the expression of the genes.In summary,there was a significant interaction between water and N on grain yield and grain filling.Adoption of medium amount of N and alternate wetting and moderate drying irrigation(MN+WMD)could enhance grain filling and increase grain weight of inferior spikelets,and consequently,increase grain yield and N use efficiency.Increases in hormonal levels,activities of the enzymes involved in starch synthesis and expression of the genes encoding these enzymes contributed to the increases in grain filling rate ad grain weight of inferior spikelets,grain yield and N use eccciency under the treatment of MN+WMD.