Interaction Between Irrigation Regimes And Nitrogen Rates On Grain Yield Of Rice And Its Physiological Basis

Both water and nitrogen(N)are two key factors determining crop yield.The effects of water and N on the growth of crops are mutual promotion and mutual restriction.Rice is one of the largest food crops in China,and it is not only the first large water consumption,but also a large crop of N demand.Although there are many scholars who have studied the effects of water or N fertilizer on rice yield and its physiological mechanism,the research on the interaction between alternate wetting and drying irrigation and N rates on rice yield and its physiological basis is very scarce.In this study,a japonica hybrid rice cultivar Yongyou 2640 was grown in field,and two irrigation regimes,conventional irrigation(CI)and alternate wetting and moderate drying irrigation(AWMD),and three N rates,LN(120 kg N/ha),MN(240 kg N/ha),HN(360 kg N/ha)were conducted.The effect of irrigation regimes and N rates and their interaction on rice growth and development,yield formation,nutrient uptake,and rice quality were investigated.The main results are as follows:1.Irrigation regimes and N rates had significant interaction effects on grain yield and its components.At the same N rate and compared with CI,AWMD significantly increased grain yield by 6.7%-18.6%because of increases in the percentage of filled grains and 1000-grain weight.In the CI regime,the grain yield at HN and MN were 21.1%-23%and 12%-15.5%higher than LN,respectively.The yield at HN was lower than MN due to the lower percentage of filled grains and lower 1000-grain weight.In the AWMD regime,grain yield showed no significance between MN and HN although both MN and HN exhibited a higher grain yield than that LN,with the increase by 23.7%-25.3%and 23.2%-24.4%,respectively.Among all the treatment combinations of irrigation regimes and N rates,the combination of AWMD+MN showed the highest grain yield.The reason for the high yield of AWMD+MN was the synergetic increase in total number of spikelets,the percentage of filled grains and 1000-grain weight.2.When compared with Cl,AWMD significantly reduced irrigation water by 18.1%-23.2%and increased water use efficiency(WUE)by 27.3%-55.3%.The WUE was highest when AWMD combined with MN.AWMD significantly increased the partial factor productivity of N by 4.9%-14.8%,internal N use efficiency by 11.1%-18.6%.The AWMD also significantly increased N transportation rate and N harvest index.Higher root oxidation activity,higher biomass,and higher activities of N metabolism enzymes accounted for the higher WUE and N use efficiency(NUE)in the AWMD regime.3.Irrigation regimes and N rates had significant interaction effects on rice quality.At the same irrigation regime,the chalk size,chalkiness and protein content of rice were increased with the increase in N rates,indicating that an increase in N rate could reduce rice appearance and taste quality and increase nutrition quality.At the same N rate,when compared with CI,AWMD significantly increased appearance quality through reducing rice chalk size,chalkiness degree,and improved starch viscosity property by reducing setback and increasing peak viscosity and breakdown.The AWMD+MN treatment combination showed the highest milled rice rate,head rice rate and the lowest amylose content,and the best rice quality among all the treatment combination of irrigation regimes and N rates.4.Compared with CI,AWMD showed fewer unproductive tillers,higher percentage of productive tillers,greater leaf area index especially effective leaf area index,and better rice population structure.AWMD increased non-structural carbohydrates(NSC)in the stem and sheath at heading stage and NSC transportation rate from heading to maturity.AWMD increased root-shoot ratio,root length especially deep root length,root oxidation activity,the amount of root exudates and leaf photosynthetic rate,activities of anti-senescence enzymes during maturity,and prolonged the period of high photo synthetic rate.The traits above were best when AWMD coupled with MN,which would be the important t agronomic and physiologic reasons for the highest yield and best quality for the AWMD+MN treatment combination.