Effects Of Nitrogen Application On Yield And Physiology Of Rice (Oryza Sativa L.) Under Salt Stress

Rice(Oryza sativa L.)is one of the main food crops that are widely planted in China.It can adapt to diverse ecological environment.The area of saline soil in Jiangsu Province covers an area of more than 687000 ha,which is the main farming land resource for agricultural development.Soil with high salinity level usually has negative effects on the growth and physiology of rice,resulting in low productivity.Nitrogen is the most important input of crop production,playing critical roles in the physiological metabolism,stress tolerance,and the utilization of other nutrient elements in the plant.In this thesis,experiments were done to investigate the effects of nitrogen application on the growth and physiology of rice grown under salt stress.This study is of importance for providing theoretical base for high-yielding production of rice grown in saline soils along the coastal line.The experimental material Nanjing 9108 is a late maturing middle japonica line with high yield and good quality,which was selected by Jiangsu Academy of Agricultural Sciences.Both field experiment and the pot experiment were conducted during the rice growing seasons of 2016 and 2017.For the field experiment,the total amount of nitrogen was set at 300 kg/ha for all the treatments.The ratio of base fertilizer to tiller fertilizer had two levels(0:10 and 7:3).The ratio of basal to panicle fertilizers had three levels(8:2,6:4 and 4:6).There were six treatments(0:8:2,0:6:4,0:4:6,5.6:2.4:2.0,4.2:1.8:4.0 and 2.8:1.2:6.0)in total for the field experiment,which was arranged in a randomized block design with three replicates.Each plot size was 5 m x 6 m.For the pot experiment,three salinity levels,including 0 ‰,0.75 ‰ and 1.5 ‰,were designed.The total amount of nitrogen applied was 2 g/pot.The ratio of nitrogen application to the basal tiller fertilizer and the panicle fertilizer was set at four levels(7:3,6:4,5:5 and 4:6).For each treatment,there were ten pots.For each pot,there were three hills and three seedlings for each hill.In the field experiment,with the increase in nitrogen applied amount,plant height decreased.Proper increase in nitrogen application to basal tiller fertilizer increased SPAD readings of rice leaves,yield,effective panicle number and grain number per panicle,but decreased setting percentage and 1000-grain weight.With the development of rice growth process,the activities of antioxidant enzymes and the content of soluble protein in leaves were increased first and then decreased with the increase in nitrogen application rate at each stage;the contents of soluble sugar and sucrose in leaves,stems and ears had the same trend at all growth stages.Proper nitrogen application can increase the activities of antioxidant enzymes and the osmotic regulation capacity,the growth and development of rice and thus rice yield.In this study,the optimum ratio was 4.2:1.8:4.0(basal fertilizer:tillering fertilizer:panicle fertilizer).In the pot experiment,the electric conductivity of saline soil was increased by the increased application of nitrogen.The increase in soil salinity significantly reduced plant height,tiller number,plant fresh weight and grain yield,Proper nitrogen application alleviated the stress effects of salinity on rice by increasing plant height,tiller number,plant fresh weight,yield components and yield.The activity of CAT,POD and SOD of rice leaves was increased as the increase in salinity level,but increased first and then decreased with the increase in nitrogen application rate.The antioxidant enzyme activity and osmotic ability were at the highest levels when the ratio of nitrogen was 6:4(base tiller fertilizer:panicle fertilizer).Under the nitrogen ratio of 6:4(base tiller fertilizer:panicle fertilizer),the content of soluble sugar was increased and the content of sucrose was increased first and then decreased with the increase in salinity level.Generally,increased nitrogen application as basal and tiller fertilizer enhanced the salt tolerance of rice plants,thus resulting into the increase in grain yield.In the pot experiment,the optimum nitrogen ratio was 6:4(base tiller fertilizer:panicle fertilizer).Combining the results of both the field experiment and the pot experiment,we found that salt stress affected rice growth,reduced plant height,tillering and rice biomass,resulting in reduced rice yield.Proper nitrogen application enhanced the resistance of rice to salt stress and alleviated the adverse effects.The activity of CAT,POD,and SOD,soluble sugar and sucrose content were increased under salt stress.These parameters were increased first and then decreased with the increase of the amount of nitrogen applied to the tillering fertilizer.Nitrogen application increased the electrical conductivity of soil.In order to improve the alleviative effects of nitrogen fertilizer on salt stress,the amount of nitrogen applied at different stages of rice growth should be controlled proportionately.The optimum nitrogen fertilizer application was 4.2:1.8:4.0 in the current study.