Effects Of Nitrate-ammonium Nutrition Regulation On The Growth And Nitrate Nitrogen Uptake And Utilization Of Sugarcane

Sugarcane is the most important sugar crop in the world.In China,the application of nitrogen fertilizer is an important practice to increase the yield of sugarcane.Currently,there is a common problem of excessive nitrogen application in sugarcane production,which has adverse impacts on the environment.Therefore,it is of significance to improve nitrogen utilization efficiency and reduce the nitrogen application rate for the green development of sugarcane industry.It has been reported that sugarcane may prefer to absorb ammonium nitrogen.In China,most of sugar is planted in dryland farmland,and due to strong nitrification,nitrate nitrogen is usually the main nitrogen supply form in soil.At present,there are few studies on the physiological and molecular mechanism of nitrate nitrogen uptake and utilization in sugarcane.Thus,this study mainly carried out two aspects of work.First,a hydroponic experiment was conducted with different nitrate-ammonium ratios（20%N,50%N,80%N,and 100%N）under adequate nitrogen supply and a control treatment without nitrogen supply（-N）.Secondly,a soil pot experiment was conducted,and four nitrogen supply form treatments（-N,0%N,50%N and 100%N）were set up under phosphorus deficiency（-P）and phosphorus supply（+P）conditions,respectively.In the tow experiments,parameters about plant growth,nutrient uptake,root morphology,nitrogen assimilation metabolism,and nitrate transporter gene expression of sugarcane seedlings were measured to elucidate the effects of different nitrate-ammonium ration supply and nitrogen-phosphorus interaction on sugarcane growth,nitrate nitrogen uptake and utilization.The main results were as follows:（1）Plant growth,nutrient uptake,root morphology,and nitrogen assimilation of sugarcane seedlings were significantly influenced by different nitrate-ammonium ratio supply.Sugarcane seedlings had the highest plant biomass when the nitrate-ammonium ratio was 80:20.Under this treatment,the total nitrogen accumulation and nitrogen use efficiency reached the highest level,while the ¹⁵N use efficiency,root morphology,root nitrogen assimilation enzyme activity,and related nitrogen metabolites were also relatively high.Compared with the mono-nitrate supply,the total nitrogen accumulation and nitrogen utilization efficiency of plants were higher under high nitrate-ammonium ratio treatments（50%or more of nitrate-to-ammonium）,while lower nitrate-ammonium ratio inhibited the root growth.Compared with nitrogen deficiency treatment,the expression of Ss NPF4.6,Ss NPF6.3 and Ss NRT2.1 were down-regulated but Ss NPF6.4 was significantly up-regulated in sugarcane roots under nitrogen supply treatments.Ss NRT2.5 was highly down-regulated when the nitrate-ammonium ratio was80%and above.（2）Nitrogen-phosphorus interaction had significant effects on the plant growth,nutrient uptake,root morphology,and nitrogen assimilation of sugarcane seedlings.Under phosphorus deficiency,plant biomass,nitrogen and phosphorus accumulation,and root morphology of sugarcane seedlings were highest under mono-ammonium treatment.The expressions of nitrate transporter genes were down-regulated under nitrate and ammonium mixed supply compared to the single nitrogen form supply.Under phosphorus supply condition,sugarcane seedlings showed higher biomass and nitrogen and phosphorus accumulation under both mono-ammonium and nitrate-ammonium supply,while the root morphology did not differ much under different nitrogen supply treatments.The expression of Ss NPF4.6,Ss NPF6.3,Ss NRT2.1and Ss NRT2.5 were all down-regulated and Ss NPF6.4 was up-regulated under ammonium-nitrate simultaneous supply compared with single nitrogen form supply.In conclusion,this study analyzed the nitrate nitrogen supply ratio suitable for sugarcane seedling growth through hydroculture and soil culture experiments,and revealed the physiological and molecular response characteristics of nitrate nitrogen uptake and utilization by sugarcane grown under different nitrate-ammonium ratio and nitrogen-phosphorus interaction supply.These results provide references for improving nitrogen use efficiency of sugarcane through agronomic or biological approaches.