Nitrogen Form Preference Of Sugarcane And Preliminary Mechanism Analysis

Sugarcane is the most important sugar crop in China,and nitrogen（N）nutrition is closely related to its yield and quality.Currently a big problem of high N application rate but low N use efficiency generally exists in sugarcane production,which needs to be resolved for green development.Supplying crop N nutrition based on its preference of N form is beneficial to improve N use efficiency and thus to reduce N application rate.However,until now,the research results on the preference of ammonium or nitrate in sugarcane are controversial,and the appropriated ammonium-nitrate ratio for sugarcane growth is still uncertain.To solve this problem,the present work chose tow representative sugarcane varieties from Yunnan and Guangxi for study.Firstly,a hydroponic experiment was conducted with variety Yunze05-51,setting 7 ammonium-nitrate ratios under adequate N supply(10 mmol L^-1),and parameters about plant growth,nutrient uptake and utilization efficiency,N assimilation physiological characteristics and ammonium transporter expression were measured,based on which the relationship between the preference of N form and ammonium-nitrate ratio for sugarcane optimum growth and related mechanism were discussed.Then,variety Liucheng05-136 was used to carry out a vermiculite-based experiment with the same ammonium-nitrate ratios to verify the results obtained from the hydroponic experiment.The main results are as follows:（1）In the hydroponic experiment,plant biomass of Yunzhe05-51 peaked when ammonium-nitrate ratio was 20:80.Under this treatment,plants showed the best root morphology and highest activity of N assimilation related enzymes,relatively higher expression of ammonium transporters as well as N uptake and utilization efficiency.When ammonium concentration in the solution exceeding 4 mmol L^-1,root growth displayed depression before shoot growth;whole plant growth and root morphology were lowest under sole ammonium supply.Compared with plants under sole nitrate supply,plants under combined ammonium and nitrate supply or sole ammonium supply had higher N concentration and uptake;moreover,N uptake per unit root length increased with the increasing ammonium ratio.Under the sole ammonium supply,plants showed the lowest N utilization efficiency,with high levels of free amino acids and soluble proteins.（2）In the vermiculite-based experiment,shoot and root of Liucheng05-136 reached the best growth under ammonium-nitrate ratios of 20:80 and 50:50.Under the two treatments,plants had relatively higher N uptake per unit root weight,N utilization efficiency,and activity of N assimilation related enzymes.Plants were significantly stunted under the sole ammonium supply,but showed higher levels of N concentration,total N uptake and root N uptake efficiency compared to that under sole nitrate supply.Similarly,plants under the sole ammonium supply had the lowest activity of N assimilation related enzymes,accompanied by high free amino acids and soluble proteins levels.In conclusion,the two selected sugarcane varieties showed high plant N concentration and root N uptake efficiency under the sole ammonium supply,but from plant biomass response,sole ammonium supply was not conducive to sugarcane growth.Under high N supply,the increase of the ammonium proportion might cause the plant to suffer from ammonium toxicity,which would reduce root morphology and activity,N assimilation enzyme activity,nutrient physiological utilization efficiency and other physiological characteristics.The effect of simultaneous supply of ammonium and nitrate on plant growth was better than that of single supply of ammonium or nitrate,and the optimal ammonium-nitrate ratio for the two sugarcane varieties was 20:80 in the present study.