Effects Of Rice Root Characters On Root Carbon And Nitrogen Costs And Their Relationship With Nitrogen Absorption And Utilization Efficiency

Rice root characteristics may reflect the root metabolic costs,which are closely correlated with root nitrogen absorption and utilization efficiency.In this study,two rice varieties with larger root system and higher root cortical aerenchyma(Yangdao6 and Shanyou63)and the other ones with smaller root system and lower root cortical aerenchyma(Nongken57 and Huanghuazhan)were hydroponic cultured and subjected to three nitrogen treatments(low N,10 ppm;medium N,40 ppm;and high N,100 ppm).The objectives of this research were to evaluate the relationships between root characteristics with nitrogen absorption and utilization efficiency and explicit the effects of root carbon and nitrogen costs on nitrogen absorption and utilization efficiency and its possible mechanism through the analysis of rice root morphological,anatomical and physiological properties.Nitrogen costs and carbon costs were defined as N content per unit root length and C content per unit root length,respectively,and comprehensively affected root construction cost.And we also used some indirect indexes to describe root metabolic costs,i.e,specific root length,root cortical aerenchyma,root biomass per unit root length,root heat per unit root length and root respiration rate.The results showed as follows:1.Nitrogen applications affected rice plant nitrogen absorption and utilization.With N application increased,there was a significant increase of rice root and shoot nitrogen contents,total N uptake,N uptake ability,nitrogen content per unit root length,and proportion of N distribution to shoot(shoot N%),but a significant decrease of nitrogen dry matter production efficiency(NUEb)and nitrogen recovery rate.Meanwhile,Huanghuazhan,Yangdao6 and Shanyou63 exhibited higher total nitrogen uptake and nitrogen recovery rate,but lower shoot N% and NUEb when compared with Nongken57 under low N treatment.2.Different N levels affected rice root morphological traits.When compared with medium N treatment,there was a significant increase of total root length,root surface area,root-shoot ratio,specific root length and specific root surface area;and the root dry weight,total root length,root surface area,root-shoot ratio of Huanghuazhan,Yangdao6 and Shanyou63 were higher than Nongken57 under low N treatment.High N treatmentrestrained root growth observably,which induced remarkable decrease of root dry weight,total root length,root surface area and root-shoot ratio.3.Different N levels affected rice root anatomical traits.When compared with medium N level,there was a remarkable increase of root cortical aerenchyma and sclerenchyma thickness,and decrease of the cortical living cell area%,but no significant changes in root diameter,stele diameter and cortical thickness under low N treatment.The four rice varieties showed much smaller root cortical arenchyma and sclerenchyma thickness,but bigger root diameter and stele diameter under high N treatment.Moreover,Yangdao6 and Shanyou63 showed higher root cortical aerenchyma than Nongken57 and Huanghuazhan,and Huanghuazhan,Yangdao6 and Shanyou63 exhibited bigger root diameter,stele diameter and cortical thickness than Nongken57.4.Different N levels affected rice root carbon and nitrogen costs.When compared with medium N level,the four rice varieties showed lower root heat,bigger root biomass and heat distribution percentage and remarkable decrease of biomass per unit root length and heat per unit root length,which showed a decreasing distribution of root materials and heat.It showed a decreasing trend of carbon content per unit root length,nitrogen content per unit root length and root construction cost,and only carbon content per unit root length and nitrogen content per unit root length decreased significantly.In contrast,root construction cost and nitrogen content per unit root length increased significantly under high N treatment,which showed an increasing root carbon and nitrogen costs as well.When compared with Nongken57,Huanghuazhan,Yangdao6 and Shanyou63 showed higher root respiration rate,carbon content per unit root length,nitrogen content per unit root length,and root construction cost,which exhibited higher root carbon costs and nitrogen costs under low N level.When compared with medium N treatment,Huanghuazhan,Yangdao6 and Shanyou63 had bigger decreasing amplitude of net photosynthesis,carbon content per unit root length,nitrogen content per unit root length and shoot N distribution than Nongken57.5.Root characteristics such as root-shoot ratio,specific root length,specific root surface area,root cortical aerenchyma,sclerenchyma thickness were significantly negatively correlated with total N uptake and shoot N%,respectively,and had obvious positive correlation with NUEb,respectively.And there was a significant positive correlation of root respiration rate,root nitrogen content with total N uptake,shoot N%,respectively,but a negative correlation with NUEb.In addition,root respiration rate,root biomass and heat per unit root length,root carbon and nitrogen content per unit root length were also remarkably positively correlated with total N uptake and shoot N%,respectively,but negatively correlated with NUEb,respectively.In conclusion,reducing root carbon and nitrogen costs had disadvantage of nitrogen uptake and shoot N% after low N treatment,and rice varieties with bigger root system had higher root construction cost,but bigger decreasing amplitude of root carbon and nitrogen costs,and finally showed higher nitrogen absorption efficiency.