Responses of Root and Leaf NR Activity in Wheat Seedling to Elevated CO_2 Concentration under Different N Levels

Hydroponics was employed to investigate the responses of root growth and organic carbon excretion,leaf NR activity in wheat seedling to elevated CO2 concentration.The seedlings of wheat variety 'Xiaoyan 22' were planted in open top chamber(OTCs) under two atmospheric CO2 concentrations(375 and 750 μmol·mol-1) and 4 nitrogen levels(2.5,5.0,10.0,15.0 mmol·L-1).The biomass of root and shoot,root length,area and organic carbon excretion rate,and leaf NR activity in wheat seedling were measured.The results showed that compared with the background CO2 concentration,elevated CO2 increased biomass of root and shoot,root length and area,root organic carbon excretion rate and content,and leaf NR activity in wheat seedling at one-leaf,two-leaf and three-leaf stages under 4 nitrogen levels,but it enhanced the ratio of root/shoot only under 10.0 mmol·L-1 N level at two-leaf and three-leaf stages.With the increase of nitrogen levels,biomass of root and shoot,root length and area,leaf NR activity were increased,while root organic carbon excretion rate were deceased,the change of root/shoot ratio was different at three stages.The ratio of root/shoot decreased with increasing nitrogen levels at one-leaf stage and obtained a higher value under 15.0 and 10.0 mmol·L-1 N level at two-leaf and three-leaf stages,respectively.The effect of different levels on NR activity was more significant at one-leaf stage.Therefore,Enhanced CO2 concentration would promote root and shoot growth,root organic carbon excretion and leaf NR activity.Increasing nitrogen level could improve the growth of root and shoot,nitrogen assimilation and root/shoot ratio in the CO2 enrichment,and reduce carbon loss of root for organic carbon excretion.