Abstract: [Objective]Citrus Sinensis×Poncirus trifoliata is one of the main citrus rootstocks. Since the species has resistance to citrus tristeza and other good traits, it is used widely in citrus cultivation. This study aims to explore the nitrogen uptake and utilization mechanism of C.sinensis×P.trifoliata further. In addition, it aims to provide scientific basis for improving nitrogen use efficiency and scientific fertilization in citrus production.[Method]C.sinensis×P.trifoliata plants, the experiment material, were grown in hydroponic culture based on Hoagland solution with adjustment in different nitrogen forms. The effect of different ratios of nitrogen forms on the seedling root growth was studied. Besides, the nitrate, and ammonium absorption kinetics characteristics were analyzed in this experiment.[Result]The results showed that the taproot length, the number of lateral roots and root activity were increased first and then decreased as the NH₄⁺ percentage increasing, and mixture of different nitrogen forms promoted taproot length than individual nitrogen forms. A certain ratio of ammonium induced lateral root formation. There was no significant difference in lateral root growth among the treatments of all nitrate, all ammonium and treatment 4(NO₃^-:NH₄⁺=3:7).Treatment 2(NO₃^-:NH₄⁺=7:3) was optimal for root activity, treatment 1(NO₃^-:NH₄⁺=10:0) and 3(NO₃^-:NH₄⁺=5:5), were the second and all ammonium treatment was least. Treatment 2(NO₃^-:NH₄⁺=7:3) was the optimal treatment forroot growth and root activity increasing, and treatment 3(NO₃^-:NH₄⁺=5:5)was the second. As for NO₃^- uptake, the parameter of V_m, K_m and α value were all raised at first time and then reduced as the NH₄⁺ percentage increasing, and the V_m, K_m and α value of treatment 2(NO₃^-:NH₄⁺=7:3) were prominently higher than those of the other treatments. For NH₄⁺ uptake,the parameters of V_m and K_m were all raised at first time and then reduced,but α valuechanged in more complex. The V_m of treatment 2(NO₃^-:NH₄⁺=7:3) was higher than others notably, and the K_m was lower than treatment 3(NO₃^-:NH₄⁺=5:5), but higher than treatment 4(NO₃^-:NH₄⁺=3:7)and all ammonium treatment. However, the α value had no difference from all ammonium treatment, but higher than others.The results indicated that the treatment 2 had a preference and competition for NH₄⁺ and NO₃^-.[Conclusion]The all treatments showed that V_mNO₃^- > V_mNH₄⁺ and K_mNO₃^- > K_mNH₄⁺, indicating that C.sinensis×P.trifoliata would be fond of mixed nitrogen and need more nitrate than ammonium.

Key words: Citrus sinensis×Poncirus trifoliata, root morphology, nitrate, ammonium, uptake kinetics