Physiologoical Investigation Of No₃^-and Nh₄⁺ Acquisition And Utilization Of Tobacco K326 And Honghuadajingyuan

K326 and HD are major tobacco varieties in China,whose nitrogen?N?-fertilizer input with obviously different application rates?K326 tolerant/adptive to higher N but HD to lower N?.To understand primary components involved in tobacco N-use physiology,we adopted these two cultivars as valuable genetic materials to assess their growth response to varied N regimes under hydroponic culture condition,in which plants were grown till 8-leaf stage with a supply of NO₃^-or NH₄⁺or NH₄NO₃ as N source at a low,middle and high concentration(LN,0.5 mmol·L^-1;MN,2 mmol·L^-1;HN,10 mmol·L^-1),respectively.Plant biomass,total nitrogen,ammonium,nitrate,arginine,GS and NR activity,nitrogen transfer-and use-efficiency as well as root uptake by using ¹⁵N-labelling were examined.Our main results are summarized as below:1)K326 and HD utilize preferentially NO₃^-or NH₄NO₃ but not NH₄⁺only,with 2mmol·L^-1 N supply being probably enough and economical to achieve their good biomass production at a vegetative stage.2)When using NO₃^-or NH₄NO₃ as N source,HD plants exhibited higher N-use and-transfer efficiency under lower N supply;under higher N,both K326 and HD showed a decreased N-use efficiency,but K326 had a higher N-transfer efficiency with a production of more upper part biomass.3)Both varieties were found to be strongly sensitive to external ammonium,due to perhaps lack of or abnormal signalling?s?related to nitrate and/or arginine rather than an impairment of N acquisition and initial assimilation;this interpretation could be supported by the analysis of the content of plant N,ammonium and activities of GS and NR.4)We revealed that the time for the movement of N from its uptake by the roots to the upper parts of the two varieties happened within only several minutes,and that at least two different systems for the N uptake exist in the tobacco roots.When external N concentrations were?1 mmol·L^-1,the process of the root N-uptake displayed an enzymatic action property;Km values(i.e.K326:84.5?mol·L^-1,HD:47.8?mol·L^-1)for NO₃^-absorbed by the roots were significantly lower than that for NH₄⁺(K326:93.5?mol·L^-1,HD:90.4?mol·L^-1).Thus,the lower Km values for NH₄⁺or NO₃^-uptake by HD would be one of critical physiological factors for a better tolerance of HD to low N than that of K326.As external N?1 mmol·L^-1,N acquisition by the roots was positively proportional to external N concentrations;the rate of NO₃^-uptake by K326 was more than 2-fold higher than that of by HD?being one of possible physiological reasons for K326 to adapt to higher N e.g.NO₃^-?,and the rate of NH₄⁺uptake by K326 was 2.4-fold lower than that of by HD;the uptake rate of K326 for NO₃^-was 3.6-fold as many as that for NH₄⁺,and the uptake rate of HD for NO₃^-was 1/2-fold as many as that for NH₄⁺.Taken together,in this study we explored and identified physiological properties involved in nitrogen acquisition and utilization of two tocacco varieties,leading to a suggestion that the growth adaptation of K326 and HD to high-or lower-N may be ascribed to their different competences for the effective N uptake/translocation as well as assimilation.Thus,the present work would provide valuable hints availing to prompt a deep investigation of molecular/genetic bases controlling plant nitrogen use efficiency,as well as fundamental knowledge useful for bettering fertilization practice in tobacco cultivars' cultivation.