^(15)N短时标记法鉴定烟草‘K326’和‘红花大金元’吸收NO3~–和NH4~＋的生理动力学特征

Physiological characterization of NO_3^- and NH_4^+ uptake kinetics of tobacco 'K326' and 'Honghuadajinyuan' using a short-time ^(15)N-substrate labeling approach

本文运用营养液培养方法、^(15)N底物标记及同位素质谱测定技术等,首先鉴定了两烟草(Mcortma品种(‘K326'和‘红花大金元')吸收NO_3^-及NH_4^+而不发生向地上部转移的最短时间,继而对其吸收的系统组成进行了较精确的测定,发现两品种吸收氮素至转移到地上部仅在数分钟之内;根细胞吸收N至少有两类体系。在N浓度≤1 mmol·L^(-1)时,根系吸收途径具有酶促反应特征;吸收NO_3^-的米氏常数(X_M)值(‘K326':84.5 mmol·L^(-1);‘红花大金元':47.8μmol·L^(-1))明显低于对NH_4^+的K_M值(‘K1326':93.5 mmol·L^(-1);‘红花大金元':90.4 mmol·L^(-1));‘红花大金元'较低的K_M值可能是该品种较‘K326'耐低N的生理原因之一。外界N浓度>1 mmol·L^(-1)时,根系对N的吸收量与外界N浓度呈正相关;‘K326'对NO_3^-的吸收速率是‘红花大金元'的2倍以上,对NH_4^+的吸收率则为‘红花大金元'的约2.4倍;‘K326'对NO_3^-的吸收速率约为对NH_4^+吸收的3.6倍,而‘红花大金元'对NO_3^-的吸收速率约为对NH_4^+吸收速率的1/2。本研究结果对探索烟草根系吸收氮素的相关分子机理提供了生理学依据。

In this article,by using a nutrient-hydroponic culture method,^（15）N-substrate labeling and mass spectrum-based element measurement techniques,we identified firstly a minimum time when N was not transported to upper parts of two tobacco（Nicotiana tabacum） varieties after their root uptake of NH4~＋and NO3^-~,and subsequently characterized processes and components of N absorption by roots.Our data show 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 should 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 μmol·L^-1;＇Honghuadajinyuan＇（＇HD＇）：47.8 μmol·L^-1]for NO3^-absorbed by the roots were significantly lower than that for NH4~＋（＇K326＇：93.5 μmol·L^-1;＇HD＇：90.4 μmol·L^-1）,suggesting that the lower KM 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 N1 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＇,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 higher than for NH4~＋,and ＇HD＇ for NO3^-was half as many as for NH4~＋.Our results may provide physiological evidence for further exploration of molecular mechanisms of N uptake by the roots of tobacco plants.