摘 要：
硝化作用是导致农田氮素损失的一个重要原因,自然界中一些植物根系能分泌抑制土壤氨氧化细菌的物质,统称为生物硝化抑制剂。高粱根系分泌的对羟基苯丙酸(简称MHPP),具有很好的抑制效果。为了研究MHPP的分泌机制,本实验用铵态氮与硝态氮两种不同氮素形态水培高粱幼苗,在一定时间内收集根系分泌物。将根系分泌物通过高效液相色谱法(HPLC)测定其中的MHPP含量,通过微量滴定法测定氢离子含量。同时,将根系采样后,用两相法分离根系细胞膜,测定质子泵活性。结果表明,在铵态氮营养下,高粱分泌MHPP的速率在3.52μmol g-1 d-1左右,而硝态氮营养下几乎不分泌MHPP。在铵态氮营养下,根系分泌氢离子的速率为16.49μmol g-1 d-1,而硝态氮营养下没有氢离子的分泌。而且,随着根系分泌物收集液中铵离子浓度的增加,也促进了根系分泌MHPP和氢离子的速率。因此,铵态氮营养促进了根系分泌MHPP。进一步研究发现,虽然在表观上MHPP的分泌受氮素形态影响,但是在这一过程中MHPP分泌与氢离子的分泌速率是偶合的。而细胞分泌氢离子是由细胞膜上质子泵活性决定的。因此,本实验结果发现,MHPP的分泌实质上是通过根系细胞膜质子泵活性来调控的:在没有氮源的情况下,只要通过壳梭孢菌素(Fusicoccin)或钒酸盐(Vanadate)改变质子泵活性就可以影响MHPP的分泌。所有实验数据的统计结果发现,MHPP分泌与质子分泌之间的比例大约为1∶4,且相关系数r=0.98。因此,铵态氮营养下高粱根系大量分泌生物硝化抑制剂MHPP在一定程度上是由于铵离子的吸收刺激了质子泵活性提高所引起的,这也是防止根际铵态氮被氧化、提高氮素利用率的一个重要生理机制。
译 名：
RELATIONSHIP BETWEEN EXUDATION OF NITRIFICATION INHIBITOR MHPP AND PLASMA MEMBRANE PROTON PUMP OF SORGHUM ROOT
作 者：
Zhou Jinqua;Zhang Mingchao;Wei Zhijun;Hu Jun;Zhu Yiyong;College of Resources and Environmental Sciences,Nanjing Agricultural University;
关键词：
Sorghum;;Nitrification inhibitor;;Plasma membrane H+ ATPase
摘 要：
Nitrification is an important biochemical process in the soil,and a cause of soil nitrogen loss. In nature,some plant species can exude from their roots some substances that may inhibit the activity of soil ammonia oxidizing bacteria. This kind of nitrification inhibitory substances is generally referred to as biological nitrification inhibitor(BNI). Sorghum is such a plant species that exudes from its roots methyl hydroxy phenyl propionate(MHPP),which has great inhibitory effect on soil nitrification. To explore underlying mechanism of sorghum roots exuding MHPP,sorghum seedlings were cultivated in hydroponic systems using ammonium and nitrate,separately. Root exudates were collected after a given period of time for analysis of MHPP content with a High Performance Liquid Chromatograph(HPLC)and hydrion content with the micro-titration method. After the collection of root exudates,Sorghum roots were also collected for isolation of plasma membrane of root cells with the two phase method for determination of H+ ATPase activity. Results show that in the hydroponic system with ammonium sorghum roots exuded MHPP at a rate of about 3.52 μmol g-1 root FW d-1 and proton at a rate of 16.49 μmol g-1 d-1,while in the other,they did almost zero. And in the former,sorghum roots exuded,while in the latter they did almost neither. Moreover,with increasing concentration of ammonium in the hydroponic medium,roots exudation of MHPP and proton increased in rate,too. So,it is obvious that ammonium nitrogen promotes release of MHPP from sorghum roots. However,it was found in further study that although apparently the release of MHPP from sorghum roots was affected by form of nitrogen,the exudation of MHPP was always correlated to that of hydrions in the process,and root cells exuding proton depended on activity of the plasma membrane H+pump. It was also found in the study that MHPP release from sorghum roots was actually regulated by plasma membrane H+ ATPase activity. In the absence of nitrogen sources, it is feasible to use fusicoccin or vanadate to affect proton pump activity and hence release of MHPP from sorghum roots. Statistic analysis of all the data acquired in the experiment shows that the release of MHPP and proton was nearly at a ratio of 1∶4 and their correlation coefficient r was 0.98. Therefore,it can be held that in the hydroponic system using ammonium nitrogen the secretion of a large volume of MHPP from sorghum roots is to a certain extent induced by enhanced plasma membrane H+ ATPase activity resulting from absorption of ammonium ion,which can be deemed as an important physiological mechanism that can be used to prevent ammonium oxidation in rhizosphere and improve nitrogen utilization efficiency.
