Heim I., Hartwig U. A., Nosberger J.
Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland.
Plant Physiol. 1993 Nov;103(3):1009-1014. doi: 10.1104/pp.103.3.1009.
Previous studies have shown that nitrogenase activity decreases dramatically after defoliation, presumably because of an increase in the O2 diffusion resistance in the infected nodules. It is not known how this O2 diffusion resistance is regulated. The aim of this study was to test the hypothesis that current N2 fixation (ongoing flux of N2 through nitrogenase) is involved in the regulation of nitrogenase activity in white clover (Trifolium repens L. cv Ladino) nodules. We compared the nitrogenase activity of plants that were prevented from fixing N2 (by continuous exposure of their nodulated root system to an Ar:O2 [80:20] atmosphere) with that of plants allowed to fix N2 (those exposed to N2:O2, 80:20). Nitrogenase activity was determined as the amount of H2 evolved under Ar:O2. An open flow system was used. In experiment I, 6 h after complete defoliation and the continuous prevention of N2 fixation, nitrogenase activity was higher by a factor of 2 compared with that in plants allowed to fix N2 after leaf removal. This higher nitrogenase activity was associated with a lower O2 limitation (measured as the partial pressure of O2 required for highest nitrogenase activity). In experiment II, the nitrogenase activity of plants prevented from fixing N2 for 2 h before leaf removal showed no response to defoliation. The extent to which nitrogenase activity responded to defoliation was different in plants allowed to fix N2 and those that were prevented from doing so in both experiments. This leads to the conclusion that current N2 fixation is directly involved in the regulation of nitrogenase activity. It is suggested that an N feedback mechanism triggers such a response as a result of the loss of the plant's N sink strength after defoliation. This concept offers an alternative to other hypotheses (e.g. interruption of current photosynthesis, carbohydrate deprivation) that have been proposed to explain the immediate decrease in nitrogenase activity after defoliation.
以往的研究表明，去叶后固氮酶活性会急剧下降，推测这是由于受侵染根瘤中氧气扩散阻力增加所致。目前尚不清楚这种氧气扩散阻力是如何调节的。本研究的目的是检验以下假设：当前的固氮作用（通过固氮酶的持续氮气通量）参与白三叶草（白车轴草L. cv Ladino）根瘤中固氮酶活性的调节。我们将阻止固氮的植株（通过使其结瘤根系持续暴露于氩气：氧气[80:20]的气氛中）与允许固氮的植株（暴露于氮气：氧气，80:20）的固氮酶活性进行了比较。固氮酶活性通过在氩气：氧气条件下释放的氢气量来测定。采用了开放流动系统。在实验I中，完全去叶并持续阻止固氮6小时后，与去叶后允许固氮的植株相比，固氮酶活性高出2倍。这种较高的固氮酶活性与较低的氧气限制相关（以最高固氮酶活性所需的氧气分压来衡量）。在实验II中，在去叶前阻止固氮2小时的植株的固氮酶活性对去叶没有反应。在两个实验中，允许固氮的植株和阻止固氮的植株中，固氮酶活性对去叶的反应程度不同。这得出结论：当前的固氮作用直接参与固氮酶活性的调节。有人认为，由于去叶后植物氮库强度的丧失，氮反馈机制引发了这种反应。这一概念为解释去叶后固氮酶活性立即下降而提出的其他假设（例如当前光合作用的中断、碳水化合物缺乏）提供了一种替代方案。
