N2 Fixation, Carbon Metabolism, and Oxidative Damage in Nodules of Dark-Stressed Common Bean Plants.

Common beans (Phaseolus vulgaris L.) were exposed to continuous darkness to induce nodule senescence, and several nodule parameters were investigated to identify factors that may be involved in the initial loss of N2 fixation. After only 1 d of darkness, total root respiration decreased by 76% and in vivo nitrogenase (N2ase) activity decreased by 95%. This decline coincided with the almost complete depletion (97%) of sucrose and fructose in nodules. At this stage, the O2 concentration in the infected zone increased to 1%, which may be sufficient to inactivate N2ase; however, key enzymes of carbon and nitrogen metabolism were still active. After 2 d of dark stress there was a significant decrease in the level of N2ase proteins and in the activities of enzymes involved in carbon and nitrogen assimilation. However, the general collapse of nodule metabolism occurred only after 4 d of stress, with a large decline in leghemoglobin and antioxidants. At this final senescent stage, there was an accumulation of oxidatively modified proteins. This oxidative stress may have originated from the decrease in antioxidant defenses and from the Fe-catalyzed generation of activated oxygen due to the increased availability of catalytic Fe and O2 in the infected region.

Reversible o(2) inhibition of nitrogenase activity in attached soybean nodules.

Various forms of stress result in decreased O(2) permeability or decreased capacity to consume O(2) in legume root nodules. These changes alter the nodule interior O(2) concentration (O(i)). To determine the relationship between O(i) and nitrogenase activity in attached soybean (Glycine max) nodules, we controlled O(i) by varying external pO(2) while monitoring internal H(2) concentration (H(i)) with microelectrodes. O(i) was monitored by noninvasive leghemoglobin spectrophotometry (nodule oximetry). After each step-change in O(i), H(i) approached a new steady state, with a time constant averaging 23 s. The rate of H(2) production by nitrogenase was calculated as the product of H(i), nodule surface area, and nodule H(2) permeability. H(2) permeability was estimated from O(2) permeability (measured by nodule oximetry) by assuming diffusion through air-filled pores; support for this assumption is presented. O(i) was nearly optimal for nitrogenase activity (H(2) production) between 15 and 150 nm. A 1- to 2-min exposure to elevated external pO(2) (40-100 kPa) reduced H(i) to zero, but nitrogenase activity recovered quickly under air, often in <20 min. This rapid recovery contrasts with previous reports of much slower recovery with longer exposures to elevated pO(2). The mechanism of nitrogenase inhibition may differ between brief and prolonged O(2) exposures.