Direct, continuous measurements of the CO2 evolution of root nodules, calibrated by direct measurements of rate of ethylene production, were utilized to determine the short-term response of nitrogenase activity to defoliation and photosynthesis in white clover. Defoliation (removal of all expanded leaflets) generally resulted in a fall in nodulated root respiration within 10 min; most respiration associated with nitrogenase activity ceased within 1–2 h. Darkening of the shoot also reduced nodulated root respiration within 10 min, but the subsequent fall in respiration, although of the same magnitude, was slower. The re-illumination of shoots largely reversed these effects. The inhibition of photosynthesis by DCMU largely simulated the effects of darkening the shoots. It is concluded that, in these white clover plants of 100 mg to 2.0 g total weight, current photosynthate provides the primary source of energy for N2 fixation. The mobilization of reserve energy substrate appeared to play only a small role. The minimum time interval of 10 min between onset of treatment and fall in nodule respiration probably reflects the time taken to exhaust the assimilate in transit between leaf and nodule.
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Ryle et al. (1985) studied this question.