STUDIES ON THE RELATION BETWEEN NET PHOTOSYNTHESIS AND NITROGENASE-LINKED RESPIRATION IN SUBTERRANEAN CLOVER

The relation between the rate of nitrogenase-linked respiration and net photosynthesis, and the effect of defoliation on this relation, was studied in plants of subterranean clover (Trifolium subterraneum L. cv. Seaton Park). Nitrogenase-linked respiration was estimated as the difference between the rate of nodulated root respiration at 21% O2 and at 3% O2.The level to which the rate of nitrogenase-linked respiration fell several hours after defoliation was directly proportional to the decline in the rate of net photosynthesis. Approximately 9% of net photosynthesis was always expended in nitrogenase activity, irrespective of whether or not the plants were defoliated. This proportion was maintained during the first 3 d of regrowth.To determine whether the decline in nitrogenase-linked respiration after defoliation was due solely to the decline in the rate of photosynthesis, a further experiment was conducted in which the pre-defoliation rate of net photosynthesis was restored immediately (with supplementary light) or within 5 min (supplementary light and CO2) after defoliation. Restoring the rate of net photosynthesis did not prevent the post-defoliation decline in nitrogenase-linked respiration. However, when photosynthesis was reduced to zero by the imposition of darkness, and the rate of nitrogenase-linked respiration allowed to decline to a steady rate after 3 h, a rapid recovery in the rate of nodulated root respiration began within 2 h of returning the plants to the light. It was hypothesized that a 'shoot factor', which was affected by defoliation, could override the apparent relation between nitrogenase-linked respiration and the rate of current photosynthesis. © 1990 Oxford University Press.