DEFOLIATION IN WHITE CLOVER - NODULE METABOLISM, NODULE GROWTH AND MAINTENANCE, AND NITROGENASE FUNCTIONING DURING GROWTH AND REGROWTH

In two experiments, the functioning and metabolism of nodules of white clover, following a defoliation which removed about half the shoot tissue, were compared with those of undefoliated plants. In one experiment, the specific respiration rates of nodules from undefoliated plants varied between 1160 and 1830 .mu.mol CO2 g-1 h-1, of which nodule ''growth and maintenance'' accounted for 22 .+-. 2 per cent, or 27 .+-. 3.6 per cent, according to method of calculation. Defoliation reduced specific nodule respiration and nodule ''growth and maintenance'' respiration by 60-70 per cent, and rate of N2 fixation by a similar proportion. The original rate of nodule metabolism was re-established after about 5 d of regrowth; during regrowth nodule respiration was quantitatively related to rate of N2 fixation: fixation: 9.1 .mu.mol CO2 .mu.mol-1 N2. With the possible exception of nodules examined 24 h after defoliation, the efficiency of energy utilization in nitrogenase functioning in both experiments was the same in defoliated and undefoliated plants: 2.0 .+-. 0.1 .mu.mol CO2 .mu.mol-1 C2H4; similarly, there was no change in the efficiency of nitrogenase functioning as rate of N2 fixation increased with plant growth from 1 to 22 .mu.mol N2 per plant h-1. Exposure of nodulated white clover root systems to a 10 per cent acetylene gas mixture resulted in a sharp peak in rate of ethylene production after 1.5-2.5 min; subsequently, rate of ethylene production declined rapidly before stabilising after 0.5-1 h at a rate about 50 per cent of that initially observed. Regression of ''peak'' rate of ethylene production on rate of N2 fixation indicated a value of 2.9 .mu.mol C2H4 .mu.mol-1 N2, for rates of N2 fixation between 1 and 22 .mu.mol N2 per plant h-1. The relationships between nitrogenase respiration, acetylene reduction rates and N2 fixation rates are discussed.