Regrowth by Swards of Subterranean Clover after Defoliation. 2. Carbon Exchange in Shoot, Root and Nodule

The carbon economy of subterranean clover swards subjected tothree defoliation treatments (removal of 30, 70 and 80% of shootdry weight) was compared with that of uncut swards. Carbon dioxideexchange in shoots and roots was measured independently 0, 4,8 and 12 d after defoliation. The respiration linked to nitrogenaseactivity was estimated by comparing root respiration measuredin an atmosphere containing 3% oxygen with the respiration in21% oxygen. Net photosynthesis fell by up to 100% immediately after defoliation.There was a decline of over 60percnt; in root respiration bythe end of the first light period, composed of a rapid declineof 70% in nitrogenase-linked respiration in all treatments anda slower decline of nearly 40% in root plus nodule growth andmaintenance respiration in the more severe treatments. Recoveryof net photosynthesis to rates achieved by uncut swards occurredover 4 d in the 30% cut treatment and at least 12 d in moresevere treatments. Whilst recovery of photosynthesis was theprinciple determinant of recovery of net positive carbon balance,the early reduction in respiration facilitated this outcome.After the immediate decline in nitrogenase-linked respiration,recovery in this component of respiration appeared to be linkedwith the recovery in net photosynthesis (approximately 10% ofnet photosynthesis). Carbon budgets revealed priorities in allocation towards leafin the first 5 d and later also towards root growth in severelydefoliated swards. Root respiration comprised a large respiratorycost (up to 75% of net photosynthesis) during early regrowth. Carbon budget, defoliation, N₂ fixation, photosynthesis, regrowth, respiration, subterranean clover, Trifolium subterraneum L