Carbon and Nitrogen Yield, and N2 Fixation in White Clover Plants Receiving Simulated Continuous Defoliation in Controlled Environments

Single plants of white clover grown in controlled environments, and dependent for nitrogen on N, fixation, were defoliated at 1 or 2 d intervals to 3, 2 and 1 expanded leaves per stolon (Expt 1), and to 1,0.5 (1 leaf on every alternate stolon) and 0 expanded leaves per stolon (Expt 2), for 43–50 days Plants adapted to severe defoliation by developing much smaller leaves with a slightly reduced specific leaf area, more stolons, a smaller proportion of weight in leaf, root and nodules and a greater proportion of weight in stolons. The daily yield (material removed by defoliation) of d. wt and nitrogen generally decreased with severity of defoliation, as did the residual plant weight. However, the ‘efficiency’ of yield (daily yield/residual weight × 100) of dry matter and nitrogen was greater in the most severely defoliated treatments, attaining a maximum of 5–6 % All plants adapted to the imposed defoliation regimes, however severe, with the result that even plants maintained without any fully expanded leaves invested a similar fraction of their metabolic resources in shoot and root as less severely defoliated plants, and continued to grow and fix N2, albeit at a very reduced rate of 1–2 mg Nd−11. The energetic cost of N2 fixation (acetylene reduction) remained constant in all treatments at 31 mole CO2 mole C2H4−1, but there was some evidence that rate of N2 fixation per unit of nodule weight declined in the most harshly defoliated treatment.