Growth, Maintenance and Nitrogen Fixation of Nodulated Plants of Subterranean Clover (Trifolium subterraneum L.)

Abstract

Plants were grown in a nitrogen-free medium in pots to form a closed canopy. Over a 50-day period, the net CO₂ exchange of the pot was measured; nitrogen fixation was estimated by acetylene reduction assay; chemical analyses were performed; and dry matter was determined. Growth and maintenance requirements were estimated in terms of CO₂. Dry matter growth rate was almost constant with time, the value half-way through the experiment agreeing with an estimate calculated from the daily CO₂-exchange rate and 37.4% carbon in the biomass. Nitrogen accumulation rate was nearly constant. The total daytime CO₂ influx for thirteen successive pots during the experiment was almost the same. By contrast, the total dark CO₂ efflux increased with time. After partitioning into growth and maintenance components, the increase was due to increase in maintenance. The proportion of the dark efflux attributed to growth was constant with increase in biomass over the range 200-600 gm^-2 but the maintenance coefficient may have decreased slightly over this range. Nitrogen fixation rate was constant during the growth period and reproducible between pots: 2.86 moles acetylene were concluded to be equivalent to each mole of nitrogen fixed. As CO₂ efflux decreased to the maintenance level in a dark period longer than the usual 12 h, acetylene reduction decreased to near zero. This suggests a close association between growth rate and the rate of nitrogen fixation.