Maintenance Requirements of White Clover at High and Low Growth Rates

Plants use a considerable fraction of their daily income of carbon from photosynthesis to maintain existing biomass. In recent carbon balance studies, it has become apparent that the maintenance requirement decreases with decreasing growth rate of the plant. The objective of this study was to determine extreme values of the maintenance requirement for one species growing at high and low growth rates. Stands of white clover (Trifolium repens L.) were grown in a whole-plant assimilation chamber where their daily carbon balance could be determined. After several days in high light conditions, during which the growth rate was 30 g. dry matter·m ⁻²· day⁻¹, the light level was reduduced so that the plants were in a maintenance condition (zero growth rate). They were kept this condition for 27 days. The maintenance requirement fell gradually from 39 to 23 mgC·gC⁻¹· day⁻¹, while the growth conversion efficiency remained constant at 0.67 gC·gC⁻¹. The protein content also remained constant at 27 %. The experimental results were interpreted in terms of Thornley's dynamic model of plant growth. The model was modified to include starch storage, along lines suggested by Challa. Good agreement with the experimental results for the high growth condition was obtained with 40 % of current photosynthate going into starch storage during the daytime and out of it at night. The model was also able to mimic the long-term response to zero growth conditions when it was assumed that the degradable part of the biomass degraded at a rate of 0.83 g·g⁻¹·ᵈᵃʸ⁻¹,and that the yield of degradable biomass carbon per unit of carbon available was 0.38 g·g⁻¹ at the high growth rate, increasing to 0.54 g·g⁻¹ at the zero growth rate. This latter assumption was justified on the grounds that more of the degradable protein would be synthesized from recycled amino acids in the zero growth rate condition, with consequent reductions in carbon costs. In the simulatlons, the degradable fraction fell from 9 to 6 % of the biomass over the first 10 days of the zero growth condition.