Carbon Balance as a function of Plant Size in Sorghum Plants1

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Abstract
In young plants, the daily respiratory loss rate is expected to be almost proportional to the daily photosynthetic input, since photosynthate is being converted to new biomass with constant efficiency and maintenance losses are small. As plants increase in size, the maintenance requirement is thought to increase in proportion to the biomass, while the photosynthetic input also increases, but at a slower rate due to mutual shading of leaves. Thus, the maintenance requirement plays an increasingly greater role and may dominate the daily carbon balances of large plants. The photosynthetic input, conversion efficiency and maintenance requirement of vegetative sorghum plants [Sorghum bicolor (L.) Moench ‘RS 610’] were determined as a function plant size in plants growing under constant conditions (28°C and 1000 μmol s−1 m−2 of photosynthetically active radiation for 12 h). Although the sorghum plants grew much faster than field‐grown plants of the same cultivar, the morphology and growth characteristics were shown to be very similar when compared at the same size of plant. The maintenance requirement per unit biomass decreased with increasing plant size, from 26 mgC gC−1 day−1 for the smallest plants to 20 mgC gC−1 day−1 for plants with a biomass of 19 gC (50g dry weight). The protein content decreased in about the same proportion (from 29 to 20% protein). The average maintenance requirement per gram of protein was 39 mgC g protein−1 day,−1 which is considerably less than that of white clover (Trifolium repens L.) growing under the same conditions (60 mgC g protein−1 day−1). It is suggested that a smaller fraction of the protein is degradable in the sorghum than in the clover. The ontogenetic changes in maintenance requirement found in this experiment were relatively small, considering the wide range of plant sizes studied. Very little error would be introduced by extrapolating from data taken on moderately‐ sized plants. However, these data should be taken over a range of different growth conditions, since it has already been shown that the maintenance requirement per unit biomass is a function of the growth rate, as it is affected by environmental conditions such as temperature, light and soil moisture.