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Ontogenetic Changes in the Respiration Coefficients of Grain Sorghum

Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843-2474

^* Corresponding author.

The daily carbon balance of a plant is determined by the gross input of C, the efficiency of new biomass, and the maintenance requirement of existing biomass. According to models of C utilization in plants, the synthesis efficiency (or yield of growth, Y₈) should depend on the nature of the product being synthesized, while the maintenance requirement per unit biomass (m) should depend mainly on the concentration and turnoverate of the protein in existing biomass. Values of Y₈ and m were determined throughout the ontogeny of grain sorghum [Sorghum bicolor (L.) Moench]. Individual plants were grown and tested under controlled environment conditions (30°C, high light). Carbon balance determinations were made from 28 to 100 d after emergence. The value of Y₈ was found to increase from 0.70 g C.g C^–1 to 0.78 g C.g C^–1, while m decreased from 24 mg C.g C^–1.d^–1 for young plants to 6 mg C.g C^–1 .d^–1 for mature plants. It is suggested that the changes in Yg reflect changes in the composition of the end products of biosynthesis. Protein concentration was found to decrease with age. The efficiency of synthesis of proteins is less than that of the carbohydrates that make up the bulk of plant tissue. A decrease in protein synthesis could therefore explain the increase in Y₈. The decrease in m after anthesis was much greater than the decrease in protein concentration. It therefore seems likely that a large fraction of the protein in the plants during grain filling was structural or storage protein that was not being rapidly turned over.

Key Words: Crop growth models • Carbon balance • Synthesis • Maintenance • Sorghum bicolor (L.) Moench

Received for publication March 3, 1987.