THE EFFECT OF CARBON-DIOXIDE CONCENTRATION ON RESPIRATION OF GROWING AND MATURE SOYBEAN LEAVES

Soybean plants were grown continuously at 350 and 700cm(3)m(-3) CO2 at constant temperature, Respiration rates of third trifoliolate leaves were measured at the growth CO2 concentration for the whole dark perio d from 5d before through to 5d after full area expansion, The short-te rm response of respiration rate to the measurement CO2 concentration w as also determined at each age, Respiration rates per unit of dry mass declined with age and were significantly less at a given age or RGR i n leaves grown and measured at the elevated CO2. The difference in res piration rate was largest in mature leaves and resulted from the diffe rent measurement CO2 concentrations, The respiratory costs of the tiss ue synthesis, estimated from the elemental composition of the tissue, did not differ substantially between CO2 treatments, The response of r espiration rate to carbon dioxide concentration was not strongly affec ted by the form of nitrogen supplied. Maintenance respiration calculat ed by subtracting growth respiration from total respiration was negati ve in rapidly growing leaves for both CO2 treatments, This indicates t hat CO2 efflux in the dark does not accurately reflect the average 24h rate of energy expenditure on growth and maintenance for soybean leav es.