RESPONSES OF RESPIRATION TO INCREASES IN CARBON-DIOXIDE CONCENTRATIONAND TEMPERATURE IN 3 SOYBEAN CULTIVARS

The purpose of this experiment was to determine how respiration of soy beans may respond to potential increases in atmospheric carbon dioxide concentration and growth temperature. Three cultivars of soybeans (Gl ycine max L. Merr.), from maturity groups 00, IV, and VIII, were grown at 370, 555 and 740 cm(3) m(-3) carbon dioxide concentrations at 20/1 5, 25/20, and 31/26 degrees C day/night temperatures. Rates of carbon dioxide efflux in the dark were measured for whole plants several time s during exponential growth. These measurements were made at the night temperature and the carbon dioxide concentra:ion at which the plants were grown. For the lowest and highest temperature treatments, the sho rt term response of respiration rate to measurement at the three growt h carbon dioxide concentrations was also determined. Elemental analysi s of the tissue was used to estimate the growth conversion efficiency. This was combined with the observed relative growth rates to estimate growth respiration. Maintenance respiration was estimated as the diff erence between growth respiration and total respiration. Respiration r ates were generally sensitive to short term changes in the measurement carbon dioxide concentration for plants grown at the lowest, but not the highest carbon dioxide concentration. At all temperatures, growth at elevated carbon dioxide concentrations decreased total respiration measured at the growth concentration, with no significant differences among cultivars. Total respiration increased very little with increasi ng growth temperature, despite an increase in relative growth rate. Gr owth respiration was not affected by carbon dioxide treatment at any t emperature, but increased with temperature because of the increase in relative growth rare. Values calculated for maintenance respiration de creased with increasing carbon dioxide concentration and also decrease d with increasing temperature. Calculated values of maintenance respir ation were sometimes zero or negative at the warmer temperatures. This suggests that respiration rates measured in the dark may not have ref lected average 24-h rates of energy use. The results indicate that inc reasing astmospheric carbon dioxide concentration may reduce respirati on in soybeans, and respiration may be insensitive to climate warming. (C) 1996 Annals of Botany Company