EFFECTS OF ELEVATED CARBON-DIOXIDE CONCENTRATION IN THE DARK ON THE GROWTH OF SOYBEAN SEEDLINGS

Previous work has shown that elevated carbon dioxide (CO2) concentrations in the dark reversibly reduce the rate of CO2 efflux from soybeans. Experiments were performed exposing soybean plants continually to concentrations of 350 or 700 cm(3) m(-3) for 24 h d(-1), or to 350 during the day and 700 cm(3) m(-3) at night, in order to determine the importance of the reduced rate of dark CO2 efflux for plant growth. High CO2 applied only at night conserved carbon and increased dry mass during initial growth compared with the constant 350 cm(3) m-3 treatment. Long-term net assimilation rate was increased by high CO2 in the dark, without any increase in daytime leaf photosynthesis. However, leaf area ratio was reduced by the dark CO2 treatment to values equal to those of plants continually exposed to the higher concentration. From days 14-21, leaf area was less for the elevated night-time CO2 treatment than for either the constant 350 or 700 cm(3) m(-3) treatments. For the day 7-21-period, relative growth rate was significantly reduced by the high night CO2 treatment compared with the 350 cm(3) m(-3) continuous treatment. The results indicate that some functionally significant component of respiration was reduced by the elevated CO2 concentration in the dark.