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

Previous work has shown that elevated carbon dioxide (CO2) concentrati ons in the dark reversibly reduce the rate of CO2 efflux from soybeans . Experiments were performed exposing soybean plants continually to co ncentrations of 350 or 700 cm(3) m(-3) for 24 h d(-1), or to 350 durin g the day and 700 cm(3) m(-3) at night, in order to determine the impo rtance of the reduced rate of dark CO2 efflux for plant growth. High C O2 applied only at night conserved carbon and increased dry mass durin g initial growth compared with the constant 350 cm(3) m-3 treatment. L ong-term net assimilation rate was increased by high CO2 in the dark, without any increase in daytime leaf photosynthesis. However, leaf are a 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 th an 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 co mponent of respiration was reduced by the elevated CO2 concentration i n the dark.