Carbon Dioxide Concentration at Night Affects Translocation from Soybean Leaves

Shedding light on CO₂ at night

There has been extensive discussion about the effects on plants of elevated night-time CO₂ concentrations. Some investigators report an inhibitory effect on respiration, while others do not find this. If night-time respiration is inhibited, is this beneficial (e.g. less loss of photosynthetically fixed carbon) or harmful (e.g. less energy for essential processes)? The careful work of James Bunce (USDA-ARS, Beltsville, pp. 399-403) has helped to answer these questions. He studied the effects of night-time CO₂ concentrations on respiration and dry matter accumulation in soybean. The key to his success was the use of very many, very similar plants kept in a strictly controlled environment. In the day, plants were supplied with 350 micromol mol^-1 CO₂. At night, the concentration range was 220-1400 micromol mol^-1. The results are abundantly clear: increasing the CO₂ concentration led to a decrease in respiration rate (respiration rates at the highest CO₂ concentration were about 80 % of those at the lowest). Daytime photosynthesis was also inhibited by high night-time CO₂ concentrations, even though all plants were exposed to the same daytime CO₂ concentration. However, increasing night-time CO₂ concentrations also led to an increase in leaf dry matter, measured over a 3-d period, especially in leaf 2 (despite the inhibition of photosynthesis). Calculation of mean net CO₂ exchange rates during the day and night, coupled with measurement of increases in leaf dry matter, enabled the author to estimate translocation rates. Remarkably, the inhibition of translocation by increased CO₂ concentrations matched very closely the inhibition of respiration. The author neatly brings most of these data together by suggesting that the increases in leaf dry matter occur because of an inhibition of translocation, and the latter results from the inhibition of respiration by high CO₂ concentrations (less energy available to bring about translocation).