The effect of high levels of carbon dioxide on dark respiration and growth of plants

Abstract Raising ambient levels of CO₂ during the night, between 350 and 950cm³m⁻³, reduced the dark respiration rate of Medicago sativum seedlings. The percentage effect was greater for maintenance respiration than for dark respiration as a whole, and when the plants were in a low photosynthate status. Twenty‐four h carbon balance studies confirmed a reduction in night time respiration and an increase of net carbon gain when night time [CO₂] was high. Growth experiments showed a small but significant increase of dry weight in Medicago sativum seedlings exposed to high [CO₂] (∼ 1200 cm³m⁻³) at night. This effect was greater for plants grown with Rhizobium nodules than for plants grown with nitrate in the absence of Rhizobium. A similar, but smaller and statistically non‐significant effect of high night time [CO₂] on growth was found for Xanthium strumarium seedlings. The significance of these findings is discussed in relation to the rising CO₂ content of the atmosphere.