Altered night-time CO2 concentration affects the growth, physiology and biochemistry of soybean

Soybean plants (Glycine max (L.) Merr. c.v. Williams) were grown in CO₂ controlled, natural-light growth chambers under one of four atmospheric CO₂ concentrations (CO₂]): (1) 250 μmol mol^–1 24 h d^–1250/250]; (2) 1000 μmol mol^–1 24 h d^–11000/1000]; (3) 250 μmol mol^–1 during daylight hours and 1000 μmol mol^–1 during night-time hours 250/1000] or (4) 1000 μmol mol^–1 during daylight hours and 250 μmol mol^–1 during night-time hours 1000/250]. During the vegetative growth phase few physiological differences were observed between plants exposed to a constant 24 h CO₂] (250/250 and 1000/1000) and those that were switched to a higher or lower CO₂] at night (250/1000 and 1000/250), suggesting that the primary physiological responses of plants to growth in elevated CO₂] is apparently a response to daytime CO₂] only. However, by the end of the reproductive growth phase, major differences were observed. Plants grown in the 1000/250 regime, when compared with those in the 1000/1000 regime, had significantly more leaf area and leaf mass, 27% more total plant dry mass, but only 18% of the fruit mass. After 12 weeks of growth these plants also had 19% higher respiration rates and 32% lower photosynthetic rates than the 1000/1000 plants. As a result the ratio of carbon gain to carbon loss was reduced significantly in the plants exposed to the reduced night-time CO₂]. Plants grown in the opposite switching environment, 250/1000 versus 250/250, showed no major differences in biomass accumulation or allocation with the exception of a significant increase in the amount of leaf mass per unit area. Physiologically, those plants exposed to elevated night-time CO₂] had 21% lower respiration rates, 14% lower photosynthetic rates and a significant increase in the ratio of carbon gain to carbon loss, again when compared with the 250/250 plants. Biochemical differences also were found. Ribulose-1,5-bisphosphate carboxylase/ oxygenase concentrations decreased in the 250/ 1000 treatment compared with the 250/250 plants, and phosphoenolpyruvate carboxylase activity decreased in the 1000/250 compared with the 1000/1000 plants. Glucose, fructose and to a lesser extent sucrose concentrations also were reduced in the 1000/250 treatment compared with the 1000/1000 plants. These results indicate that experimental protocols that do not maintain elevated CO₂ levels 24 h d^–1 can have significant effects on plant biomass, carbon allocation and physiology, at least for fast-growing annual crop plants. Furthermore, the results suggest some plant processes other than photosynthesis are sensitive to CO₂] and under ecologically relevant conditions, such as high night-time CO₂], whole plant carbon balance can be affected.