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 mu mol mol(-1) 24 h d(-1) [250/250]; (2) 1000 mu mol mol(-1) 24 h d(-1) [1000/1000]; (3) 250 mu mol mol(-1) during daylight hours and 1000 mmol mol(-1) during nighttime hours [250/1000] or (4) 1000 mu mol mol(-1) during daylight hours and 250 mmol 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.
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