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

Abstract

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–1[250/250]; (2) 1000 μmol mol–1 24 h d–1[1000/1000]; (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.