SULFUR-DIOXIDE AND CARBON-DIOXIDE INDUCED CHANGES IN SOYBEAN PHYSIOLOGY

Soybean (Glycine max L. Merrill) plants were exposed to sulfur dioxide (SO2, 0.1-mu-l/l) and carbon dioxide (CO2, 450-mu-l/l), singly and in combination, in open-top field chambers at the flowering stage. The exposure for SO2 and CO2 was for 4 h and 12 h/day, respectively, for 5 days. Photosynthetic rate (PN), specific root nodule nitrogenase activity (SNA), foliar nitrogen (N) and chlorophyll (Ch-a and Ch-b) were measured after the exposure. The plants exposed to SO2 showed reduction in PN (by 13%), SNA (by 28%), N (by 23%) without significant change in Ch-a and Ch-b. The plants exposed to CO2 showed no change in PN, reduction in SNA (by 18%) and N (by 23%) and an increase in both Ch-a and Ch-b. When the plants were exposed to a combination of these two gases PN and SNA were the same as that of the control plants, N decreased slightly (by 6%) but both Ch-a and Ch-b increased significantly. Carbon dioxide was able to compensate the negative impact of SO2.