Abstract: The climate change caused by elevated CO₂ concentration and drought are bound to affect the growth of soybean. Few studies have addressed the effects of elevated CO₂ concentration on the physiology and biochemistry of soybean under drought stress. Here, we examined the changes of photosynthetic ability, photosynthetic pigment accumulation, antioxidant level, osmotic adjustment substances, hormone levels, signal transduction enzymes and gene expression level of soybean at flowering stage under different CO₂ concentration (400 and 600 μmol·mol^-1) and drought stress (normal water: leaf relative water content was 83%-90%; drought stress: leaf relative water content was 64%-70%). The results showed that the transpiration rate, water use efficiency and net photosynthetic rate of soybean leaves were significantly increased by elevated CO₂ concentration, but the content of chlorophyll b was decreased under drought stress. Elevated CO₂ concentration significantly increased peroxidase activity and abscisic acid content of leaves under drought stress, decreased the content of proline, and did not affect the content of soluble saccharides. The increased CO₂ concentration under drought stress significantly promoted the content of calcium-dependent protein kinase and glutathione-S-transferase, and up-regulated the expression of related genes, while significantly decreased the content of mitogen-activated protein kinase and the heat shock protein, and down-regulated the expression of their genes. The results would be helpful to understand the impacts of climate change on the growth, physiology and biochemistry of soybean, and to deal with the production problems of soybean under future climate change.

Key words: elevated CO₂ concentration, drought stress, soybean, antioxidant system, signal transduction, plant hormone