Photosynthesis and plant growth at elevated levels of CO₂

In this review, we discuss the effects of elevated CO₂ levels on photosynthesis in relation to the whole plant growth in terrestrial higher C₃ plants. Short-term CO₂ enrichment stimulates the rate of photosynthesis. Plant mass is also enhanced by CO₂ enrichment. However, the effects of long-term CO₂ enrichment on photosynthesis are variable. Generally, the prolonged exposure to CO₂ enrichment reduces the initial stimulation of photosynthesis in many species, and frequently suppresses photosynthesis. These responses are attributed to secondary responses related to either excess carbohydrate accumulation or decreased N content rather than direct responses to CO₂. Accumulation of carbohydrates in leaves may lead to the repression of photosynthetic gene expression and excess starch seems to hinder CO₂ diffusion. Therefore, the species which have the sink organs for carbohydrate accumulation do not show the suppression of photosynthesis. The suppression of photosynthesis by CO₂ enrichment is always associated with decreases in leaf N and Rubisco contents. These decreases are not due to dilution of N caused by a relative increase in the plant mass but are the result of a decrease in N allocation to leaves at the level of the whole plant, and the decrease in Rubisco content is not selective. Leaf senescence and plant development are also accelerated by CO₂ enrichment. However, they are independent of each other in some species. Thus, various responses to CO₂ observed at the level of a single leaf result from manifold responses at the level of the whole plant grown under conditions of CO₂ enrichment.