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 C-3 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.