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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 …