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.

The N-use efficiency for photosynthesis was higher in a C₄ plant, maize, than in a C₃ plant, rice, including rbcS antisense rice with optimal ribulose-1,5-bisphosphate carboxylase (Rubisco) content for CO₂-saturated photosynthesis, even when photosynthesis was measured under saturating CO₂ conditions. The N cost for the C₄ cycle enzymes in maize was not large, and the lower amount of Rubisco allowed a greater N investment in the thylakoid components. This greater content of the thylakoid components as well as the CO₂ concentrating mechanism may support higher N-use efficiency for photosynthesis in maize.

Changes in chlorophyll fluorescence, P700⁺-absorbance and gas exchange during the induction phase and steady state of photosynthesis were simultaneously examined in rice (Oryza sativa L.), including the rbcS antisense plants. The quantum yield of photosystem II (ΦPSII) increased more rapidly than CO₂ assimilation in 20% O₂. This rapid increase in ΦPSII resulted from the electron flux through the water–water cycle (WWC) because of its dependency on O₂. The electron flux of WWC reached a maximum just after illumination, and rapidly generated non-photochemical quenching (NPQ). With increasing CO₂ assimilation, the electron flux of WWC and NPQ decreased. In 2% O₂, WWC scarcely operated and ΦPSI was always higher than ΦPSII. This suggested that cyclic electron flow around PSI resulted in the formation of NPQ, which remained at higher levels in 2% O₂. The electron flux of WWC in the rbcS antisense plants was lower, but these plants always showed a higher NPQ. This was also caused by the operation of the cyclic electron flow around PSI because of a higher ratio of ΦPSI/ΦPSII, irrespective of O₂ concentration. The results indicate that WWC functions as a starter of photosynthesis by generating ΔpH across thylakoid membranes for NPQ formation, supplying ATP for carbon assimilation. However, WWC does not act to maintain a high NPQ, and ΦPSII is down-regulated by ΔpH generated via the cyclic electron flow around PSI.

The green revolution’s breeding of semi-dwarf rice cultivars in the 1960s improved crop yields, with large increases in the use of nitrogen (N) fertilizer. However, excess N application has caused serious environmental problems, including acid rain and the eutrophication of rivers and oceans. To use N to improve crop yields, while minimizing the associated environmental costs, there is a need to produce crops with higher N-use efficiency and higher yield components. Here we show that transgenic rice overproducing ribulose 1,5-bisphosphate carboxylase–oxygenase (Rubisco)—the key enzyme of photosynthesis—exhibits increased yields with improved N-use efficiency for increasing biomass production when receiving sufficient N fertilization in an experimental paddy field. This field experiment demonstrates an improvement in photosynthesis linked to yield increase due to a higher N-use efficiency in a major crop.