Oxygen and CO(2) sensitivity of C(4) photosynthesis: From a leaf to the whole plant

The effects of O₂ and CO₂ on net photosynthesis and growth of plants assimilating CO₂ through the C₄ pathway were examined. For maximum net CO₂ assimilation, C ₄ plants have an optimum O₂ concentration of 5–10 kPa, above and below which net photosynthesis is inhibited. This dual effect of O₂ on C₄ photosynthesis is present in species representing the three C₄ sub-types. The high O₂ requirement of C ₄ photosynthesis is due to the C₄ cycle activity while the supra optimal O₂ inhibition is due to photorespiration. In C ₄ photosynthesis, O₂ contributes, through pseudo-cyclic photophosphorylation, to the extra ATP required by the C₄ cycle to regenerate phospho enolpyruvate. Consistently, growth of Amaranthus edulis Speg., a C₄ plant was stimulated by 30% at 20 kPa relative to 5 kPa O₂. In C₄ plants Photosystem II activity is associated mainly with CO₂ fixation. O₂ acts as a final electron acceptor both in photorespiration and in the Mehler reaction. However, under normal atmospheric conditions, these two processes account for less than 10% of the Photosystem II activity. Biomass accumulation in maize (Zea mays L.), a C₄ plant was stimulated 20% when plants were grown, for 30 days after emergence, under 3 times current ambient CO₂ (110 Pa). Plants grown under elevated CO₂ had lower chlorophyll content lower total soluble protein, lower photosynthetic capacity and carboxylation efficiency. However, under elevated CO₂, net photosynthesis rates were higher than on plants growing under ambient CO ₂. Growth of maize benefits from elevated CO₂ through acclimation in the content and activity of certain photosynthetic enzymes. Increased capability to acquire CO₂ (by carbonic anhydrase), to synthesize photosynthetic end-products (sucrose by fructose-1,6-bisphosphatase and starch by ADP-glucose pyrophosphorylase), to utilize photosynthetic end-products to produce extra energy (by respiration) and to allocate N from the photosynthetic apparatus (e.g. rubisco and malate dehydrogenase) to other sinks (leaf expansion) may enhance growth of C₄ plants under elevated CO₂.