Preferential C4-dicarboxylic acid synthesis, the postillumination CO2 burst, carboxyl transfer step, and grana configurations in plants with C4-photosynthesis

Abstract

An oxygen-insensitive postillumination CO₂ burst occurs in many plants with the C₄-pathway of photosynthesis. C₄-species with a postillumination burst synthesize mainly aspartate during very short term photosynthesis and have low levels of 'malic' enzyme (EC. 1.1.1.40) activity. It is postulated that the burst is CO₂ which normally participates in the carboxyl transfer step of C₄-photosynthesis. The burst is absent in C₄-species that produce mainly malate in short-term photosynthesis. This group of plants has very high 'malic' enzyme activity. Transport of malate to the bundle sheath layer and its metabolism by 'malic' enzyme provides reducing power (NADPH₂) as well as CO₂ to the Calvin cycle. This combination of products may attenuate the postillumination burst. Some malate formers show extreme grana reduction in bundle sheath chloroplasts which, is associated with a low capacity for generating reducing power. Malate transport compensates for this lack of reducing power in sheath cells with agranal chloroplasts. Transport of aspartate or oxaloacetate to the sheath cells does not transfer reducing power. The sheath chloroplasts of aspartate formers have well-developed grana but some reduction in grana within mesophyll chloroplasts is apparent in certain species. This reduction may reflect a low demand for reducing power where the major C₄-acid synthesized is aspartate rather than malate.