C₄ photosynthetic efficiency under low light in Amaranthus cruentus L.: The relationship between CO₂ leakiness and in vivo activities of C₄ photosynthetic enzymes

Abstract

Under low-light conditions, C₄ photosynthesis would be partly limited due to dysfunction of the CO₂ concentrating mechanism. In this study, we investigated the light dependency of CO₂ leakiness and in vivo activities of C₄ photosynthetic enzymes in Amaranthus cruentus L. (NAD-ME subtype, dicot).
CO₂ leakiness was estimated at 0.3 and was constant at irradiances over 200 μmol photon m^-2 s^-1. On the other hand, at irradiances less than 200 μmol photon m^-2 s^-1, CO₂ leakiness increased with the decrease of irradiances, and it was 0.45 at 80 μmol photon m^-2 s^-1. In contrast, the ratio of Rubisco initial activity to its total activity (Rubisco activation state) decreased with the decrease of irradiances at irradiances below 200 μmol photon m^-2 s^-1. These results indicate that the increase in the CO₂ leakiness under low light conditions would be explained by the decrease of Rubisco activation state.