Quantum Yield of Photosystem II and Efficiency of CO₂ Fixation in Flaveria (Asteraceae) Species under Varying Light and CO₂

Abstract

The quantum yields of electron transport from photosystem II (PSII) (Φ_e, determined from chlorophyll a fluorescence), and CO₂ assimilation (Φ_CO2, photosynthetic rate/light intensity) were measured simultaneously in vivo with representative species of Flaveria which show a progression in development between C₃ and C₄ photosynthesis and in reduction of photorespiration. These were F. pringlei (C₃), F. sonorensis (C₃-C₄, but lacking a C₄ cycle), F. floridana (C₃-C₄, with partially functional C₄ cycle), F. brownii (C₄-like) and F. bidentis (C₄). The level of PSII activity with varying C_I under 210 mbar O₂ was very similar in all species. However, the progressive development of C₄ characteristics among the species produced an increased efficiency in utilisation of PSII derived energy for CO₂ assimilation under 210 mbar O₂, due to reduced photorespiratory losses at low CO₂ levels. In all species, when photorespiration was limited by low O₂ (20 mbar), there was a linear or near linear relationship between the quantum yield of PSII v. the quantum yield of CO₂ fixation with varying intercellular levels of CO₂ (C_i) indicating that CO₂ fixation in this case is linked to PSII activity. When switching from 20 to 210 mbar O₂ at atmosphere levels of CO₂, there was a similar decrease in the efficiency in utilising PSII activity for CO₂ assimilation at different light intensities, but the degree of sensitivity to O₂ progressively decreased among the species concomitant with the development of C₄ photosynthesis. These results may help explain why there is an advantage to evolution of C₄ photosynthesis in environments where C_i becomes limiting.