Lessons from relatives: c4 photosynthesis enhances CO2 assimilation during the low-light phase of fluctuations

Comparing photosynthesis in 3 pairs of closely related C-3 and C-4 species across 3 fluctuating light regimes shows that C-4 photosynthesis has a systematic advantage under the low-light phase. Despite the global importance of species with C-4 photosynthesis, there is a lack of consensus regarding C-4 performance under fluctuating light. Contrasting hypotheses and experimental evidence suggest that C-4 photosynthesis is either less or more efficient in fixing carbon under fluctuating light than the ancestral C-3 form. Two main issues have been identified that may underly the lack of consensus: neglect of evolutionary distance between selected C-3 and C-4 species and use of contrasting fluctuating light treatments. To circumvent these issues, we measured photosynthetic responses to fluctuating light across 3 independent phylogenetically controlled comparisons between C-3 and C-4 species from Alloteropsis, Flaveria, and Cleome genera under 21% and 2% O-2. Leaves were subjected to repetitive stepwise changes in light intensity (800 and 100 & mu;mol m(-2) s(-1) photon flux density) with 3 contrasting durations: 6, 30, and 300 s. These experiments reconciled the opposing results found across previous studies and showed that (i) stimulation of CO2 assimilation in C-4 species during the low-light phase was both stronger and more sustained than in C-3 species; (ii) CO2 assimilation patterns during the high-light phase could be attributable to species or C-4 subtype differences rather than photosynthetic pathway; and (iii) the duration of each light step in the fluctuation regime can strongly influence experimental outcomes.

Automated summary

Comparing photosynthesis in 3 pairs of closely related C-3 and C-4 species across 3 fluctuating light regimes shows that C-4 photosynthesis has a systematic advantage under the low-light phase. However, there is still lack of consensus regarding C-4 performance under fluctuating light due to neglect of evolutionary distance between selected C-3 and C-4 species and use of contrasting fluctuating light treatments.