Lessons from relatives: C4 photosynthesis enhances CO2 assimilation during the low-light phase of fluctuations.

Despite the global importance of species with C4 photosynthesis, there is a lack of consensus regarding C4 performance under fluctuating light. Contrasting hypotheses and experimental evidence suggest that C4 photosynthesis is either less, or more efficient in fixing carbon under fluctuating light than the ancestral C3 form. Two main issues were identified that may underly the lack of consensus: neglect of evolutionary distance between selected C3 and C4 species and use of contrasting fluctuating light treatments. To circumvent these issues, we compared photosynthetic responses to fluctuating light across three independent phylogenetically controlled comparisons between C3 and C4 species from Alloteropsis, Flaveria, and Cleome genera under 21% and 2% O2. Leaves were subjected to repetitive stepwise changes in light intensity (800 and 100 {micro}mol m-2 s-1 PFD) with three contrasting durations: 6, 30 and 300 seconds. These experiments reconcile the opposing results found across previous studies showing that 1) stimulation of CO2 assimilation in C4 species during the low light phase was both stronger and more sustained than in C3 species; 2) CO2 assimilation patterns during the high light phase were genus-specific rather than impacted by photosynthetic pathway; and 3) the duration of each light step in the fluctuation regime can strongly influence experimental outcomes. One sentence significance statementComparing photosynthesis in three pairs of closely related C3 and C4 species across three fluctuating light regimes showed that C4 photosynthesis has a systematic advantage under the low light phase not related to suppression of photorespiration, while the comparative efficiency under the high light phase was not determined by photosynthetic pathway.