A comparison of stomatal conductance responses to blue and red light between C3 and C4 photosynthetic species in three phylogenetically-controlled experiments

Introduction: C-4 photosynthesis is an adaptation that has independently evolved at least 66 times in angiosperms. C-4 plants, unlike their C-3 ancestral, have a carbon concentrating mechanism which suppresses photorespiration, often resulting in faster photosynthetic rates, higher yields, and enhanced water use efficiency. Moreover, the presence of C-4 photosynthesis greatly alters the relation between CO2 assimilation and stomatal conductance. Previous papers have suggested that the adjustment involves a decrease in stomatal density. Here, we tested if C-4 species also have differing stomatal responses to environmental cues, to accommodate the modified CO2 assimilation patterns compared to C-3 species.Methods: To test this hypothesis, stomatal responses to blue and red-light were analysed in three phylogenetically linked pairs of C3 and C4 species from the Cleomaceae (Gynandropsis and Tarenaya), Flaveria, and Alloteropsis, that use either C3 or C4 photosynthesis.Results: The results showed strongly decreased stomatal sensitivity to blue light in C-4 dicots, compared to their C-3 counterparts, which exhibited significant blue light responses. In contrast, in C-3 and C-4 subspecies of the monocot A. semialata, the blue light response was observed regardless of photosynthetic type. Further, the quantitative red-light response varied across species, but the presence or absence of a significant stomatal red-light response was not directly associated with differences in photosynthetic pathway. Interestingly, stomatal density and morphology patterns observed across the three comparisons were also not consistent with patterns commonly asserted for C-3 and C-4 species.Discussion: The strongly diminished blue-light sensitivity of stomatal responses in C-4 species across two of the comparisons suggests a common C-4 feature that may have functional implications. Altogether, the strong prevalence of species-specific effects clearly emphasizes the importance of phylogenetic controls in comparisons between C-3 and C-4 photosynthetic pathways.