Leaf pigments and photosystems stoichiometry underpin photosynthetic efficiency of related C3, C3-C4 and C4 grasses under shade

The quantum yield of CO2 assimilation (QY) is generally lower in C3 relative to C4 plants at warm temperatures, and differs among the C4 subtypes. Here, we investigated whether variations in QY are linked to light absorption or conversion efficiency. We grew six representative grasses with C3, C3-C4 and C4 photosynthesis under full (control) or 20% (shade) sunlight, and measured the in vivo activity and stoichiometry of PSI and PSII, leaf spectral properties and pigment contents, and photosynthetic enzyme activities. Overall, shade reduced leaf photosynthesis, absorptance, especially in the green region, as well as carotenoids/chlorophylls and chlorophyll a/b ratios in C4 more than non-C4 species. Amongst C4, NADP-ME species had the highest QY and cyclic electron flow (CEF), and the NAD-ME species underwent the greatest reduction in leaf absorptance and pigments and PSII contents under shade, whist CEF and PSII/PSI were unaffected. These results demonstrate that the greater efficiency of the CO2 concentrating mechanism in NADP-ME grasses at low light depends on light absorption and harvesting properties in addition to coordination between the C3 and C4 cycles. This is important for maximising light energy absorption and providing the right ATP/NADPH ratio while minimising photoinhibition under variable light conditions. SUMMARY STATEMENTChanges in leaf absorptance, pigment contents and photosystems stoichiometry underpin photosynthetic efficiency and responses of closely related C3, C3-C4 and C4 grasses under shade