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

The quantum yield of photosynthesis (QY, CO2 fixed per light absorbed) depends on the efficiency of light absorption, the coupling between light absorption and electron transport, and the coupling between electron transport and carbon metabolism. QY is generally lower in C-3 relative to C-4 plants at warm temperatures and differs among the C-4 subtypes. We investigated the acclimation to shade of light absorption and electron transport in six representative grasses with C-3, C-3-C-4 and C-4 photosynthesis. Plants were grown under full (control) or 25% (shade) sunlight. We measured the in vivo activity and stoichiometry of PSI and PSII, leaf spectral properties and pigment contents, and photosynthetic enzyme activities. Under control growth-light conditions, C-4 species had higher CO2 assimilation rates, which declined to a greater extent relative to the C-3 species. Whole leaf PSII/PSI ratios were highest in the C-3 species, while QY and cyclic electron flow (CEF) were highest in the C-4, NADP-ME species. Shade significantly reduced leaf PSII/PSI, linear electron flow (LEF) and CEF of most species. Overall, shade reduced leaf absorptance, especially in the green region, as well as carotenoid and chlorophyll contents in C-4 more than non-C-4 species. The NAD-ME species underwent the greatest reduction in leaf absorptance and pigments under shade. In conclusion, shade compromised QY the least in the C-3 and the most in the C-4-NAD-ME species. Different sensitivity to shade was associated with the ability to maintain leaf absorptance and pigments. This is important for maximising light absorption and minimising photoprotection under low light.

Automated summary

The quantum yield of photosynthesis depends on the efficiency of light absorption, electron transport, and carbon metabolism. C-3 plants usually have lower quantum yield compared to C-4 plants at warm temperatures, and there are differences among the various C-4 subtypes. This study investigated how six representative grass species with different photosynthesis types acclimated to shade conditions. The results showed that shade significantly reduced light absorption, electron transport, and quantum yield in most species, particularly in C-4 plants.-