Cyclic electron flow and light partitioning between the two photosystems in leaves of plants with different functional types

Cyclic electron flow (CEF) around photosystem I (PSI) is essential for generating additional ATP and enhancing efficient photosynthesis. Accurate estimation of CEF requires knowledge of the fractions of absorbed light by PSI (f(I)) and PSII (f(II)), which are only known for a few model species such as spinach. No measures of f(I) are available for C-4 grasses under different irradiances. We developed a new method to estimate (1) f(II) in vivo by concurrently measuring linear electron flux through both photosystems LEFO2 in leaf using membrane inlet mass spectrometry (MIMS) and total electron flux through PSII (ETR2) using chlorophyll fluorescence by a Dual-PAM at low light and (2) CEF as ETR1-LEFO2For a C-3 grass, f(I) was 0.5 and 0.4 under control (high light) and shade conditions, respectively. C-4 species belonging to NADP-ME and NAD-ME subtypes had f(I) of 0.6 and PCK subtype had 0.5 under control. All shade-grown C-4 species had f(I) of 0.6 except for NADP-ME grass which had 0.7. It was also observed that f(I) ranged between 0.3 and 0.5 for gymnosperm, liverwort and fern species. CEF increased with irradiance and was induced at lower irradiances in C-4 grasses and fern relative to other species. CEF was greater in shade-grown plants relative to control plants except for C-4 NADP-ME species. Our study reveals a range of CEF and f(I) values in different plant functional groups. This variation must be taken into account for improved photosynthetic calculations and modelling.