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

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 C₄ 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 [Formula: see text] 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-[Formula: see text]. For a C₃ grass, f_I was 0.5 and 0.4 under control (high light) and shade conditions, respectively. C₄ 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 C₄ 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 C₄ grasses and fern relative to other species. CEF was greater in shade-grown plants relative to control plants except for C₄ 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.