Abstract
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 $$\left( {{\text{LEF}}_{{{\text{O}}_{ 2} }} } \right)$$ LEF O 2 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— $${\text{LEF}}_{{{\text{O}}_{ 2} }}$$ LEF O 2 . For 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.