Evolutionary diversity of light partitioning between photosystems (FI) in leaves of higher plants

Sagun JV1, Badger MR2, Chow WS2 and Ghannoum O1

  1. ARC Centre of Excellence for Translational Photosynthesis, Hawkesbury Institute for the Environment, Western Sydney University, Hawkesbury Campus, Locked Bag 1797, Penrith, NSW 2751, Australia.
  2. ARC Centre of Excellence for Translational Photosynthesis, Research School of Biology, Australian National University, Canberra ACT, 2601.

The cyclic electron flux (CEF) around PSI is essential for efficient photosynthesis because it generates ATP thus balancing ATP/NADPH energy budget, and also protects PSI and PSII against photoinhibition. However, it is difficult to quantify CEF due to the absence of net product of cyclic electron flux. ΔFlux is an upper estimate of CEF and can be calculated as the difference between the total electron flux through PSI (ETR1) and the linear electron flux (LEFO2) through both photosystems. In this study, a method was developed to concurrently measure ETR1 and LEFO2 in leaf discs in CO2-enriched air by combining membrane inlet mass spectrometry with a Dual-PAM/F. Most importantly, the method also allowed for the calculation of fI which represents the fraction of absorbed light that goes to either PSI. A correct value of fI is important for estimation of ETR1 which can be calculated by multiplying fI by the absorbed irradiance (I) and leaf absorptance (~0.85) and PSI photosynthetic efficiency (YI). Value of fI is usually assumed to be 0.5 which means that 50% of the absorbed light is portioned to PSI. But it was hypothesized that this value is higher in C4 plants compared to C3 plants because C4 plants have two photosynthetic tissues. Values of fI are only known for a few plant species and have not been calculated for C4 species with different biochemical subtypes and other species such as ferns, liverworts and other gymnosperms. In line with previously published work, our study showed that fI for spinach is 0.5. For the C3 grass Panicum bisulcatum, fI was 0.4 and 0.5 in high and low light grown plants, respectively. Both high-light and shade-grown C4 species (Maize, Panicum antidotale, Panicum milaceum and Panicum maximum) had fI of 0.6. Values of fI from other species were also determined. Gingko biloba (gymnosperm), Marchantia polymorpha (liverwort) and Polypodium sp. (fern) had fI of 0.5, while, Wollemi nobilis (gymnosperm) had fI of 0.3. Using the new fI values increased ETR1 by 12% - 20% in C4 species and decreased ETR1 by 16% in the C3 grass. This also caused the calculated value for ΔFlux at 1,000 µmol m-2 s-1 to increase by 20% - 30% in C4 species and decrease by 30% in the C3 grass species. The method developed in this study to calculate fI appears to be reliable for screening several plant species. The values obtained can be used to correctly quantify CEF and further used for photosynthesis modelling.