Fluorescence lifetime imaging shows the PSI/PSII antenna size ratio of grana and stroma inside a leaf

Emilie Wientjesa,b, Piet Swinkelsa, Jan Willem Borstb,c, Herbert van Amerongena,c
aLaboratory of Biophysics, Wageningen University, Wageningen, The Netherlands; bMicrospectroscopy Center Wageningen, Wageningen, The Netherlands; cLaboratory of Biochemistry, Wageningen University, Wageningen, The Netherlands

In plants photosynthesis is driven by Photosystem I (PSI) and Photosystem II (PSII). Under high illumination the fluorescence decay of PSII occurs on a timescale of (sub)nanoseconds, while PSI decays in less than 100 ps. We used this difference to quantify the PSI / PSII emission ratio in chloroplasts inside a leaf based on fluorescence lifetime imaging data. The excitation was with 633 nm light, at this wavelength the extinction coefficient of chlorophyll a and chlorophyll b is approximately equal, as such the PSI / PSII emission ratio is directly related to the relative PSI / PSII antenna size. This allows us to compare the PSI / PSII antenna size in:

  • Grana and stroma membranes
  • Wild type A. thaliana and the LHCII reduced antenna size ΔLhcb2 mutant
  • Top and bottom of a leaf
  • Plants acclimated to different light conditions

Furthermore, we can show that the three PSII lifetimes are not equally present in stroma and grana membranes. This method opens a range of possibilities to study (changes in) the PSI / PSII antenna sizes and PSII lifetimes in chloroplasts inside a leaf with a spatial resolution of a few hundred nanometers.  

Figure 1: Fluorescence lifetime imaging of chloroplasts inside a WT and ΔLhcb2 A. thaliana leaf. The contribution of the 85ps PSI component to the fluorescence decay at 705-740nm is colour coded. A stronger PSI contribution is observed in the DLhcb2 mutant compared to WT and in the stroma compared to the grana.