P700 reduction/oxidation kinetics
Far red or red light induced photooxidation of P700 was used to determine rates of CEF as described by Morgan-Kiss et al. (2002b). A volume of exponential phase cultures representing 25 μg Chl a was dark adapted for 10 min and then filtered onto 25 mm GF/C filters (Whatman, Cat No. 1822-025). Filters were measured on the Dual-PAM 100 instrument using the leaf attachment. The proportion of photooxidizable P700 was determined by monitoring absorbance changes at 820 nm and expressed as the parameter (∆A820/A820). The signal was balanced and the measuring light switched on. Far red (FR) light (λmax=715 nm, 10 Wm−2, Scott filter RG 715) was then switched on to oxidize P700. For the red light measurement, cells were pre-treated in the dark for 10 min in the presence of 10 µM DCMU to inhibit electron transfer from PSII to PSI. After steady-state oxidation levels were reached, the FR or red light was switched off to re-reduce P700. The half time for the reduction of P700+ to P700 (t ½red) was calculated as an estimate of relative rates of PSI-driven CEF (Ivanov, Morgan, Gray, Velitchkova, & Hüner, 1998). The re-reduction time for P700 was calculated using the program OriginPro 8.5.1 using first order exponential decay kinetics.