Analysis of the Results
Effects of herbicides on the DFIC
By inhibiting the electron transport at the acceptor side of PS2, the herbicides drastically alter the properties of the millisecond DF. In the presence of herbicides the overall intensity of DF is suppressed because of two reasons:
- The blocked PS2 reaction centres are rapidly closed upon illumination and remain in this state because the electrons cannot advance further than QA. The DF emission is then suppressed because it is proportional to the number of open reaction centres.
- Transmembrane electrochemical potential (proton gradient) can no longer be generated by linear electron transport. DF is lower because of the lack of proton gradient.
Fig. 14. DFIC in the presence of herbicides 
The DFIC of pea leaves treated with the herbicide atrazine in different concentrations can be seen in Fig. 14. The most obvious effect is that by adding higher concentrations the DFIC amplitudes gradually decrease until reaching saturation (above 10 mM of atrazine).
The curves also show that, apart of the general suppresion of DF, there are notable differences in the shape of the DFIC. The slow phase maximum I4, which is dependent on the proton gradient is more sensitive to the herbicide than the fast phase maximum I1, supposedly generated due to charge separation in the reaction centres.
It is interesting to note that unlike the PFIC, which is a flat line in fully inhibited samples, the DFIC never loses the I1 and (especially) I4 peak entirely. When the PS2 electron transport is blocked, I4 is most likely a result of cyclic electron transport around PS1.
Which parameters can be applied to quantitatively assess the extent of inhibition? Although I4 is not the most sensitive peak in the DFIC and does not go to zero at full inhibition, its amplitude is easy to determine accurately and is a suitable parameter to measure the herbicide effect.