Figure 8. A schematic presentation of electron transport within PSII and the resulting TL bands.

The Q_A and Q_B quinones, P₆₈₀ and the Mn cluster S-states are shown. Forward electron transport is represented by plain arrows and back electron transport by blue arrows. Changes in the peak temperature observed in TL experiments can be used as an estimate for the recombination energy of the Q_B ⁻S₂/Q_BS₁ and Q_B ⁻S₃/Q_BS₂ (B-band) or Q_A ⁻S₂/Q_AS₁ (Q-band) redox pairs. To observe the Q band emission, reduction of the Q_B site was blocked by herbicides that bind specifically to this site. The electron released from P₆₈₀ by light excitation at subzero temperatures can reach the Q_A site and, in the presence of herbicides that bind to the Q_B site, recombine upon warming with the oxidized S₂ state by back electron flow producing the Q band. Since the energy gap between Q_A ⁻ and P₆₈₀ ⁺ is smaller than that from Q_B ⁻ to P₆₈₀ ⁺ (B band), lower activation energy is required for this recombination and thus the Q band is observed at lower temperatures.