Actinic intensity was increased in ten steps from 10 to 1,600 μmol quanta m−2 s−1 of 635 nm light. Leaf pre-illuminated for 1 h at 600 μmol m−2 s−1, with 10 min dark time before start of recording. Screenshot of the original recording (Dual-PAM user software). b Deconvolution of the ΔpH and ΔΨ components of the overall pmf by the DIRK method. Zoomed detail of the data set presented in a, showing dark-interval relaxation kinetics after turning off 200 μmol m−2 s−1 (light step 5 in a). c Partitioning of overall proton motive force (pmf)

into ΔpH and ΔΨ components as a function of light intensity during the course of the experiment depicted in a. ΔpH and ΔΨ were determined as explained in b As has been discussed click here extensively find more by Kramer and co-workers (for reviews see Kramer et al. 2004a, b; Cruz et al. 2004; Avenson et al. 2005b), the pmf and its ΔpH and ΔΨ components play a dual role in photosynthesis, namely at the level of energy transduction (synthesis of ATP from ADP GSK872 order and Pi at the thylakoid CF0–CF1 ATP synthase) and at the level of regulation. In particular, the ΔpH has been known to regulate the efficiency of light capture in PS II via dissipation of excess energy, which otherwise would lead to photodamage (Demmig-Adams

1992; Niyogi 1999). The observed increase of the ΔpH component above 300 μmol m−2 s−1 on the cost of the ΔΨ component (Fig. 2c) may serve as an example for the adaptive flexibility Thymidylate synthase of the photosynthetic apparatus. While ΔΨ contributes substantially to overall pmf at moderate PAR, where the efficiency of light capture is decisive, maximal ΔpH is approached at high light intensities only, where down-regulation of PS II becomes essential. Very recently Johnson and Ruban (2013) questioned the existence of a substantial ΔΨ components in plant

leaves during steady-state illumination, as suggested by Kramer and co-workers, on the grounds of experiments with nigericin-infiltrated leaves of wild-type Arabidopsis and with leaves of Arabidopsis mutants deficient in energy-dependent fluorescence quenching (qE). These authors argue that the apparent ECS in normal leaves during steady-state illumination is not due to a genuine 515 nm change, i.e., is not caused by ΔΨ, but in fact reflects an overlapping qE-related absorption change, the position of which varies depending on the xanthophyll content of the leaves between 525 and 540 nm (Johnson et al. 2009). It may be pointed out that all measurements of Johnson and Ruban (2013) were carried out using 700 μmol m−2 s−1 red light, i.e., at a high intensity of absorbed light, where also our data show a rather small ΔΨ component (Fig. 2c).