Wild type plants were measured under fluctuating light (
A) and the ECS measurements taken 10 s after each intensity fluctuation from lower to higher light (
A, open squares). The resulting deconvoluted ΔA
520 nm ECS signals show rapid, transient ECS 'spikes' induced by rapid Δ
ψ transients before the ΔpH component can be altered (
B). The upward spike represents the effect of rapidly increasing the light intensity. The downward spikes seen towards the ends of the traces reflect the transients that occur when the actinic light is switched to the lower light intensity at the end of the fluctuation. A rough estimate of the extent of the Δ
ψ imposed by the spikes can be obtained by comparing the ECS signals with the calibration and results presented in
Takizawa et al. (2007), which estimated that the Δ
ψ imposed by a saturating, single-turnover flash to thylakoids was about 40 mV. Based on this calibration value, the extent of basal
pmf formed by equilibration with ATP (i.e. the dark
pmf level or
pmfd) was estimated to be about 112 mV (
Takizawa et al., 2007), 60 mV of which is stored in Δ
ψ if the partitioning is 0.5. From the dark-interval relaxation kinetics of ECS under steady-state conditions (see
Figure 7B and
Figure 5—figure supplement 1), we then expect an additional light-driven
pmf under steady-state conditions to range from 150–200 mV, and given that the fraction of this
pmf stored as Δ
ψ ranged from 0.20–0.60, we estimate a range for steady-state light-driven Δ
ψ between 30 and 120 mV. The transient spikes in Δ
ψ generated during light fluctuations are likely to be essentially all stored in Δ
ψ (see main text) and range in amplitude between 60–80 mV, so that the highest amplitude Δ
ψ imposed during these conditions likely falls in the broad range between 150–260 mV. ECS units were defined as the deconvoluted ΔA
520 μg chlorophyll
−1 cm
2.