Endogenous probes of light-induced transthylakoid
proton motive force (
pmf), membrane potential (Δ
ψ) and ΔpH were used
in vivo to assess in
Arabidopsis the lumen pH responses of regulatory components of photosynthesis. The accumulation of zeaxanthin and
protonation of PsbS were found to have similar p
Ka values, but quite distinct Hill coefficients, a feature allowing high antenna efficiency at low
pmf and fine adjustment at higher
pmf. The onset of “energy-dependent’ exciton quenching (
qE) occurred at higher lumen pH than slowing of plastoquinol oxidation at the cytochrome
b6f complex, presumably to prevent buildup of reduced electron carriers that can lead to photodamage. Quantitative comparison of intrinsic probes with the electrochromic shift signal
in situ allowed quantitative estimates of
pmf and lumen pH. Within a degree of uncertainly of
![not, vert, similar](http://www.sciencedirect.com/scidirimg/entities/223c.gif)
0.5 pH units, the lumen pH was estimated to range from
![not, vert, similar](http://www.sciencedirect.com/scidirimg/entities/223c.gif)
7.5 (under weak light at ambient CO
2) to
![not, vert, similar](http://www.sciencedirect.com/scidirimg/entities/223c.gif)
5.7 (under 50 ppm CO
2 and saturating light), consistent with a ‘moderate pH’ model, allowing antenna regulation but preventing acid-induced photodamage. The apparent p
Ka values for accumulation of zeaxanthin and PsbS
protonation were found to be
![not, vert, similar](http://www.sciencedirect.com/scidirimg/entities/223c.gif)
6.8, with Hill coefficients of about 4 and 1 respectively. The apparent shift between
in vitro violaxanthin deepoxidase
protonation and zeaxanthin accumulation
in vivo is explained by steady-state competition between zeaxanthin formation and its subsequent epoxidation by zeaxanthin epoxidase. In contrast to tobacco,
Arabidopsis showed substantial variations in the fraction of
pmf (0.1–0.7) stored as Δ
ψ, allowing a more sensitive qE response, possible as an adaptation to life at lower light levels.