Modulation of energy-dependent quenching of excitons in antennae of higher plants

Abstract

Energy-dependent exciton quenching, or q(E), protects the higher plant photosynthetic apparatus from photodamage. Initiation of q(E) involves protonation of violaxanthin deepoxidase and PsbS, a component of the photosystem II antenna complex, as a result of lumen acidification driven by photosynthetic electron transfer. It has become clear that the response of q(E) to linear electron flow, termed "q(E) sensitivity," must be modulated in response to fluctuating environmental conditions. Previously, three mechanisms have been proposed to account for q(E) modulation: (i) the sensitivity of q(E) to the lumen pH is altered; (ii) elevated cyclic electron flow around photosystem I increases proton translocation into the lumen; and (iii) lowering the conductivity of the thylakoid ATP synthase to protons (g(H+)) allows formation of a larger steady-state proton motive force (pmf). Kinetic analysis of the electrochromic shift of intrinsic thylakoid pigments, a linear indicator of transthylakoid electric field component, suggests that, when CO(2) alone was lowered from 350 ppm to 50 ppm CO(2), modulation of q(E) sensitivity could be explained solely by changes in conductivity. Lowering both CO(2) (to 50 ppm) and O(2) (to 1%) resulted in an additional increase in q(E) sensitivity that could not be explained by changes in conductivity or cyclic electron flow associated with photosystem I. Evidence is presented for a fourth mechanism, in which changes in q(E) sensitivity result from variable partitioning of proton motive force into the electric field and pH gradient components. The implications of this mechanism for the storage of proton motive force and the regulation of the light reactions are discussed.

Fig. 1.

Energy-dependent antenna down-regulation (q_E) as a function of LEF. Measurements of q_E exciton quenching and LEF were performed on intact leaves of tobacco plants over light intensities ranging from 32 to 820 μmol of photons·m^–2·s^–1, as described in the text. Gas compositions were 372 ppm CO₂/21% O₂ (□), 50 ppm CO₂/21% O₂ (▵), and 50 ppm CO₂/1% O₂ (•). The sizes of the circles surrounding the symbols have been set proportional to the conductivity of the ATP synthase to protons (g_H+) as estimated by the inverse of the decay lifetime of the ECS signal, as described in Materials and Methods. The largest diameter symbol was ≈61.3 s^–1, whereas the smallest was ≈15.7 s^–1.

Fig. 2.

Total light-induced pmf as a function of the pmf attributable to LEF. The ECS_t parameter was taken as a measure of light-induced pmf, whereas the independent measure of pmf or pmf_LEF (LEF/g_H+) was derived from analysis of fluorescence and the kinetics of ECS decay upon a rapid light–dark transition (see Materials and Methods). The symbols and conditions are the same as in Fig. 1. The error bars represent standard deviations for n = 3–5.

Fig. 3.

Energy-dependent antenna down-regulation (q_E) as a function of light-induced pmf, as estimated by the ECS_t parameter. Light-induced pmf (ECS_t) values were derived from analysis of ECS decay kinetics as described in Materials and Methods. The symbols and conditions were as in Fig. 1. The error bars represent standard deviation for n = 3–5.

Fig. 4.

The relationship between energy-dependent antenna down-regulation (q_E) and the ΔpH component of light-induced pmf, as estimated by the inverted ECS signal parameter. The symbols and conditions are the same as in Fig. 1. The error bars represent SD for n = 3–5. (Inset) Kinetic traces of the ECS signal, deconvoluted as described in Materials and Methods, upon a light–dark transition from steady-state illumination. The extents of the steady-state signal and the inverted region of the signal, which are thought to be proportional to the light-induced Δψ and ΔpH components of pmf, respectively, are indicated by the vertical arrows. The traces were taken at actinic light intensity of 520 μmol of photons·m^–2·s^–1 at ambient (trace A) and LEA (trace B) conditions.