Nonphotochemical Chlorophyll Fluorescence Quenching: Mechanism and Effectiveness in Protecting Plants from Photodamage

Abstract

We review the mechanism underlying nonphotochemical chlorophyll fluorescence quenching (NPQ) and its role in protecting plants against photoinhibition. This review includes an introduction to this phenomenon, a brief history of major milestones in our understanding of NPQ, definitions, and a discussion of quantitative measurements of NPQ We discuss the current knowledge and unknown aspects in the NPQ scenario, including the following: ΔpH, the proton gradient (trigger); light-harvesting complex II (LHCII), PSII light harvesting antenna (site); and changes in the antenna induced by ΔpH (change), which lead to the creation of the quencher We conclude that the minimum requirements for NPQ in vivo are ΔpH, LHCII complexes, and the PsbS protein. We highlight the most important unknown in the NPQ scenario, the mechanism by which PsbS acts upon the LHCII antenna. Finally, we describe a novel, emerging technology for assessing the photoprotective "power" of NPQ and the important findings obtained through this technology.

Figure 1.

A, Typical PAM fluorescence trace of an Arabidopsis leaf showing induction and relaxation of NPQ. Fm and F_o are the maximum and minimum fluorescence levels in the dark before actinic light illumination (1000 µmol m⁻²s⁻¹), respectively. Fs is the steady-state fluorescence level. F_m’ is maximum fluorescence during actinic light illumination. Pulses of light (10,000 µmol m⁻²s⁻¹) were applied to close all RCIIs and were used to estimate F_m and F_m’. qE and qI are quickly and slowly reversible components of NPQ, respectively. B, Model of NPQ development (NPQ scenario) showing key factors triggering and regulating the process (for more details, see the text). The formula for the minimum component requirement for NPQ is shown below the diagram.

Figure 2.

The structure of PSII antenna components. S, M, and L are the major LHCIIs that are strongly, moderately, and loosely bound to the RCII core trimers, respectively. CP24, 26, and 29 are the minor monomeric antenna complexes. PSII core dimer is shown in red. PsbS dimer is shown with a dashed line pointing to the putative preferential interaction site in the dark.

Figure 3.

Schematic representation of putative PSII arrangements in the grana membrane in the dark (A) and NPQ (B) states. A, 18 PSII C2S2M2 complexes (outlined by yellow lines) with peripheral LHCII trimers (L trimers; after Kouřil et al., 2011). The total LHCII trimer-to-RCII monomer ratio is approximately 5. B, 18 PSII core dimers rearranged/clustered into the NPQ state (following Johnson et al., 2011). C2S2 structure is shown (outlined with a dashed red line; see the inset) preserved in the three supercomplexes shown in the far left corner. A mix of unquenched (black contour) and quenched (red contour) S, M, and L trimers and monomers of the minor antenna (not specified here) is shown.

Figure 4.

A, Part of the gradually increasing illumination procedure used in PAM measurements of Arabidopsis leaves. The formula at the top shows how qP_d is calculated. F_o’_act. and F_o’_calc. are the measured and calculated (Oxborough and Baker, 1997) dark fluorescence levels, respectively. P1, 2, and 3 are saturating pulses, AL and FR are actinic and far red light, respectively, and 625 and 820 are the intensities of actinic light in µmol m⁻²s⁻¹. B, The relationships between the PSII yield, qP_d, and NPQ in the dark over the course of the gradually increasing actinic light intensity procedure (Ruban and Belgio, 2014). The formula shows the relationship between PSII yield, qP, and NPQ. C, Light intensity (in µmol m⁻²s⁻¹) tolerated by 50% of the Arabidopsis mutant plants examined: –Zea (npq1); –PsbS (npq4); +PsbS (wt), and ++PsbS (PsbS overexpressor, L17).