Higher plants must dissipate absorbed light energy that exceeds the ph
otosynthetic capacity to avoid molecular damage to the pigments and pr
oteins that comprise the photosynthetic apparatus. Described in this m
inireview is a current view of the biochemical, biophysical and bioene
rgetic aspects of the primary photoprotective mechanism responsible fo
r dissipating excess excitation energy as heat from photosystem II (PS
II). The photoprotective heat dissipation is measured as nonphotochemi
cal quenching (NPQ) of the PSII chlorophyll a (Chi a) fluorescence. Th
e NPQ mechanism is controlled by the trans thylakoid membrane pH gradi
ent (Delta pH) and the special xanthophyll cycle pigments. In the NPQ
mechanism, the de-epoxidized endgroup moieties and the trans-thylakoid
membrane orientations of antheraxanthin (A) and zeaxanthin (Z) strong
ly affect their interactions with protonated chlorophyll binding prote
ins (CPs) of the PSII inner antenna. The CP protonation sites and step
s are influenced by proton domains sequestered within the proteo-lipid
core of the thylakoid membrane. Xanthophyll cycle enrichment around t
he CPs may explain why changes in the peripheral PSII antenna size do
not necessarily affect either the concentration of the xanthophyll cyc
le pigments on a per PSII unit basis or the NPQ mechanism. Recent time
-resolved PSII Chi a fluorescence studies suggest the NPQ mechanism sw
itches PSII units to an increased rate constant of heat dissipation in
a series of steps that include xanthophyll de-epoxidation, CP-protona
tion and binding of the xanthophylls to the protonated CPs; the concer
ted process can be described with a simple two-step, pH-activation mod
el. The xanthophyll cycle-dependent NPQ mechanism is profoundly influe
nced by temperatures suboptimal for photosynthesis via their effects o
n the trans-thylakoid membrane energy coupling system. Further, low te
mperature effects can be grouped into either short term (minutes to ho
urs) or long term (days to seasonal) series of changes in the content
and composition of the PSII pigment-proteins. This minireview conclude
s by briefly highlighting primary areas of future research interest re
garding the NPQ mechanism.