Non-photochemical quenching. A response to excess light energy.

Plants and algae have a love/hate relationship with light. As oxygenic photoautotrophic organisms, they require light for life; however, too much light can lead to increased production of damaging reactive oxygen species as byproducts of photosynthesis. In extreme cases, photooxidative damage can cause pigment bleaching and death, a phenomenon all too familiar to anyone who has tried to move a houseplant outdoors into full sunlight. The quantity of the light in natural environments can vary over several orders of magnitude and on a time scale that ranges from seconds to seasons. Because light is such an important environmental parameter, plants have evolved numerous biochemical and developmental responses to light that help to optimize photosynthesis and growth. For example, plants rely on photoreceptors such as phytochrome for shade avoidance responses. Some plants are able to adjust their capacity for harvesting sunlight through leaf and chloroplast movements. During long-term acclimation to changes in light intensity many algae and plants regulate the size of their lightharvesting pigment antennae through changes in gene expression and/or proteolysis. Large antennae are necessary for efficient light capture in limiting light, but they can be a liability when light is abundant or excessive. On a daily as well as seasonal basis most plants receive more sunlight than they can actually use for photosynthesis. Under these circumstances, regulation of light harvesting is necessary to balance the absorption and utilization of light energy, thereby minimizing the potential for photooxidative damage. Besides adjusting light absorption, algae and plants have ways of getting rid of excess light energy that has already been absorbed. This update will focus on protective non-photochemical mechanisms that quench singlet-excited chlorophylls (Chl) and harmlessly dissipate excess excitation energy as heat. These non-photochemical quenching (NPQ) processes occur in almost all photosynthetic eukaryotes, and they help to regulate and protect photosynthesis in environments in which light energy absorption exceeds the capacity for light utilization. We will summarize progress in understanding NPQ that has been made since the last Update article on NPQ appeared in this journal (Horton et al., 1994).