As we saw earlier in the case of mitochondria, reactive oxygen species (ROS) generated during electron transport (see Figure 12-35) can both serve as signals to regulate organelle function and cause damage to a variety of biomolecules. The same is true of chloroplasts. For example, hydrogen peroxide (H₂O₂) regulates gas-exchange pores (stomata) in plants subjected to drought stress to prevent dehydration and controls cyclic electron flow, a process we will describe shortly.

Even though the PSI and PSII photosystems, with their associated light-harvesting complexes, are remarkably efficient at converting radiant energy to useful chemical energy in the form of ATP and NADPH, they are not perfect. Depending on the intensity of the light and the physiological conditions of the cells, a relatively small—but significant—amount of the energy absorbed by chlorophyll in the light-harvesting antennas and reaction centers results in the chlorophyll being converted to an activated state called triplet chlorophyll. In this state, the chlorophyll can transfer some of its energy to molecular oxygen (O₂), converting it from its normal, relatively unreactive ground state, called triplet oxygen (³O₂), to a very highly reactive (ROS) singlet state, ¹O₂. Some of this ¹O₂ can be used for signaling to the nucleus to communicate the metabolic state of the chloroplast to the rest of the cell. However if the majority of the ¹O₂ is not quickly quenched by reacting with specialized ¹O₂ “scavenger molecules,” it will react with, and usually damage, nearby molecules. This damage, called photoinhibition, can suppress the efficiency of thylakoid activity.

Carotenoids (polymers of unsaturated isoprene groups, including β-carotene, which gives carrots their orange color) and α-tocopherol (a form of vitamin E) are hydrophobic small molecules that are known to play important roles as ¹O₂ quenchers to protect plants. For example, inhibition of tocopherol synthesis in the unicellular green alga Chlamydomonas reinhardtii by the herbicide pyrazolynate can result in greater light-induced photoinhibition. The carotenoids, which very efficiently siphon off energy from dangerous triplet chlorophyll when they are in close proximity to it, are the quantitatively most important molecules for preventing ¹O₂ formation. The PSII monomer from the cyanobacterium Thermosynechococcus elongatus contains about 11 carotenoid molecules compared with its 35 chlorophylls, indicating the importance of carotenoid-mediated quenching of ¹O₂.

Under intense illumination, photosystem PSII is especially prone to generating ¹O₂, whereas PSI will produce other ROS, including superoxide, hydrogen peroxide, and hydroxyl radicals. One of the protein subunits in the PSII reaction center, called D1, is subjected to almost constant ¹O₂-mediated damage even under low light conditions. A damaged reaction center moves from the grana to the unstacked regions of the thylakoid, where the D1 subunit is degraded by a protease and replaced by newly synthesized D1 protein in what is called the D1 protein damage-repair cycle. This rapid replacement of damaged D1, which requires a high rate of D1 synthesis, helps PSII recover from photoinhibition and maintain sufficient activity. An important component in the damage-repair cycle is a chaperone of the HSP70 family (see Chapter 3) called HSP70B. This chaperone binds to the damaged PSII and helps prevent loss of the other components of the complex as the D1 subunit is replaced. The extent of photoinhibition can depend on the amount of HSP70B available to the chloroplasts.