chlorophylls can follow one of several competitive pathways: transformation into chemical energy via photochemistry and photosynthetic electron transport, transfer to oxygen to form ROS, re-emission as fluorescence from excited chlorophyll molecules, or dissipation as heat via NPQ. This last pathway of heat dissipation functions like a “safety valve” for photosynthesis (Niyogi, 2000) that prevents or reduces damage from excess light.
Some carotenoids function in NPQ and directly quench ROS such as singlet oxygen (Baroli et al., 2000). Importantly, a strong correlation between zeaxanthin accumulation and a rapidly inducible form of NPQ, known as energy-dependent quenching (qE; Horton et al., 1996; Niyogi, 2000), has been demonstrated in several tracheophyte species (Demmig-Adams, 1990; Demmig-Adams and Adams, 1996). Sustained NPQ mechanisms, often referred to as photoinhibitory quenching (qI), result in a decrease in the quantum efficiency of photosynthesis and can also be associated with zeaxanthin, though possibly through a different, pH-independent mechanism (Verhoeven et al., 1996). Desert plants might be expected to undergo the qE form of NPQ for diurnal fluctuations in light intensity as well as qI or other sustained NPQ forms, e.g. qH (Malnoë, 2018), to deal with seasonal changes in light. Indeed, desiccation-tolerant mosses have been shown to exhibit strong, sustained mechanisms of NPQ after exposure to high light or desiccation (Yamakawa et al., 2012; Yamakawa and Itoh, 2013).
In addition to changes in overall light intensity, plants, like other organisms, are sensitive to UV radiation, an important stressor that plants must cope with in nature (Jansen et al., 1998; Wolf et al., 2010). An array of cellular components are damaged by absorption of UV-B radiation (280 – 315 nm), including components of the photosynthetic apparatus (Teramura and Sullivan, 1994; Jansen et al., 1998). UV-B
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