Introduction
Drylands represent the largest terrestrial biome, accounting for at least 35% of Earth’s land mass (Middleton and Thomas, 1992; Peel et al., 2007). These ecosystems experience extreme daily and seasonal fluctuations in light, temperature, and water availability, often concomitantly. Interaction between extreme environmental conditions such as low water availability and high light represents a particular challenge for plants. To survive dry periods, many dryland bryophytes and a smaller number of vascular plants have evolved vegetative desiccation tolerance, defined as the ability to equilibrate to dry air and resume metabolic activity after rehydration (Gaff, 1977; Proctor et al., 2007; Stark, 2017). Yet, while desiccation tolerance allows these plants to survive dry periods by limiting metabolic activity to periods of adequate moisture availability, this adaptation implicates long periods of exposure to high light intensity during full sun, including unusable photosynthetically active radiation (PAR) and direct UV radiation, both of which may be harmful.
Plants may respond to radiation stresses via photosynthetic pigments and antioxidants (Demmig-Adams and Adams, 1996; Frohnmeyer and Staiger, 2003; Li et al., 2009; Liguori et al., 2017). During periods of high-light exposure excess energy absorbed by chlorophylls forms reactive oxygen species (ROS), which react with and damage sensitive molecular machinery (Li et al., 2009). Plants, therefore, face the trade- off of maximizing light absorbance for use in photosynthesis while also providing adequate photoprotection to minimize ROS damage; desert mosses need to balance these requirements both when metabolically active, and when desiccated. One of the major photoprotective mechanisms in plants is the dissipation of excess light energy as heat, a set of processes collectively known as non-photochemical quenching (NPQ; Müller, Li & Niyogi 2001; Ruban 2016; Malnoë 2018). Light energy absorbed by 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 triggers the production of carotenoids (Middleton and Teramura, 1993), and some of the same high-light photoprotective mechanisms can also protect plants from UV radiation. For example, it was demonstrated that zeaxanthin contributes to UV stress protection and damage prevention in tobacco (Götz et al., 2002). Additionally, some plants have evolved UV-absorbing chemical sunscreens such as flavonoids that reduce the amount of UV reaching sensitive molecules (Tohge and Fernie, 2017).
Exposure to UV radiation does not always yield a negative effect for photosynthetic organisms, however. Recently there has been a paradigm shift in understanding UV as a regulatory signal rather than solely a stressor, as UV perception is involved in critical metabolic functions (Rozema et al., 1997; Davey et al., 2012; Hideg et al., 2013; Morales et al., 2013; Singh et al., 2014; Williamson et al., 2014; Neugart and Schreiner, 2018). Researchers have begun to instead classify UV radiation as a “eustress,” (Hideg et al., 2013). In this framework, UV-B is understood to stimulate a state of alert that includes activation defenses, especially if the radiation is experienced in small doses. For example, UVR8, the UV-B receptor in plants, mediates the accumulation of transcripts encoding early light-inducible proteins (ELIPs) (Singh et al., 2014), which function in photoprotection (Hutin et al., 2003). Furthermore, low doses of UV radiation can induce protective responses that increase a plant’s tolerance to other abiotic and biotic stressors (Frohnmeyer and Staiger, 2003). For instance, ELIPs are also important for desiccation tolerance in resurrection plants (Zeng, 2002; Oliver et al., 2004; Van Buren et al., 2019).
Although many mosses are found in cool, low light environments, several species are abundant in drylands where they are common and important members of biological soil crusts (biocrusts). Biocrusts are complex communities of bryophytes, lichens, fungi, cyanobacteria, and other microorganisms living on the surface of soil in drylands (Belnap et al., 2003). These communities provide critical ecosystem services such as reducing erosion, increasing soil fertility and water infiltration, and even facilitating germination of native seeds while reducing germination of large-appendaged exotic seeds (Harper and Belnap, 2001; Belnap, 2002, 2006; Belnap et al., 2003; Hawkes, 2004; Li et al., 2005; Su et al., 2007). Mosses play important roles in biocrusts, such as contributing to both soil stability via rhizoids and soil formation via capture of nutrient- rich fine particles (Seppelt et al., 2016). Moreover, in some dryland ecosystems, biocrust mosses control the overall carbon balance by reaching peak photosynthetic activity during winter months when surrounding shrubs are dormant (Zaady et al., 2000; Jasoni et al., 2005). However, Mojave Desert mosses are faced with being quiescent during hot, dry summers and are thus unable to use any of the intense solar radiation for photosynthesis (Stark, 2005). Furthermore, while many plants have morphological mechanisms to reduce absorption of excess light, such as altering leaf angle or the production of a waxy cuticle, mosses both lack thick cuticles (Jeffree, 2007) and are unable to alter leaf angle once desiccated. Although their dry state is often a curled state thought to be a protective adaptation for minimizing light absorption (Zotz and Kahler, 2007), it may not alone be enough to protect desert mosses from the long-term excess light and intense UV radiation they face while quiescent.
Studies on UV protection in mosses have been limited with most focus on Antarctic mosses and UV-B supplementation in greenhouses or growth chambers (Searles et al., 2001; Gwynn-Jones et al., 1999; Searles et al., 1999; Lud et al., 2002; Martínez-Abaigar et al., 2003; Newsham, 2003; Green et al., 2005; Núñez-Olivera et al., 2005; Robinson et al., 2005; Dunn and Robinson, 2006; Björn, 2007; Turnbull et al., 2009). Thus, there is a need for a better understanding of the effects of natural levels of UV radiation in a field setting. While nearly all mosses tested in nature appear to be minimally damaged by ambient UV levels, (Boelen et al., 2006), in some species UV protection appears to be physiologically constitutive and in others it is plastic. For example, the Antarctic mosses Ceratodon purpureus and Bryum subrotundifolium exhibit sun forms that are tolerant to UV and shade forms that are not but can be acclimated to UV within a week in natural sunlight (Green et al., 2005). On the other hand, in the mosses Sanionia uncinata, Chorisodontium aciphyllum, Warnstorfia sarmentosa, and Polytrichum strictum, also from Antarctica, UV-B absorbing compounds are not induced by enhanced UV-B radiation (Boelen et al., 2006). Similarly, field-collected plants of Syntrichia ruralis, a dryland moss, was unaffected by supplemental UV-B radiation, based on chlorophyll fluorescence (Takács et al., 1999; Csintalan et al., 2001). Yet while this species appears to have sufficient UV protection, it is unclear whether it is constitutive or inducible, whether with UV or another environmental cue. Studies have shown that UV tolerance correlates with desiccation tolerance (Takács et al., 1999), and that desiccation itself confers extra protection from UV in two Antarctic mosses (Turnbull et al., 2009). Both habitat and genetics are strong predictors of UV tolerance in bryophytes but there is much within- and among-genera variability (Hespanhol et al., 2014). Thus, the need to study each species in its own environment is critical to understanding how UV is tolerated in nature.
The desert moss Syntrichia caninervis is a highly desiccation-tolerant (Proctor et al., 2007; Stark, 2017) important member of western North American dryland biocrust communities, including in the Mojave Desert (Stark et al., 1998; Bowker et al., 2000; Coe et al., 2012; Antoninka et al., 2016; Seppelt et al., 2016). This species frequently forms continuous or semi-continuous carpets in exposed, intershrub desert soil crusts and tolerates high levels of solar radiation while dry. Interestingly, mature shoots of S. caninervis develop a dark brown or black coloration in nature (Fig. 1A) but remain bright green when grown in dim, artificial laboratory light (personal observation), suggesting a plastic pigment-accumulation reaction in response to light exposure. Accumulation of dark pigmentation varies in nature, too. S. caninervis plants are greener in very low-light microhabitats (Ekwealor and Fisher, 2020) and when UV is filtered out of natural sunlight (unpublished data). This apparent “suntan” pattern suggests the possibility of an adaptive response for UV protection, though that function has not yet been tested in S. caninervis.
To this end, we conducted an integrated, four-part experiment to test how desert mosses withstand solar radiation while quiescent under natural and extreme fluctuations in climate and solar radiation characteristic of the Mojave Desert. We deployed a year-long, controlled UV-reduction manipulation on twenty in-situ microsites of S. caninervis to test the hypotheses that: (1) natural S. caninervis plants undergo sustained NPQ while desiccated and after rehydration, (2) if UV radiation is a stressor then a reduction of natural levels of UV will result in improved recovery of maximum PSII quantum efficiency (Fv/Fm) but (3) one year of UV removal will de- harden plants and thus increase vulnerability to UV damage, indicated by a reduction in Fv/Fm after an laboratory UV treatment. In order to better understand the mechanisms of photoprotection, UV tolerance, and recovery from desiccation, we measured relative abundance of photosynthetic pigments and antioxidants in field-manipulated plants, and quantified differential transcript abundance on UV-reduced plants and controls. Finally, to understand the effects of the high light and desiccating natural environment on the pigment and antioxidant profiles, we compared field-collected, un-manipulated S. caninervis plants to those cultured in a laboratory growth chamber.