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
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