Altered Fv/Fm recovery following UV-filtering
Surprisingly, F v/F m was not affected in desiccated S. caninervis when natural levels of UV were reduced, but the recovery ofF v/F m was impaired during at least 192 hours in winter recovery conditions (Figure 2). In contrast, many plants respond to supplemental UV radiation with reducedF v/F m (Bradshaw, 1965; Strid et al. , 1990; He et al. , 1993; Pukacki & Modrzyński, 1998; Ranjbarfordoei et al. , 2011; but see Takácset al. , 1999; Csintalan et al. , 2001; Lau et al. , 2006; Basahi et al. , 2014). Furthermore, relative abundance of the xanthophylls zeaxanthin and lutein was also increased in UV-filtered plants (Figure 3), a response also typically seen with UV supplementation (Agrawal, Singh & Agrawal 2009). Why should removal of UV radiation, presumably a stressor, result in altered recovery ofF v/F m and more antioxidant xanthophylls in S. caninervis ? One possible explanation for the observed defect in F v/F mrecovery is that removal of UV somehow causes an impairment in relaxation of sustained NPQ. As with un-manipulated field-collected plants, the observed F v/F mincrease over the recovery period for UV-filtered and UV-transmitted plants was driven by an increase in F m and thus is consistent with relaxation of sustained NPQ. Indeed, the increased abundance of zeaxanthin with removal of UV is consistent with the hypothesis that UV filtering induces a sustained zeaxanthin-related NPQ (Verhoeven et al. 1996).
It is possible that UV radiation is a photomorphogenic (Gitz & Liu-Gitz 2003) or regulatory signal rather than (or in addition to) being a stressor and the absence of this signal affects NPQ recovery. For example, UV may induce production of enzymatic antioxidants or phenolics (Cooper-Driver, Bhattacharya & Harborne 1998; Clarke & Robinson 2008; Waterman et al. 2017) that may have roles beyond UV protection, such as in desiccation tolerance (Gitz & Liu-Gitz 2003; Poulson, Boeger & Donahue 2006; Robson, Hartikainen & Aphalo 2015). Without these protections, the natural desiccation regime in the field might cause more photo-oxidative stress. In addition to increased VAZ pool size (Figure 3), the relative abundance of tocopherols increased with removal of UV from S. caninervis in the field (Figure 5 and Table 4), consistent with increased ROS activity. Tocopherols quench singlet oxygen from the PSII reaction center (Trebst, Depka & Holländer-Czytko 2002; Trebst 2003; Krieger-Liszkay 2005), and α-tocopherol has been shown to confer antioxidant protection to thylakoid membranes in UV-B exposed spinach plants (Delong & Steffen 1998). There are a number of stress protection mechanisms that are mediated by UVR8, the UV-B sensing protein receptor (Singh et al. 2014), many of which could result in slower F v/F m recovery and increased antioxidant abundance with removal of UV. In fact, the UV-B response pathway and the photomorphogenesis pathway have substantial overlap (Stanley & Yuan 2019).