Abstract
Seagrasses inhabiting the intertidal zone experience periodically repeated cycles of air exposure and rehydration. However, little is known about the photoprotective mechanisms in photosystem (PS)II and PSI, as well as changes in carbon utilization upon air exposure. The photoprotective processes upon air exposure in Halophila beccarii Asch., an endangered seagrass species, were examined using the Dual-PAM-100 and non-invasive micro-test technology. The results showed that air exposure enhanced non-photochemical quenching (NPQ) in both PSII and PSI, with a maximum increase in NPQ and Y(ND) (which represents the fraction of overall P700 that is oxidized in a given state) of 23 and 57%, respectively, resulting in intensive thermal energy dissipation of excess optical energy. Moreover, cyclic electron transport driven by PSI (CEF) was upregulated, reflected by a 50 and 22% increase in CEF and maximum electron transport rate in PSI to compensate for the abolished linear electron transport with significant decreases in pmfLEF (the proton motive force [pmf]) attributable solely to proton translocation by linear electron flow [LEF]). Additionally, H+ fluxes in mesophyll cells decreased steadily with increased air exposure time, exhibiting a maximum decrease of six-fold, indicating air exposure modified carbon utilization by decreasing the proton pump influxes. These findings indicate that efficient heat dissipation and CEF confer daily air exposure tolerance to the intertidal seagrass H. beccarii and provide new insights into the photoprotective mechanisms of intertidal seagrasses. This study also helps explain the extensive distribution of H. beccarii in intertidal zones.
Highlights
Seagrass ecosystems have a global distribution, offering indispensable ecosystem services (Hemminga and Duarte, 2000; Barbier et al, 2011), including carbon sequestration (Duarte and Krause-Jensen, 2017), sediment stabilization (Bos et al, 2007), and provision of food and habitats, as well as the reduction of pathogenic microorganisms (Lamb et al, 2017)
Our results demonstrated that the increase in transient postillumination did not affect chlorophyll fluorescence when subjected to air exposure treatment (Figure 2A)
We tentatively propose that cyclic electron transport contributed substantially to air exposure tolerance for intertidal H. beccarii (Figure 9), which helped explain the broad distribution of H. beccarii in intertidal zones
Summary
Seagrass ecosystems have a global distribution, offering indispensable ecosystem services (Hemminga and Duarte, 2000; Barbier et al, 2011), including carbon sequestration (Duarte and Krause-Jensen, 2017), sediment stabilization (Bos et al, 2007), and provision of food and habitats, as well as the reduction of pathogenic microorganisms (Lamb et al, 2017). NPQ is a method for heat dissipation associated with quenching of excitation energy similar to overloading optical energy (Müller et al, 2001; De Carvalho et al, 2011; Yamakawa et al, 2012; Yamakawa and Itoh, 2013) It mainly involves a low thylakoid lumen pH- and zeaxanthin-dependent quenching mechanism (Xu et al, 2015). It has been reported that carotenoids and zeaxanthin that accumulate upon desiccation are crucial to the activation of NPQ, as well as protection of the thylakoid membranes from peroxidation (Havaux et al, 2007; Du et al, 2010; Beckett et al, 2012) Another protection mechanism mentioned above is involved in the compensation for abolished linear electron flow (LEF) (Gao et al, 2011). It has been proposed that both NPQ and CEF contribute largely to the protective and adaptive mechanisms of intertidal plants (Gao et al, 2011; Zia et al, 2016; Tan et al, 2017)
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