Desiccation tolerant (DT) plants engage and disengage sustained forms of energy dissipation in response to desiccation and rehydration. This project sought to characterize the role of zeaxanthin and thylakoid protein phosphorylation status in sustained energy dissipation during desiccation in bryophytes with varying DT. Tolerant (Polytrichum piliferum, Dicranum species, Calliergon stramineum) and sensitive (Grimmia species, Schistidium rivulare, Sphagnum species) moss were desiccated in darkness or natural light conditions for up to three weeks. Desiccation caused pronounced reductions in Fv/Fm in all cases which was enhanced by light exposure during desiccation. Desiccation in darkness resulted in no accumulation of Z in any species, however, in natural light conditions there was significant accumulation of Z in tolerant but not sensitive species. Desiccation in natural light, relative to darkness, resulted in more pronounced reductions in Fo in tolerant but not sensitive species. Recovery of Fv/Fm upon rehydration occurred in two phases, a rapid phase (minutes) and a slower phase (hours). Increased time of desiccation, and light exposure, resulted in a reduction in the rapid phase. Desiccation in light conditions resulted in some accumulation of the phosphorylated form of the major light harvesting trimer (LHCII). Data are consistent with two mechanisms of sustained quenching, neither of which requires Z. However, when desiccation occurs in natural light conditions, accumulation of Z likely contributes to one or both of the sustained forms of dissipation. Increases in LHCII phosphorylation during desiccation are consistent with increased connectivity between the photosystems. The absence of Z formation in sensitive species may contribute to their lack of desiccation tolerance.

Desiccation and low temperatures inhibit photosynthetic carbon reduction and, in combination with light, result in severe oxidative stress, thus, tolerant organisms must utilize enhanced photoprotective mechanisms to prevent damaging reactions from occurring. We sought to characterize the desiccation tolerance of the fern Polypodium virginianum and to assess the role of the xanthophyll cycle and sustained forms of thermal dissipation in its response to desiccation, as well as to low temperatures during winter. Our results demonstrate that P. virginianum is desiccation‐tolerant and that it increases its utilization of sustained forms of zexanthin (Z)‐dependent thermal dissipation in response to desiccation and low temperatures during winter. Experiments with detached fronds were conducted in dark and natural light conditions and demonstrated that some dark‐formation of Z occurs in this species. In addition, desiccation in the light resulted in more pronounced declines in maximal photochemical efficiency (Fv/Fm) and higher Z levels than desiccation in the dark, indicating a substantial fraction of the sustained reduction in Fv/Fm is due to Z‐dependent sustained dissipation. Recovery from desiccation and from low temperatures exhibited biphasic kinetics with a more rapid phase (1–4 h), which was accompanied by an increase in minimal fluorescence yield (Fo) but no change in Z, and a slower phase (up to 24 h) correlating with reconversion of Z to violaxanthin. These data suggest that two mechanisms of sustained thermal dissipation occur in response to desiccation and low temperatures, possibly corresponding to sustained forms of the energy‐dependent and zeaxanthin‐dependent mechanisms of dynamic thermal dissipation.