RESUMO
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.
