Seasonal changes in photosynthesis in the desiccation-tolerant fern Polypodium virginianum

Sustained zeaxanthin-dependent thermal dissipation is induced by desiccation and low temperatures in the fern Polypodium virginianum

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 (F_v /F_m ) and higher Z levels than desiccation in the dark, indicating a substantial fraction of the sustained reduction in F_v /F_m 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 (F_o ) 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.

Photosynthesis limitations in three fern species

Maximum photosynthesis rates in ferns are generally lower than those of seed plants, but little is known about the limiting factors, which are crucial to understand the evolution of photosynthesis in land plants. To address this issue, a gas exchange/chlorophyll fluorescence analysis was performed in three fern species spanning high phylogenetic range within Polypodiopsida (Osmunda regalis, Blechnum gibbum and Nephrolepis exaltata) to determine their maximum net photosynthesis (AN ), stomatal (gs ) and mesophyll (gm ) conductances to CO2 , and the maximum velocity of carboxylation (Vc,max ). The in vitro Rubisco specificity factor (SC /O ) was also determined. All three species had values for SC /O similar to those typical of seed plants, but values of AN , gs , gm and Vc,max were within the lowest range of those observed in seed plants. In addition, gs was unresponsive to light and CO2 , as already described in other fern species. On the contrary, gm varied with changes CO2 . A quantitative photosynthesis limitation analysis suggested that early land plants (ferns) presented not only stomatal limitations-which were less adjustable to the environment-but also restricted gm and Vc,max , resulting in limited maximum photosynthesis rates.

Are correlations among foliar traits in ferns consistent with those in the seed plants?

Broad-based studies of gymnosperms and angiosperms reveal consistent and functionally significant correlations among foliar traits such as leaf mass per area (LMA), maximum photosynthetic rate (A(area)), foliar nitrogen (N(area)), foliar chlorophyll (Chl) and leaf longevity. To assess the generality of these relationships, we studied 20 fern species growing in the understorey of a temperate deciduous forest. We found that foliar N(area) increases with LMA, and that foliar N(area) and A(area) are positively correlated with one another, as are foliar N(area) and Chl. The ferns in general have very low LMA compared with most seed plants; A(area), N(area) and Chl are below median values for seed plants but are not extreme. Species with overwintering fronds have significantly higher LMA than species with fronds that senesce at the end of the growing season, as well as a significantly higher C : N ratio in frond tissue and relatively high foliar N on an areal basis. Correlations among foliar traits associated with gas exchange in these forest understorey ferns are in accordance with patterns reported for seed plants, suggesting a high degree of functional constraint on the interrelationships among key elements in foliar design.