Abstract
The photoprotective mechanisms of desiccation tolerance and freezing tolerance and their relation to molecular mobility (cell vitrification) were assessed in a single model: the exceptional subalpine and resurrection plant Ramonda myconi. Dehydrated leaves showed a drop in maximal photochemical efficiency of PSII (F-v/F-m) accompanied by synthesis of zeaxanthin (Z), even in the dark, which was limited by cell vitrification after complete desiccation. The recovery of F-v/F-m after a severe drying treatment (7 days at 50% relative humidity) confirmed the tolerance of R. myconi leaves to desiccation. In winter, R. myconi plants showed a highly dynamic component of photoinhibition. Interestingly, the potential activity of the enzyme violaxanthin de-epoxidase (VDE) occurred at -7 degrees C, below the freezing temperature range of the leaves (-2 +/- 2 degrees C) and even in the dark. This suggests that, in nature, the enzyme can still be active in frozen leaves, as long as they are above the glass transition temperature. The drop in F-v/F-m and increase in Z were reversible upon rehydration and thawing, respectively, but were not perfectly matched, suggesting that both Z-independent and Z-dependent forms of sustained dissipation are occurring. Overall, our data reinforce the light-independent activity of the VDE enzyme under stress and suggest that Z-accumulation could occur in darkness in a scenario when temperatures drop dramatically in the night under natural conditions.