Desiccation and freezing of photosynthetic tissues are among the most relevant abiotic factors that result in an imbalance between absorption of light energy and its utilization for photosynthetic carbon gain. Tolerance to desiccation is a rare strategy among angiosperms (so far less than 200 species) and among them only a couple of genera (i.e.: Ramonda) occur in environments experiencing regular freezing events. These species provide a valuable opportunity for the comparative study of the photosynthetic and photoprotective responses to both extreme stresses. In this study, we explore the photoprotective processes that Ramonda myconi, a subalpine resurrection plant with perennial leaves endemic to the Pyrenees, develops in response to cold versus desiccation stress. Specifically, we followed changes in photochemical efficiency and pigment composition during desiccation/rehydration and during freezing/de-freezing cycles in field- and lab-experiments. Additionally, we performed an Infrared Differential Thermal Analysis (IDTA) of thermal images during controlled cooling of leaves to determine the temperature at which ice formation occurs.
Our results demonstrated that desiccation in darkness caused a significant reduction in photochemical efficiency (Fv/Fm) that corresponded to dark-production of the photoprotective pigment zeaxanthin (Z). Fv/Fm recovered rapidly upon rehydration, in concert with reconversion of Z to violaxanthin (V). Results from winter and freezing treatments show that Fv/Fm decreases as a function of leaf temperature, with values dropping steadily as temperatures drop below 0˚C. Leaves collected in the frozen state, with low Fv/Fm, maintained high levels of Z. Upon artificially warming leaves in the lab Fv/Fm recovered quickly, with similar kinetics as leaves recovering from desiccation, and this corresponded to reconversion of Z to V. Experiments doing controlled cooling of winter acclimated leaves showed that leaves freeze around -3°C. This temperature also represents the threshold at which Fo experiences an abrupt decrease even when leaves are cooled in the dark. However in contrast to desiccation in the dark, controlled freezing of leaves in the dark did not induce significant Z accumulation. The fast acclimation of the photosynthetic apparatus to the changes in the availability of liquid water within the leaf (as result of desiccation or freezing) are critical for allowing plants to cope with extreme variations in the environment.