Effects of different light intensities, CO 2 concentrations, temperatures and drought stress on photosynthetic activity in two paleoendemic resurrection plant species Ramonda serbica and R. nathaliae

Abstract

Ramonda serbica and R. nathaliae are rare resurrection flowering plants of the northern hemisphere. Although their habitats on the north-exposed slopes of canyons and gorges are similar, those inhabited by R. nathaliae are more varied in terms of humidity, temperature and type of bedrock. These fine differences in ecological preference lead us to ask whether diploid R. nathaliae is ecologically more plastic than hexaploid R. serbica. Photosynthetic performance and fluorescence parameters are central in evaluating a plant's capacity to withstand extreme environmental conditions. We have compared the effects of various light intensities, CO2 concentrations and temperatures on the photosynthetic response of well hydrated plants of the two species. We also tracked changes in chlorophyll a fluorescence, photochemical efficiency and non-photochemical quenching during plant desiccation. The results have shown that both species have a low compensation point and low dark respiration that are indispensable for their survival at shaded and humid sites; only there are they able to maintain an appropriate equilibrium between positive net carbon gain and optimal water status. Under conditions of saturating light and CO2, net CO2 uptake was always higher in R. nathaliae that showed lower sensitivity to temperatures between 25 °C and 40 °C. Moreover, the leaves of R. nathaliae are amphistomatic, with smaller but more numerous stomata than those in the hypostomatic leaves of R. serbica. These xeromorphic characters of R. nathaliae suggest the better regulation of gas exchange and efficiency of water use, under the conditions of everyday short periods of direct sunlight and higher temperatures, making it possible for this species to inhabit more open, warm and dry habitats than R. serbica. Pronounced drought stress induced, in both species, strong decline in fluorescence parameters, photochemical quenching and chain electron transport rate. At the same time the exceptionally high non-photochemical quenching provided the main and highly effective mechanism of energy dissipation and protection against over-excitation. Although being homoiochlorophyllous, chloroplasts in desiccated plants are well protected from photodamage in two ways. First, the leaves are curled inward with the palisade tissue oriented to the inner side of the leaf. Second, the lower epidermis, that hence becomes exposed to sunlight, is densely covered with dark non-glandular hairs and is rich in carotenoids, anthocyanins and phenolics. All this protects desiccated plant tissues against excessive light induced damage.