Abstract
CO2 exchange (CO2 assimilation, dark respiration), potential photochemical activity and blue and red fluorescence ratios (F450/F690, F690/F735) of the desiccation-tolerant (DT) ectohydric moss (Tortula ruralis) and two DT lichens (Cladonia convoluta and C. furcata) from a temperate semidesert grassland during desiccation and rehydration are reported. Optimal water content for carbon fixation (RWC = 100%) was less than 50% of the saturation water content in all investigated species. The water-content compensation point of CO2 assimilation was also low and ranges 6-8% of saturation water content. Thylakoid membrane activity as measured by slow chlorophyll fluorescence is unchanged through a broader range of water content. On rehydration photosynthesis recovered fully and rapidly within 1 h in all three species. The low water content requirement for net CO2 assimilation in the three DT species allows them to revive periodically and prosper during the dry seasons. These plants can gain water from dew or air humidity at dawn and maintain slight positive carbon balance day by day. During the desiccation-rehydration cycles blue/(far)red and red/far red fluorescence ratios (F450/F690, F450/F735, F690/F735) suggest changes in cell volumes and in the optical properties of the plants rather than shifts in the photosynthetic pigment and secondary metabolite levels.
