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

Seasonal changes in photosynthesis were examined in the desiccation-tolerant fern Polypodium virginianum growing in a forest understory along cliff edges of the Niagara Escarpment in southern Ontario, Canada. For plants growing in situ, the photosynthetic response to irradiance was examined on a seasonal basis, to determine the degree to which the utilization of light changed over the growing season. Experiments were executed on control plants, on previously desiccated then rehydrated plants, and on continuously hydrated plants to determine if prior desiccation influenced the response to light. Soil and xylem water potential and temperature were monitored and used as covariates in analyses. The results showed that carbon gain in the spring greatly exceeded that of any other season. Despite this, there was little change in the photosynthetic response to light on a seasonal basis even though plants were exposed to highly variable and highly limited light most of the time. Prior desiccation had a slight influence on photosynthetic rate but not on other photosynthetic parameters such as the light compensation point and Lhalf. Temperature was a significant seasonal covariate and additional experiments conducted in the laboratory showed that the response of photosynthesis to temperature was broad. Xylem water potential was correlated with seasonal changes in relative humidity. The results suggest that P. virginanum persists in hostile cliff-edge habitats by being able to exploit high-light periods in the spring and by thereafter maintaining a low but relatively constant rate of carbon despite prior exposure to fluctuating supplies of light and water.