PHOTOSYNTHETIC RESPONSE TO SUNFLECKS IN THE DESICCATION-TOLERANT FERN POLYPODIUM-VIRGINIANUM

The photosynthetic response to sunflecks was investigated in the fern Polypodium virginianum growing on cliff edges of the Niagara Escarpment, in southern Ontario, Canada. Ferns were exposed to natural precipitation. continuous watering, or continuous drought prior to exposure to artificial sunflecks. The results were analyzed using temperature, humidity and previous exposure to light as covariates. The response to sunflecks was extremely rapid and showed no measurable induction period in the field. Controlled experiments completed in the laboratory showed exceptionally short induction periods. Previous exposure to sunflecks, and desiccation did not influence the time constants of response to sunflecks although drying did reduce maximum rates of CO2 exchange. Polypodium virginianum shows an extremely rapid response to fluctuations in the supply of light. and this is interpreted to be advantageous to this species because of the array of extremely hostile conditions that occur at cliff edges.