Adaptations to extreme low light in the fern Trichomanes speciosum

Trichomanes speciosum is a threatened species restricted to sheltered, very humid sites. Uniquely amongst European ferns, differing ecological toleran ces of the gametophyte and sporophyte generations are manifested as widely differing distributions. The perennial, vegetatively propagating gametophyt e persists in drier, colder, darker habitats than the sporophyte. In sites where the gametophyte grows, light availability was found to be <1 <mu>mol m(-2) s(-1) for approx. 85% of daylight hours, rarely or tin some sites) ne ver rising above 10 mu mol m(-2) s(-1). Much of the time, light was <0.01% of full sunlight. Measurements of gas exchange and chlorophyll fluorescence yield show that these plants have optimal photosynthesis at light intensit ies c. 5-10 <mu>mol m(-2) s(-1), the highest light to which they are normal ly exposed to in their natural environment. The absence of any capacity for reversible nonphotochemical fluorescence quenching means that there is lit tle or no protection of the photosynthetic apparatus from light-induced dam age. We conclude that these plants are able to create what are essentially monocultures in their. extreme environments only because of a combination o f low metabolic rate (at low temperatures) and an ability to make efficient use of what little light is available to them by morphological and physiol ogical means.