Photosynthesis in Desiccated Shoots of Polytrichum

S ummary The desiccation responses of four species of Polytrichum were examined. The relationship between CO 2 assimilation and shoot water content was similar in Polytrichum commune and Polytrichum piliferum , the species most dissimilar in ecological requirements. There were, however, considerable differences between species in recovery following various desiccation treatments. Continuous desiccation was found to be more damaging than the same total period of desiccation interrupted by periods of wetting. Polytrichum piliferum was least affected by any of the desiccation treatments. Desiccation to 76 % r.h. was found to suppress the photosynthetic rate of both Polytrichum commune and Polytrichum piliferum after rewetting more than treatment at 55% or 90% r.h. Subsequent recovery was generally better following the 90% r.h. treatment. Slow desiccation of Polytrichum commune was substantially less damaging than rapid drying.