PHOTOSYNTHETIC CAPACITY CHANGES IN PELTIGERA I. THE SYNTHESIS OF ADDITIONAL PHOTOSYNTHETIC UNITS IN P. PRAETEXTATA

S ummary Changes in the photosynthetic capacity of Peltigera preaetextata (Somm.) Vain, over the course of a year have been examined using photosynthetic‐illumination curves. The results have been interpreted using the photosynthetic response models originally proposed by Prézelin. The low net photosynthetic rates obtained following prolonged winter snow cover are interpreted as resulting from the interruption of energy transduction in some photosynthetic units. In spring, full energy transduction is rapidly re‐established by the storage of experimental replicates at a warm temperature and results in a marked increase in net photosynthetic rates. As expansion of the tree canopy takes place both apparent quantum efficiency and photosynthetic capacity increase markedly, whilst respiration decreases significantly, providing high rates of net photosynthesis even under light levels as low as 75 μmol m −2 s −1 . This change in summer capacity is interpreted as being due to the synthesis of additional photosynthetic units in response to the low light conditions on the woodland floor. The data are discussed both in relation to comparable events in other lichen species and to the potential significance of seasonal changes in photosynthetic capacity in the ecology of P. praetextata.