Photosynthetic Capacity Changes in Peltigera

S ummary The seasonal net photosynthetic response matrix to moisture, temperature and light has been examined in two populations of Peltigera rufescens (Weis) Mudd, geographically widely separated and representative of arctic and temperate climates, respectively. The arctic population shows a seasonally constant net photosynthetic rate whereas the temperate population exhibits a marked summer increase of photosynthetic capacity under high temperatures and levels of illumination. This change in capacity is shown by use of photosynthetic‐illumination curves to be due to temperature acclimation, presumably involving a seasonal enzymatic change in the Calvin cycle. Experimental induction of the summer or winter response can be achieved by storage of air‐dry thalli under an appropriate combination of ambient temperature and day length. The data are discussed both in relation to the ecology of the two contrasting populations and to seasonal changes in the photosynthetic capacity of other lichen species.