Photosynthetic capacity and PSII efficiency of the evergreen alpine cushion plant Saxifraga paniculata during winter at different altitudes

The effect on photosynthetic capacity, photochemical efficiency of photosystem II (PSII), and frost resistance to environmental changes during winter in the evergreen alpine cushion plant Saxifraga paniculata were investigated at different altitudes. S. paniculata was more resistant to cold induced photoinhibition during midwinter than other evergreen subalpine species. Photochemical efficiency (F-v/F-m) was only reduced to 0.62. Photosynthetic capacity was only reduced if weekly mean leaf temperatures dropped below +5 degrees C and remained positive during the entire investigation period. Growth cessation was accompanied by low values of photosynthetic capacity to maintain photostasis, i.e., the balance between energy input through photochemistry and subsequent energy utilization through metabolism. Even in January after night temperatures down to -22 degrees C, photosynthetic capacity still averaged +4.0 +/- 0-8 mu mol CO2 m(-2) s(-1). Initial frost damage did not commence until leaf temperatures dropped below -27.6 degrees C. This was sufficient to survive absolute air temperature minima. As an evergreen species, the sustained high efficiency of PSII in winter potentially enables S. paniculata to immediately utilize periods with moderate temperature conditions for photosynthesis, particularly at the end of winter when there is sufficient water on rocks due to melting snow.