COLD-INDUCED PHOTOINHIBITION LIMITS REGENERATION OF SNOW GUM AT TREE-LINE

Cold-induced photoinhibition was studied in relation to the distribution of juvenile Eucalyptus pauciflora Sieb. ex. Spreng near the low temperature limits of its distribution in southeastern Australia. In early autumn, there were no differences in the photosynthetic properties of leaves from juvenile plants naturally growing in canopy-shaded or fully exposed sites, with all leaves having photosynthetic characteristics typical of leaves from high light environments. In mid-winter, cold-induced photoinhibition, as indicated by loss of quantum yield, increased with increase in exposure to high irradiances, with quantum yield in the most exposed leaves averaging 51% less than that expected in fully functional leaves of C3 species. However, decrease in photosynthetic efficiency at limiting photon flux densities was not accompanied by a parallel decrease in photosynthetic capacity at higher light intensities. This implies that cold-induced photoinhibition in E. pauciflora was due to protective dissipation of absorbed light energy rather than damage to photosystem II. The density of juveniles was highest in canopy-shaded sites on the eastern to south-western sides of trees where juveniles received intermittent high intensity irradiance from sunflecks but were protected from prolonged exposure to bright sunlight, particularly in winter when solar angle is low and the sun follows a more northerly path. Thus, the distribution of juvenile plants was correlated with the occurrence and severity of cold-induced photoinhibition.