Abstract

Long-term acclimation of photo- and pigment-chemistry was investigated in a naturally-regenerating stand of Acacia melanoxylon R.Br. ex Ait. A pronounced decrease in photochemical efficiency of A. melanoxylon saplings was observed between autumn and winter in both thinned and unthinned treatments, but the decrease was more severe in the thinned treatment. Associated pigment changes in the unthinned treatment included a decrease in total chlorophyll content and a rise in chlorophyll a : b. Similar acclimation occurred in the thinned treatment with additional increase in zeaxanthin per unit chlorophyll observed. Saplings in the thinned treatment were exposed to lower minimum temperatures, more hours of frost and higher light intensities in the mid- to lower-crown. Growth chamber studies of the short-term acclimation of photo- and pigment-chemistry were conducted in a low / high light and cold / warm temperature factorial experiment. Photochemical efficiency and quantum yield adjusted within one day and then remained constant for 10 d in response to the imposed treatments. Chlorophyll concentration had decreased in all treatments by day 2 in the growth chambers, and subsequently increased in warm, but not in cold, treatments, irrespective of light level by day 10 in the growth chambers. The concentration of lutein-5,6-epoxide decreased in response to the cold-high light treatment and increased in response to other treatments by day 10 in the growth chambers, consistent with a function in sustained photoprotection in leaves of shade-adapted species. Our experiments indicated that A. melanoxylon is susceptible to cold-induced photoinhibition under cool temperatures (2–8°C) and moderate light intensities (450 μmol m^–2 s^–1).