Photosynthetic gas exchange and temperature-induced damage in seedlings of the tropical alpine species Argyroxiphium sandwicense.

The capacity of Argyroxiphium sandwicense (silverword) seedlings to acclimate photosynthetic processes to different growing temperatures, as well as the tolerance of A. sandwicense to temperatures ranging from -15 to 60° C, were analyzed in a combination of field and laboratory studies. Altitudinal changes in temperature were also analyzed in order to explain the observed spatial distribution of A. sandwicense. A. sandwicense (Asteraceae) is a giant rosette plant that grows at high elevation on two Hawaiian volcanoes, where nocturnal subzero temperatures frequently occur. In addition, the soil temperatures at midday in the open alpine vegetation can exceed 60° C. In marked contrast to this large diurnal temperature variation, the seasonal variation in temperature is very small due to the tropical maritime location of the Hawaiian archipelago. Diurnal changes of soil and air temperature as well as photosynthetic photon flux density were measured on Haleakala volcano during four months. Seedlings were grown in the laboratory, from seeds collected in ten different A. sandwicense populations on Haleakala volcano, and maintained in growth chambers at 15/5, 25/15, and 30/25° C day/night temperatures. Irreversible tissue damage was determined by measuring electrolyte leakage of leaf samples. For seedlings maintained at each of the three different day/night temperatures, tissue damage occurred at -10° C due to freezing and at about 50° C due to high temperatures. Tissue damage occurred immediately after ice nucleation suggesting that A. sandwicense seedlings tend to avoid ice formation by permanent supercooling. Seedlings maintained at different day/night temperatures had similar maximum photosynthetic rates (5 μmol m-2 s-1) and similar optimum temperatures for photosynthesis (about 16° C). Leaf dark respiration rates compared at identical temperatures, however, were substantially higher for seedlings maintained at low temperatures, but almost perfect homeostasis is observed when compared at their respective growing conditions. The lack of acclimation in terms of frost resistance and tolerance to high temperatures, as well as in terms of the optimum temperature for photosynthesis, may contribute to the restricted altitudinal range of A. sandwicense. The small seasonal temperature variations in the tropical environment where this species grows may have prevented the development of mechanisms for acclimation to longterm temperature changes.