CARBOHYDRATE MOBILIZATION AND MOVEMENT IN ALPINE PLANTS

Starch retention and disappearance from leaves and carbon movement under various temperatures were studied in two alpine species, Oxyria digyna (L.) Hill and Phleuln alpinuin L., and in two lowelevation species, Helianthus annus L. and Elyrnus canadensis L. The alpine species exhibited starch disappearance from the leaf following cool night temperatures, whereas starch retention was noted under similar conditions for the low-elevation species. The alpine species, Oxyria, exhibited the highest rates of starch disappearance from the leaf under cool temperatures as well as the highest carbohydrate translocation under cool temperatures. The low-elevation species had low rates of starch disappearance and carbohydrate translocation under low temperatures, but exhibited relatively higher rates with an increase in temperature. Such a mechanism whereby alpine species can maintain relatively high rates of translocation under cold temperature represents a major form of physiological adaptation to the short, cool, growing season in the alpine tundra. ALPINE plants are adapted for survival in a uniquely severe environment characterized by a short growing season, low mean air and soil temperatures, great diurnal temperature fluctuations, high wind, and high light intensity. Because of the short growing season, few alpine plants are annuals and the majority overwinter as perennial rhizomes, roots or bulbs (Mooney, 1972; Billings and Mooney, 1968). In order to maintain this perennial growth form, much of the carbon fixed must be rapidly translocated to the storage organ under conditions of low air and soil temperatures. Although the precise mechanism of sugar transport remains to be elucidated, the Munch pressure flow theory as modified by Crafts appears to be better supported by currently gathered data than by other proposed theories. Current views of the pressure flow mechanism state that sugars are moved from the mesophyll cells to an intercellular "free space" from whence they are actively loaded into the intermediary or companion cells of the phloem (Geiger et al., 1974). The resultant lowering of the water potential of the sieve-element companion cell complex causes a flux of water into the sieve tube, setting up a turgor pressure gradient which is the immediate driving force for flow to a sink area where active unloading occurs. Thus, movement of carbohydrates down the sieve tube itself is probably a passive process, while loading and unloading of carbohydrates are active processes I Received for publication 27 September 1976; revision accepted 7 August 1978. This work was partially supported by National Science Foundation Grant GB-36150 to A. T. Harrison. 2 Present address: Syracuse University, Biological Research Labs, 130 College Place, Syracuse, New York 13210. dependent upon the availability of biochemical energy (Sovonick, Geiger, and Fellows, 1974). The primary question which we wished to consider here is: Given the high rates of carbon storage exhibited by alpine species under low temperatures, do alpine plants possess a special adaptive mechanism whereby high rates of translocation can be maintained under low temperatures? Several workers have provided data which would support such a hypothesis though none have made direct measurements of translocation rates in alpine species. The arctic grass species Dupontia fischeri, labeled with "4CO2 under ambient field conditions, showed significant amounts of translocation label appearing up to 30 cm from the labeled leaf within 24 hr (Allessio and Tieszen, 1975). Thus, high translocation rates under low temperatures could represent a major form of adaptation for survival in the alpine tundra. MATERIALS AND METHODS-The species under consideration were an alpine herb, Oxyria digyna (L.) Hill (alpine sorrel) and an alpine grass Phleurn alpinurn (L.) (alpine timothy), and two low-elevation species, Elymnus canadensis L. (canadian wildrye-a typical prairie grass species), and Helianthus annuus L. (sunflower). The alpine species were transplanted from Libby Flats in the Medicine Bow Mountains at 3600 m elevation, Elymnus was transplanted from near the Laramie River at Wheatland, Wyoming at 1500 m, and Helianthus was grown from commercially available seed. Plants were grown in pots in small programmable greenhouses at the University of Wyoming in Laramie under thermal regimes which simulated summer alpine and lowland field conditions.