Cold hardiness and supercooling along an altitudinal gradient in andean giant rosette species

Abstract

Factors affecting supercooling capacity and cold hardiness were investigated in leaves of ten giant rosette species of the genus Espeletia (Compositae). These species grow along a 2,800-4,200 m elevation gradient in the Venezuelan Andes. In this high tropical environment, freezing frequently occurs every night, particularly above 3,300 m, but lasts for only a few hours. Supercooling capacty is linearly related to leaf water potential (Ψ ) in all species; however supercooling is more responsive to Ψ changes in Espeletia species from high paramos. The rate of change in the species-specific supercooling point and the rate of change of average annual minimum temperature along the elevation and climatic gradient follow the same trend (approximately -0.6 K per 100 m elevation). At a given elevation, the expanded leaves of the different species tend to supercool 8-10 K below minimum air temperatures. Experimentally-induced freezing was accompanied by the formation of intracellular ice and tissue damage. The relative apoplastic water content (RAWC) of the leaves, which may influence the ice nucleation rate or the facility by which ice propagates, was determined by pressure-volume methods. Species from higher sites tend to exhibit lower RAWC (2%-7%) than species from lower sites (7%-36%). A causal relationship between supercooling capacity and RAWC is suggested. In the high tropical Andes, the temperature oxotherm plateau of Espeletia leaves seems to be sufficiently fow to avoid freezing.

Freezing tolerance in grasses along an altitudinal gradient in the Venezuelan Andes

“…Grasses in the páramo grow under more unfavorable ground level thermal conditions due to nocturnal surface heat loss (Azócar and Monasterio 1980;Azócar et al 1988;Earnshaw et al 1990;Squeo et al 1991), compared to the species of Espeletia (Goldstein et al 1985;Rada et al 1987) and Polylepis sericea (Rada et al 1985b), which grow well above ground. Under such environmental conditions, supercooling itself does not seem to be an eVective mechanism to resist night freezing temperatures.…”

“…There are adaptations allowing protection of growing structures in giant Andean, Afroalpine and Hawaiian rosettes (Beck et al 1982(Beck et al , 1984Goldstein et al 1985;Rada et al 1985a;Melcher et al 1994), but in many other life forms, including grasses, tolerance arises as the convenient strategy to confront the thermal constraint present on tropical high mountains. This opposes Hedberg's (1964) statement that both the bunch arrangement of almost all páramo grasses and the aboveground dead leaves serve as cold isolation morphological strategies, a clear avoidance mechanism.…”

“…Additionally, it is possible to think of cold resistance mechanisms as a function of plant water responses. Species of Espeletia (Goldstein et al 1985) and P. sericea (Rada et al 1985b) which show strong avoidance mechanisms to water stress also show freezing avoidance. A close relationship between water potential and the ability to avoid freezing through supercooling has been reported (Goldstein et al 1985;Rada et al 1987).…”

“…Supercooling capacity, a decrease in tissue temperature well below the equilibrium freezing point without the occurrence of extracellular freezing of water (Beck 1994;Körner 1999), has been frequently reported in páramo plant species: giant rosettes of the genus Espeletia (Asteraceae) (Goldstein et al 1985;Rada et al 1985a;Rada et al 1987), Polylepis sericea (Rosaceae), a tree (Rada et al 1985b), Hinterhubera lanuginosa (Asteraceae), a small shrub, and Hypericum laricifolium (Clusiaceae), a shrub (Squeo et al 1991). …”

Water relations and gas exchange in Coespeletia moritziana (Sch. Bip) Cuatrec., a giant rosette species of the high tropical Andes

“…Night-time low temperature resistance has been studied for many of the giant rosettes of the Venezuelan high Andes (Goldstein et al 1985b, Rada et al 1985. In all cases, a high supercooling capacity of the leaves together with isolating marcescent leaf layers protecting stems, permit these rosettes to avoid freezing of their tissues.…”