Equilibrium freezing of leaf water and extracellular ice formation in Afroalpine ?giant rosette? plants

Abstract

The water potentials of frozen leaves of Afroalpine plants were measured psychrometrically in the field. Comparison of these potentials with the osmotic potentials of an expressed cellular sap and the water potentials of ice indicated almost ideal freezing behaviour and suggested equilibrium freezing. On the basis of the osmotic potentials of expressed cellular sap, the fractions of frozen cellular water which correspond to the measured water potentials of the frozen leaves could be determined (e.g. 74% at -3.0° C). The freezing points of leaves were found to be in the range between 0° C and -0.5° C, rendering evidence for freezing of almost pure water and thus confirming the conclusions drawn from the water-potential measurements. The leaves proved to be frost resistant down to temperatures between -5° C and -15° C, as depending on the species. They tolerated short supercooling periods which were necessary in order to start ice nucleation. Extracellular ice caps and ice crystals in the intercellular space were observed when cross sections of frozen leaves were investigated microscopically at subfreezing temperatures.

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

“…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.…”

“…There seems to be a close relationship between plant height and life form: taller plants, including rosettes, display predominantly avoidance mechanisms, while smaller ones exhibit tolerance (Squeo et al 1991(Squeo et al , 1996. However, Afroalpine (Beck et al 1982(Beck et al , 1984 and Hawaiian giant rosettes (Melcher et al 1994), present freezing tolerance.…”

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

“…Additionally, these authors report that these low water potentials did not have any effects on leaf conductance. It is important to note that Beck et al (1984) have described extremely low Ψ L (-6.7 MPa) in afroalpine giant rosettes when leaves are frozen at night. Since the freezing process involves water movement from intracellular to intercellular spaces in leaves, it is necessary that plants be extremely resistant to water deficits in order to survive.…”

“…This stem holds a large central parenchymatic pith and is protected by a large layer of marcescent leaves. It is interesting to point out that this life form, presenting very similar structures, has evolved in other ---tropical high mountain regions of the world (Hedberg and Hedberg 1979, Halloy 1983, Smith and Young 1987, Bussmann 2006, suggesting that they have developed as a response to a tropical environment characterized by extreme daily temperature fluctuations, frequent night frosts, seasonal and daily water availability limitations and high incoming radiation inputs.…”

“…However, giant rosettes species of the African tropical mountains (Dendrosenecio and Lobelia spp.) are able to tolerate ice formation in leaf tissues (Beck et al 1984). With respect to daytime temperature, water availability, and evaporative demand studies, Goldstein et al (1984) described how adult giant rosettes depend on a high capacitance generated by the water-storage pith in order to resist water deficit periods.…”