Frost Resistance in Giant Rosette Plants 1241 Afro Alpine Plants Freezing Tolerance

As already noted above (Sect. 12.2), formation of ice crystals in the cytoplasm is always disastrous. This is predominantly due to effects of ice competing with the ordered superficial water film of membranes. In addition ice crystals may puncture membranes and organelles. Thus, freezing tolerance is only possible with extracellular ice formation. For the Afro-alpine giant-rosette plants the phenomenon has been studied in detail by E. Beck and collaborators (Beck 1983).

For the formation of extracellular ice, the basic laws of cell water relations (see Box 6.1) apply as follows:

where ^cell is the water potential of the cell, P and n are turgor and osmotic pressure respectively. During freezing the cell looses water, and ice forms outside the plasmalemma in the intercellular spaces. Thus, the protoplast shrinks and turgor becomes zero (P = 0). Therefore

and at equilibrium of the protoplasts with the extracellular ice

a situation called "equilibrium freezing" (Fig. 12.14). Essentially, ice forms gradually as water moves out of the symplast, and not abruptly as occurs after supercooling (see Sect. 12.4.3). Therefore, the occurrence of nucleating agents in the apoplast, which may include mucilage (Goldstein and Nobel 1994), are important in eliciting ice-crystal formation and avoiding supercooling (Krog et al. 1979).

According to (12.3), the concentrations of solutes in the protoplast, which determine n, are therefore given by the water potential of the ice, which is linearly dependent on the subfreezing temperature. The increased cytoplasmic concentrations of solutes may damage or protect membranes and proteins. Often special cry-oprotective solutes decrease the injurious effects of high ion concentrations on the membranes. In Afro-alpine plants, sucrose is most likely to fulfil such a role (Beck 1994a). Cryoprotectants are similar to compatible solutes discussed above in relation to osmotic stress (Sect. 7.4 and Box 7.1). In fact extracellular ice formation is nothing more than a dramatic osmotic stress, however, at low temperatures.

In this way, for example in Lobelia keniensis at —6 0C, 85% of the tissue water is frozen. When the ice thaws in the morning, the water is immediately taken up

Fig. 12.14 Equilibrium freezing: water potentials of frozen leaves (^cell = •) of S. kenio-dendrion and L. keniensis and frozen expressed cellular sap (—n = 0) of S. keniodendron compared to ice at subfreez-ing temperatures (lines: fice calculated by two different methods A and B; see (12.3)) give the same relationships. (Beck et al. 1984)

Table 12.1 Frost tolerance of leaf segments of four species of Afro-alpine megaphytes al. 1982)

Species

Frost tolerance (0 C)

50% damage (0C)

Senecio keniodendron

— 8

—10

Senecio brassica

— 10

—15

Lobelia keniensis

Lower than —20

Lower than — 20

Lobelia telekii

Lower than —20

Lower than — 20

osmotically into the cells again, and full competence of photosynthesis is regained rapidly. Using this mechanism, the Afro-alpine giant-rosette plants achieve frost resistance at temperatures of —8 0C down to —20 0C (Table 12.1).

There may also be deviations from the ideal behaviour given by (12.3) due to the osmotic contribution of extracellular solutes, which allow lower external water potentials (non-ideal equilibrium freezing) (Goldstein and Nobel 1991; Zhu and Beck 1991). At water losses > 50% a matrix potential is also generated which prevents intrusion of air between the cell wall and the plasmalemma, and in this way the wall may get under tension (negative turgor).

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