sodininkystė ir daržininkystė 25(4)

Only genotype 1 was able to survive temperatures < -20°C (Fig. 3). Temperatures
at which seedling survival was reduced by 50% were -13.6, -15.7 and -17.5°C
for genotypes 2, 3 and 1, respectively. For these three genotypes it seems that the
best screening temperatures for differentiating the population were between -10 and
-20°C but this is specific to the genotypes undergoing evaluation.
Fig.3 Mortality of three apple rootstock seedlings following controlled
acclimation and freezing
3 pav. Obelø su trimis skirtingais poskiepiais þuvimas kontroliuojamomis
aklimatizacijos ir ðaldymo sàlygomis
Discussion. Mortality, shoot, trunk and root regrowth results were summarized
in an attempt to segregate populations of different apple rootstocks according
to their cold hardiness (Table 1). Low mid- winter injury is the type of
cold stress for which this method is best suited. Most of the results from the
present study agree with the literature (Czynczyk and Holubowicz, 1984; Quamme,
1990) and since they were well correlated with in vivo field data, we recommend
this whole-plant recovery evaluation method for cold hardiness screening
of apple rootstocks.
Freeze-thaw cycling was very detrimental to either cell viability or regrowth.
Using the Z-technique, the extracellular resistance parameter (R o
), was very responsive
to freeze-thaw cycling, decreasing exponentially with increasing cold
stress. The decrease in R o
is mostly due to release of intracellular electrolytes to
extracellular space (Zhang et al., 1992). It is speculated that the release of electrolytes
to extracellular space is due to membrane rupture and reseal during freezethaw
cycling, but the event of rupture-reseal does not necessarily involve membrane
functional loss in non-lethal freeze-thaw stresses (Zhang et al., 1994). Membrane
rupture could have been caused by the piercing of the cellular membranes by
ice crystals or by the osmotic contraction and expansion of the membranes due to
the drastic difference in solute concentrations between the frozen and un-frozen
state. This latter explanation seems most likely because most evidence seems to
suggest that solute loading and vesicle expansion are very important to plant freezing
tolerance (Kaye and Guy, 1995). Freeze-thaw cycling caused more root inju-
101