Since the early ’70s there were many investigations on the
possibility of modifying the natural balance of growth regulators at
the end of the active growth stage of trees and perennials by
exogenous application of synthetic growth regulators. It was thus
hoped to affect the innate range of resistance to low temperatures
(Alden and Hermann, 1971). Also there were many reports indicating the
positive or adverse effects of growth regulator applications. For
instance, spraying some annual, perennial plants including trees, with
growth retardants such as CCC might result in an increase of frost
hardiness whereas applying stimulators such as GA lowered the cold
hardiness or made the plants more hardy (Irving, 1967; Modlibowska,
1968).
Detailed microscopic analysis has shown that the addition of growth
retardants inhibited the enlargement of the cells, and the rate of
their mitotic divisions. They were also smaller then the cells of the
control callus after the same period of growth. It was also found that
the smaller cells were more resistant to low temperature than large
and fast growing cells (Holubowixz, 1982).
