Agriculture Reference
In-Depth Information
−
◦
C can have a similar effect. For apple, buds at first bloom and pink
bud appear to be rather more sensitive than those at full bloom (Proebsting
and Mills,
to
). In general the decline in frost resistance as
buds develop is related to their increasing water content and the influence
of this in decreasing their ability to supercool (Modlibowska
; Hamer,
a).
Low temperature in itself is not lethal and supercooling, i.e. when the liquids
in plant tissues fall below the normal freezing point without crystallization,
protects from frost killing. The death of cells results from intracellular ice
formation causing mechanical damage and excessive dehydration leading to
irreversible physico-chemical changes in the protoplasm.
Exposure to low temperatures and dry conditions prior to the incidence of
frost induces a degree of hardening and resistance to frost. Hardened flowers
arecharacteristicallysmall,withsmallcells,lowwatercontentandhighcontent
of sugars, starch, hemicellulose and glucosides (Modlibowska,
,
a).
The styles are usually highly sensitive and after fertilization damage to
the placenta and ovules is of most importance. Damage is characteristically
shown by browning and blackening made obvious when the flowers are cut
longitudinally. Death of only a proportion of the ovules results in misshapen
fruits due to the uneven supply of seed-produced hormones. Pre-blossom frosts
may affect the structure and elasticity of the skin, giving lines or bands of russet,
'frost eyes' and green blotches. Ice formation under the skin of the receptacle
may protect the latter from freezing but result in blemishes on the fruit later.
Frosts are generally more frequent and more severe the earlier it is within
the spring season (Hamer and Jackson,
), so one method of reducing
the risk of frost damage is to delay budbreak and blossoming. This is most
effectively done by selection of late-flowering cultivars, including cultivars that
can crop heavily from late-blossoming axillary flowers on one-year-old shoots.
These characteristics are highly heritable and can be bred for (Spiegel-Roy and
Alston,
). Late-flowering 'sports' of a number of cultivars have also arisen
by mutation and been adopted commercially. Some degree of blossom delay
can be achieved with plant growth regulators. Paclobutrazol delays budbreak
and full bloom by
-
days on apple trees treated in a previous season (Miller
and Swietlik,
) and this chemical increases fruit bud formation on the later
flowering one-year-old wood of both apple (Buban,
) and pear (Dheim
and Browning,
). Day-time sprinkling with water in late winter and spring
can delay blossoming as a result of evaporative cooling (Anderson and Seeley,
), but may fail to reduce frost damage because it increases the water
content of the buds at each developmental stage (Hamer,
).
Direct physical protection from frost damage depends on maintaining the
bud temperatures above that at which the buds will freeze. On cloudy, windy
nights bud temperatures are near to air temperatures but under clear condi-
tions typical of radiation frosts they may be about
,
◦
C lower because of
.
