Agriculture Reference
In-Depth Information
position in April during flowering. If branches that had been horizontal were
moved to the vertical in April, this resulted in a short EPP. These responses
to changes in branch position at flowering time suggest that effects on ovule
fertility and EPP are controlled by current hormonal relationships rather than,
in this case, effects via influences on flower development in the previous year.
Temperatures simulating a warm English February, March and April re-
sulted in EPPs for 'Cox' apple of
days in consecutive years, whereas
under conditions simulating a cool English spring the EPP exceeded
and
days
in each year (Miller,
). In general, low temperatures in the post-anthesis
period increase ovule longevity (Sedgley,
).
Pollination increases embryo sac viability in such a way that even those
pollengrainsthatdonotfertilizeovulesplayanimportantpartinassistingother
pollen grains to do so. It stimulates a wave of cytoplasmic and biochemical
activity in the pistil and also stimulates development within the ovary, readying
it for fertilization. Herrero and Gascon (
) showed that in unpollinated pear
flowers the ovules degenerate between
days after anthesis whereas
in cross-pollinated flowers the degeneration is postponed by about
and
days.
This increase in EPP can also be induced by GA , which may explain why
GA treatment can increase the set of seeded fruits and suggests that GA may
be involved in the pollination effect on ovule fertility.
Flowers of 'Comice' pear treated with paclobutrazol at full bloom contain
twice as many degenerating embryo sacs as untreated flowers (Dheim and
Browning,
). This leads to complete inhibition of fruit set, which can be
reversed if the flowers are treated with gibberellic acid within
days.
Putrescine applied at full bloom delays ovule senescence and can extend the
EPP of 'Comice' pear by up to
). Putrescine is a
polyamine, a class of chemicals apparently present in cells of all tissues capable
of normal growth and development and with a regulatory role as distinct from
a purely nutritional requirement (Galston and Kaur-Sawhney,
days (Chrisosto et al. ,
).
Fruit set
Fruit set is the process that is necessary for pre-anthesis growth of the flower
parts to be followed by sustained post-anthesis growth of the fruit. At anthesis
all the chemical factors for growth - auxin, kinins and gibberellins as well
as ABA - are already present in the flower (Martin et al. ,
; Miki et al. ,
). Despite this, further growth and continued development after anthe-
sis are usually limited to only a small proportion of the flowers. Typically
only
% of the flowers give harvestable fruits, although the proportion
may vary from more than
-
% to less than
%. The rest fail to set and are
shed.
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