Agriculture Reference
In-Depth Information
Most 'sports' show a level of self-fertility that is consistent across clones and
with the 'parent' cultivar. This is not the case with 'Cox', where Stott and
Campbell (
) found appreciable variation in self-compatibility in clones
of this cultivar. Clones with a high level of self-fertility and that could also
pollinate the standard 'CoxEMLA' were subsequently selected from trees
that had been subjected to gamma-irradiation to increase the incidence of
mutations (Campbell and Lacey,
) found
evidence that when the 'self-fertile' 'Cox' is self-pollinated the pollen tubes
grow the full length of the style but fertilization only takes place between
gametes with different S-alleles.
Cross-compatibility between cultivars had been considered to be the norm
(Modlibowska,
; Lacey
et al.
,
). Alston (
), but cross-incompatibility has become more prevalent
as more inter-related cultivars are grown. Alston (
) showed that although
'Cox' and 'Idared' are fully compatible, much lower levels of fruit set are
achieved following crossings between their progeny with fully incompatible
pairs of alleles in common. These crosses between siblings also gave much
lower levels of fruit set than when they were back-crossed with the 'Cox'
parent with which they had only one incompatibility allele in common. Even
cultivars of uncertain ancestry may have incompatibility alleles in common,
to the detriment of cross-pollination. 'Jonathan' is less effective than 'Golden
Delicious' as a pollinator of 'Topred' (a 'Red Delicious' sport) as a result of a
common incompatibility allele (S
) in both 'Jonathan' and 'Topred' (Goldway
et al.
,
). Under many circumstances, however, semi-compatible cultivars
give adequate pollination for each other.
Triploid apple cultivars are less frequently self-incompatible and the self-
incompatibility is usually less strongly expressed than in diploids. For this
reason selfed triploids usually set more fruits than selfed diploids even though
they often show gametic sterility (Modlibowska,
). They produce both
seeded and seedless fruits.
Diploid European pear cultivars are almost invariably self-sterile but set
parthenocarpic fruits more readily than do apples, either naturally or following
hormone sprays. They are generally cross-fertile.
Japanese pears are largely self-incompatible and require cross-pollination,
the self-incompatability being controlled by S-alleles in the pollen and pistil.
Different cultivars have different S-alleles (Hiratsuka
et al.
,
). A mutant of
'Nijisseiki', 'Osa-Nijisseiki', is self-fertile as a result of stylar-part-mutation of
the S
-allele (Sassa
et al.
,
).
Female flower fertility
Successful fertilization depends on the pollen grains reaching the ovules before
they degenerate. Williams (
) defined the concept of an effective pollination
