Agriculture Reference
In-Depth Information
penetration in the apple style, Kobel (
) classified pollination responses as
indicating compatibility (
).
In the last case about half of the pollen tubes penetrate to the base of the style
(Spiegel-Roy and Alston,
), incompatibility (
) and half-compatibility (
/
).
Self-incompatibility in apple is under the control of an S gene system. Kobel
et al.
(
) proposed that this is based on S gene alleles. The essential feature
of the incompatibility system is that pollen is inhibited in a style or ovary con-
taining the same incompatibility alleles. The single, multi-allelic gene encodes
ribonucleases (S-RNases) which are present in the pistil of the mature flower
where they recognize and inhibit developing pollen. Several alleles of the S
gene of apple have been cloned and characterized (Broothaerts
et al.
,
) and
stylar ribonuclease isoenzymes corresponding to a large number of S-alleles
detected (Boskovic and Tobutt,
).
shows the incompatibility genotypes of many of the currently im-
portant cultivars of apple and others representing types likely to be important
as parental material for the future. It is to be expected that where the cul-
tivars have identical stylar ribonuclease patterns, corresponding to identical
S-alleles, they will be mutually incompatible. Where the cultivars have one
S-allele in common only
Table
.
% of the pollen grains will be compatible, i.e. the
cultivars will be semi-compatible. Where both alleles differ between diploid
pollen donor and acceptor cultivars then they will be fully compatible.
In general the pistils of triploids are more likely to have one or more alleles
in common with the donor pollen because they have more alleles. Cultivars
that are related are likely to have at least one incompatibility allele in common.
'Jonagold', a triploid arising from a cross between 'Jonathan' and 'Golden De-
licious' has two S-alleles in common with 'Golden Delicious' and one with
'Jonathan'. 'Elstar' ('Ingrid Marie'
'Golden Delicious') has only one incom-
patibility allele in common with 'Golden Delicious'.
Self-compatibility varies from very low (highly self-incompatible) to appre-
ciable levels. 'Red Delicious' and its sports are generally self-unfruitful (Dennis,
×
; Kemp,
). De Witte
et al.
(
) found 'Fuji' and 'Golden Delicious'
to give only
% set, respectively, following self-pollination under
conditions where pollination with the ornamental apple
Malus Baskatong
gave
% and
.
% set. This low level of self-fertility is important in view of the pre-
dominance of these cultivars both in production and as parental material. De
Witte
et al.
(
% and
) found much higher levels of self-fertility in 'Idared' (
.
%)
and 'Elstar' (
%) although, importantly, very few of the fruits so produced
were seeded. 'Cox's Orange Pippin' has given very variable set when self-
pollinated, ranging from
.
% (of a sample of over
flowers) found by
Modlibowska (
)to
% (under conditions where
M. Baskatong
pollination
gave
%) found by De Witte
et al.
(
). In the latter case the 'Cox' fruits
from self-pollination averaged
.
seeds per fruit.
