Agriculture Reference
In-Depth Information
Table
.
Pollen germination percentage of different cultivars after
and
hours at
various temperatures
-
◦
C
-
◦
C
-
◦
C
-
◦
C
-
◦
C
Cultivar
'Cox'
'Idared'
'Spartan'
'Redsleeves'
Data from Petropoulou and Alston (). Reproduced with permission.
Table
.
Pollen tube growth rates for typical pollinators of 'Cox' apple at different
temperatures. An accumulated total of
indicates completion
Mean temp. (
◦
C)
over h period
Pollen tube growth
indexper h
From Williams (b). Reproduced with permission.
b) used data from several cultivars to calculate the time for
pollen tubes to penetrate to the ovary for a diploid apple such as 'Cox' over
the relatively cool range of temperatures typical of England (Table
Williams (
.
).
Incompatibility
Inapplesandpears,asinmostfloweringplants,themaleandfemaleorgansare
in close proximity within the same flower (Figure
). Self-incompatibility has
developed as one of the mechanisms that prevents successive self-fertilizations
and deleterious inbreeding. It has long been recognized that most apple and
pear cultivars are effectively self-incompatible, or very largely so, and that fruit
set usually depends on cross-pollination between genetically different cultivars.
The site of incompatibility is in the style or ovary, because of physiological
reactions occurring between the pollen tube and the stylar and ovarian tis-
sue (Modlibowska,
.
). In general incompatible pollen tubes grow slowly
down the style. They show heavy deposits of callose tissue along and at the
end of the tube. In some cases the reaction occurs at an early stage, pollen
tube growth being inhibited in the style. In others it takes place later, even
when the tubes have reached ovarian tissue. Compatible pollen tubes, on the
other hand, grow rapidly down the style, are characterized by small, widely-
spaced, intermittent callose plugs and the absence of a terminal plug, and can
effect fertilization (Modlibowska,
; Stott,
). On the basis of pollen
