Agriculture Reference
In-Depth Information
Windbreaksresultingreatlyincreasedinsectpopulationsintheshelteredzones
(Lewis and Smith,
). Other flowers may compete for the attention of bees
(Benedek and Nagy,
) and may need to be controlled in the orchard or
the neighbourhood.
The particular flower structure of a cultivar may reduce the effectiveness
of pollination. 'Delicious' flowers have basal gaps between stamen filaments
through which bees can extract nectar without touching any stigma (Dennis,
).
The effectiveness of pollination by bees in general can be increased by
inserting pollen packs in hives in such a way as to dust the bees with pollen
on leaving (Williams,
; Schneider
et al
.,
a). Alternatively, pollen may be applied to flowers
by dusting them with a mixture of pollen and inert material.
Pollen germination and pollen tube growth
The pollen grain is a dormant, resistant structure that contains lipid reserves
for its germination and early growth but is dehydrated and must absorb water
to germinate when it reaches the stigma. In apples and pears the stigma has a
wet surface composed of extracelluar secretions of its papilla cells and of the
contents of the cells themselves which collapse after anthesis (Sedgley,
).
Thissurface,whilewet,providesthemoistenvironmentforpollengermination
(Heslop-Harrison,
). After the hydrated pollen grain has germinated in
the secretion pool on the stigma surface, the emergent pollen tube begins to
grow through the interstitial material of the transmitting tract. 'Recognition'
of incompatibility or otherwise takes place here.
Pollen germination rate depends on temperature and varies with the source
ofpollen.PetropoulouandAlston(
)found'Spartan'pollentogiveahigher
germination percentage than that of 'Cox' or 'Idared' at all temperatures
tested, but the difference was likely to be most important at
◦
C and
-
◦
C. At these latter temperatures pollen of 'Redsleeves' was outstanding in
its germination percentage and initial pollen tube growth (Table
-
.
). Among
Japanese pears Rohitha and Klinac (
) found 'Shinsui' pollen germination
◦
C whereas for the cultivars 'Hosui',
'Kosui', 'Nijisseiki' and 'Shinseiki' the optimum temperature was between
◦
C and
to have a temperature optimum at
◦
C.
The major factors influencing the speed of pollen tube growth are gene-
tic compatibility (see below) and temperature, factors which interact.
Modlibowska (
◦
C, speeds up the
incompatibility reaction and thus results in an early inhibition of the growth
of incompatible tubes. The growth of compatible tubes, in contrast, increased
over the range from
) found that high temperature, i.e.
-
◦
C for both diploids and triploids although
the latter had a lower growth rate at any given temperature.
◦
Cto
