Agriculture Reference
In-Depth Information
their relative growth and vigour. Walsh and Miller (
) identified vari-
ability between spur-types of different cultivars in their lateral budbreak
on both current season's and second-year wood, in their internode length
and in the formation of 'long spurs' between
and
cm in length. Faust
and Zagaja (
) reported that although short internodes were more fre-
quent in the low-vigour than the high-vigour seedlings arising from crosses
between short-internode parents of apple, there were some long-internode,
low-vigour and some short-internode, high-vigour progeny. The same was
true for pears. Wide crotch angles tended to be associated with low vigour.
Grochowska
et al.
(
), contrasting low- and high-vigour seedlings from
crosses between spur-types, found much higher auxin-like and gibberellin-like
activity in the phloem of low-vigour seedlings and concluded that their lack
of growth was a consequence of production of a growth inhibitor. Faust and
Zagaja (
) concluded that expansion growth is generally under polygenic
control.
The situation appears much simpler in the 'compact columnar' apples now
being bred using the 'Wijcik McIntosh' mutant as a parent. This arose as a
naturalmutationintheorchardandisanextremespur-typewhichseldomgives
laterallongshoots.Thisgrowthhabitisgovernedbythecompact(
Co
)gene.Itis
highlyheritable,with
%oftheprogenyofcrossesinvolving'Wijcik'being
compact and columnar (Lapins,
-
). The shoot
tips of 'Wijcik McIntosh' are low in polar gibberellins, have high endogenous
cytokinin levels and exhibit very high tolerance to benzyladenine in tissue
culture (Looney and Lane,
; Lapins and Watkins,
).
Effects of
rol
genes on shoot growth
Holefors
et al.
(
' apple with
Agrobacterium tumefasciens
containing the
rol
A gene, which may operate through
a decrease in gibberellin or conjugated polyamine content and increased sen-
sitivity to auxin, led to transformed plants with short internodes. Bell
et al.
(
) have shown that infection of 'M.
) found that the
rol
C gene reduced tree height and number of nodes of
transgenic 'Beurre Bosc' pear trees (cf. pp.
-
).
Effects of rootstocks on shoot growth
Even non-cropping scions on dwarfing rootstocks show reduced total shoot
growth, both because fewer of their buds give rise to shoots and because their
shoots extend more slowly in the later part of the season and cease to grow
sooner than those on more vigorous rootstocks (Avery,
). Rootstock effects
are also discussed in detail in Chapters
and
.
