Agriculture Reference
In-Depth Information
in single stem apple trees into which it was injected (Robitaille and Carlson,
). There is also evidence of growth inhibitors, which have not been charac-
terized chemically, in apple rootstocks and in dwarf scions (Grochowska
et al.
,
).
There are a large number of plant gibberellins (GAs) and some of these are
active in promoting apple shoot growth.
GAs applied to a wide range of intact plants induce elongation of stem
tissue, this effect being more pronounced in rosette or in dwarf species ( Jones,
). The increase in length of the extending shoot results
from increase in length of existing as well as newly divided cells and may also
be accompanied by an increase in cell number. There appear to be at least
three sites of GA biosynthesis in higher plants, in developing fruits and seeds,
in elongating shoot apical regions, and in roots (Graebe and Ropers,
;Metraux,
;
Sponsel,
). They seem to be transported both in the xylem and in the
phloem. In apple and pear the application of inhibitors of GA biosynthesis
such as cyclocel and paclobutrazol results in a reduction in shoot elongation,
with short internodes. Such application has been, and is, of considerable com-
mercial importance and has also provided a tool for studying control of growth
by endogenous gibberellins. Paclobutrazol (
RS,
RS)-
-(
-chlorophenyl)-
,
-
dimethyl-
-ol inhibits three steps in the oxi-
dation of the GA precursor
ent
-kaurene to
ent
-kaureneoic acid (Hedden and
Graebe,
-(
H-
,
,
-triazol-
-yl) pentan-
) and its effects on apple can be partially reversed by GA
ap-
plication (Steffens
et al.
,
). It is transported acropetally in the xylem when
applied to young stem internodes and, to a lesser extent, to the youngest un-
rolled leaf (Richardson and Quinlan,
). When applied to the soil it is taken
up by the roots and transported upwards in the xylem stream. When applied in
this way it can be relatively slow to act and may be reversibly bound to vascu-
lar tissues, to be released and to check extension growth in the following year.
Richardson and Quinlan (
) applied paclobutrazol to different regions of
apple rootstock shoots growing in a heated greenhouse in late spring, with
the effects shown in Table
. These results show that paclobutrazol applied
directly to, or translocated to, the apical region of the shoot was much more
effective in retarding shoot extension growth than the chemical localized in
other parts of the shoot. Similarly, growth of internodes and leaves produced
from a treated shoot tip was reduced. This indicates that the shoot tip, and
possibly the young leaves, are important sites for the synthesis of gibberellins
influencing shoot development.
Repeated use of paclobutrazol results in the development of a spur-type
habit of growth with lateral buds giving rise to short fruiting spurs rather
than long shoots (Elfving and Proctor,
.
), which suggests
that whether a shoot becomes a long shoot or a short shoot is at least partly
controlled by gibberellin biosynthesis.
; Tukey,
