Agriculture Reference
In-Depth Information
after full bloom inhibited flower induction, but application at
weeks after
full bloom did not. These results suggest that gibberellins inhibit the early
stages of flower initiation, but Luckwill and Silva (
) were not able to find
any effect on the plastochron.
Injection of the cytokinins zeatin and benzyladenine
weeks after
full bloom into cut petioles on spurs increased flowering in the following year,
zeatin being particularly effective (Ramirez and Hoad,
,
or
).
-dimethyl hydrazide) is very
effective in both checking shoot extension growth and increasing the num-
ber of flower buds formed. These effects appear to be independent (Tromp,
The growth retardant SADH (succinic acid
,
). SADH application does not reduce the amount of extractable GA in
apple shoots and fruits (Hoad,
). Application of
SADH after cessation of shoot growth can increase flowering in the following
year (Tromp,
; Ramirez and Hoad,
) and it seems likely that the effect is on late phases
of the flower bud formation process. SADH was found by Luckwill and Silva
(
,
) to increase the number of floral buds without influencing the rate of
node production in the buds.
Paclobutrazol inhibits three steps in the oxidation of the gibberellin pre-
cursor ent -kaurene to ent -kaurenoic acid (Hedden and Graebe,
). It retards
extension growth and also stimulates flowering. Dheim and Browning (
)
found that its application to shoot tips was as effective in initiating flowering as
were whole tree sprays. They concluded that active shoot tip meristems slowed
node production in the axillary and spur buds to below the threshold rate
for floral initiation and that paclobutrazol, by slowing shoot apical activity,
reduced this inhibitory effect. Their results were thus consistent with paclo-
butrazol increasing flower initiation by reducing apical dominance.
Application of paclobutrazol can increase flowering in the subsequent year
both on spurs and on one-year-old extension shoots (Buban,
; Dheim and
Browning,
).
Nitrogen effects on flowering
Flower production can be reduced when nitrogen fertilizer application pro-
longs the period of extension shoot growth and delays terminal bud formation.
Excessive stimulation of shoot growth and leaf production can also have ad-
verse effects on flower production as a result of shading.
Conversely, N deficiency results in poor leaf development on spurs early in
theseasonandinreducedflowerinitiation.Goodleafdevelopmentthroughout
the season as a result of adequate nitrogen supply can be expected to enhance
root activity and cytokinin production and the upward flow of cytokinins and
nutrients in the transpiration stream. Faust (
) quotes data from Buban
et al. (
) to show very large effects of N nutrition on the cytokinin (zeatin)
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