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14620316.1985.11515618

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Journal of Horticultural Science
ISSN: 0022-1589 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/thsb19
Fruit thinning Victoria plums (Prunus domestica
L.): preliminary studies with paclobutrazol
A. D. Webster & Linda Andrews
To cite this article: A. D. Webster & Linda Andrews (1985) Fruit thinning Victoria plums (Prunus
domestica L.): preliminary studies with paclobutrazol, Journal of Horticultural Science, 60:2,
193-199, DOI: 10.1080/14620316.1985.11515618
To link to this article: http://dx.doi.org/10.1080/14620316.1985.11515618
Published online: 27 Nov 2015.
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Date: 28 October 2017, At: 23:10
Journal of Horticultural Science (1985) 60 (2) 193-199
Fruit thinning Victoria plums (Prunus domestica L.):
preliminary studies with paclobutrazol
Downloaded by [UNSW Library] at 23:10 28 October 2017
By A. D. WEBSTER
East Mailing Research Station, Maidstone, Kent ME19 6BJ, UK
and LINDA ANDREWS
Luddington Experimental Horticulture Station, Stratford-upon-Avon,
Warwickshire CV37 9SJ, UK
SUMMARY
Sprays of 1 000 or 2 000 mg I- 1 paclobutrazol (PP333) thinned and increased the fruit size
of Victoria plums on two sites when applied to mature trees at full bloom, in early/midMay or early June in 1983. Sprays of 125,250,500 or 1 000 mg I- 1 paclobutrazol applied in
late May 1982 to young, lightly cropping Victorias also thinned fruitlets but failed to
increase fruit size. Significant reductions in shoot growth were recorded only on trees
sprayed at full bloom in one of the experiments in 1983.
VICTORIA plum trees (Prunus domestica L.) frequently set abundantly and retain excessive
numbers of fruitlets until harvest. This results
in small fruits at harvest, branch breakage
under the crop weight and poor flower production in the subsequent season.
Hand thinning of flowers or fruitlets has
become prohibitively expensive and chemical
methods of reducing fruit set or of promoting
flower or fruitlet abscission have been sought.
Although chemicals such as DNOC (dinitro
orthocresol), lime sulphur and ethephon have
shown initial promise as plum flower thinners
(Dodd, 1967; Martin, et al., 1975; Webster,
1980) these have often produced variable
results.
Sprays of 3-CPP amide or ethephon at the
onset of cytokinesis in the seed endosperm have
thinned Victoria plums very effectively (Webster, 1980) but the thinning response to 3-CPP
amide may be slow and the thinning activity of
both chemicals is influenced by temperature at,
and shortly after, spraying.
Recent experiments with the triazole growth
retardant paclobutrazol (PP333 or Cultar; ICI
Plant Protection Division) showed that foliar
sprays applied in May stimulated fruitlet abscission of the plum cv Grove's Late Victoria
(Webster and Quinlan, 1984). The objective of
the experiments described in this paper was to
evaluate paclobutrazol, ((2RS, 3RS)-1-(4-chlorophenyl)-4, 4, dimethyi-2-(IH-1 ,2,4-triazol-
1-yl) pentan-3-ol)), as a potential flower or
fruitlet thinner for the plum cv Victoria.
MATERIALS AND METHODS
Experiment 1
Six-year-old Victoria trees on Pixy rootstock
planted at East Mailing Research Station were
used for the initial evaluation of paclobutrazol
as a plum thinner in 1982. Trees sprayed with
125, 250, 500 or 1 000 mg I- 1 a.i. paclobutrazol
were compared with unsprayed controls. The
treatments were applied at high volume (HI
per tree), with a Cooper Peglar Falcon sprayer
using a hand lance, on 26 May when the largest
fruitlets were 20 to 25 mm long. This stage has
previously been shown to approximate to the
time of onset of cytokinesis in the seed endosperm of Victoria (Way and Skene, 1973). No
surfactants or additives were included in the
sprays. The experiment was designed in eight
randomized blocks of single-tree plots.
Records of yield, fruit size, colour, internal
gumming and soluble solids content were taken
from each tree at harvest. Effects on the time of
fruitlet abscission were monitored on sample
branches on each tree from initial set until harvest. Extension shoot growth was also recorded
on sample branches on each tree both at the
time of spraying and after the completion of
growth.
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194
Plum fruit thinning with paclobutrazol
Experiment 2
In 1983, 26-year-old Victoria trees on St.
Julien A rootstock were used at East Mailing
Research Station. The trees were sprayed (2! I
per tree) with either 1 000 mg J- 1 or 2 000 mg J- 1
paclobutrazol on one of four dates, 21 April
(full bloom), 12 May, 2 June or 23 June and
compared with unsprayed controls. No surfactants or additives were included in the sprays.
The experiment was designed in six
randomized blocks of single-tree plots.
Records of yield, fruit size, colour, internal
gumming and soluble solids content were taken
at harvest. Floral-bud numbers and fruit set at
harvest were recorded on sample branches on
each control and on trees treated at full bloom.
Fruitlet abscission between initial set and harvest was monitored on sample branches on all
trees. At the end of the growing season the new
extension growth on 20 randomly selected
branches on each tree was measured.
RESULTS
Experiment 1
Initial fruit set was very poor on these young
Victoria/Pixy trees in 1982 and was reflected in
the poor yields harvested on both control and
treated trees (Table I). All treatments significantly reduced the weight of fruit harvested per
tree but no differences between the various
treatments were recorded.
Records of final fruit set per 100 floral buds
on sample branches showed that all treatments
reduced final fruit set compared with untreated
trees, although the effect of the 1 000 mg J- 1
treatment was not statistically significant.
Weekly observations of fruit retention on
sample branches showed that fruitlet abscission
resulting from paclobutrazol treatments
occurred between 8 and 16 July, i.e. six to seven
weeks after spraying (Figure 1).
There were no significant differences in fruit
size at either picking date (Table 1), or in fruit
colour, internal gumming or soluble solids content recorded at the second picking date. From
93 to 96% of all fruits harvested contained well
developed seeds.
At the time of spraying the new extension
shoots were between 7 and 21 em long with 7 to
11 fully expanded leaves. After the completion
of growth no differences in either the mean or
adjusted total shoot length were recorded on
the sample branches.
Experiment 3
Eleven-year-old Victoria trees on St. Julien
A rootstock were used for this experiment, at
Luddington Experimental Horticulture Station
in 1983. The trees were sprayed (2 I per tree)
with either 1 000 mg J- 1 or 2 000 mg J- 1 a.i.
paclobutrazol on one of three dates, 26 April
(full bloom), 13 May or 7 June and compared
with unsprayed controls. No surfactants or
additives were included with the sprays. The
experiment was designed in six randomized
blocks of single-tree plots; two unsprayed control trees were included in each block.
Records of total yield and individual fruit
weight were taken at harvest and extension
shoot growth recorded in the following winter.
Experiment 2
Yields per tree were reduced by both 1 000
mg I- 1 and 2 000 mg J- 1 paclobutrazol treatments on 12 May and 2 June; sprays applied
either at full bloom or on 23 June produced only
TABLE
I
The effects of paclobutrazo/ sprays on the yteld and fruit size of Victoria plums in 1982. Experiment I
Spray
cone.
(mg )-')
1000
500
250
125
Unsprayed (Control)
SED
Df
Total we1ght
/tree
(kg)
2.34**
3.07*
2.97'
2.7P*
4.28
0.53
28
Individual frmt weight (g)
Jst p1ck
2nd pick
47 4
44.5
45.7
47.8
47.1
NS
(3.2)
26 (2MV)
43.1
44 3
43.0
41.5
43.7
NS
(1.2)
27 (lMV)
Levels of significance of difference between control and treated values: * P
= 0.05. ••
P
= 0.01.
Frmts harvested
/100 floral
buds
6.3
5.4*
3.3**
3.6**
10.5
2.4
28
195
A. D. WEBSTER and LINDA ANDREWS
100
~
\
90
\
\
\
\
Q)
~
80
.---=:::...::-! ----6,
o- 0 ~i~,
c:
,
0
Q)
c:
·-·------·-·-·
\ c.,
~B---e.
70
o---o
co
~
Q)
"'
~
60
~0
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E
0
-#.
50
0
1000 mgl- 1 PP333
500 mgl- 1 PP333
250 mgl- 1 PP333
•
6
125 mgl· 1 PP333
Unsprayed control
0
•
40
-c.
~
.""'"
0
·-·-·
'\ ·"·--·-·
0
0
0
\
..........._
-0\
0
o-o--o-o
30
28/5
716
15/6
25/6
Initial
set
2/7
8/7
16/7
22/7
30/7
4/8
Date
FIG. I
The mHuence of PP333 sprays on the percentage of the Initial frmt set retained on young Victona/Pixy trees in 1982:
Expenment 1
a slight and non-significant yield reduction
(Table II). The 2 June treatments were the most
effective in stimulating fruit abscission. The
main effect of spray concentration was statistically significant with mean yields of 103 kg and
83 kg on trees sprayed with 1 000 and 2 000
mg 1- 1 respectively. The main effect of spray
timing was very highly significant; trees sprayed
on 12 May or 2 June produced only 43% or 13%
of control yields respectively, and sprays at full
bloom induced more moderate yield reductions
(82% of control yields) whereas sprays on 23
June produced no significant effect. A significant statistical interaction on tree yields was
recorded between spray timings and
concentrations.
Final fruit set per 100 floral buds, recorded
only on the controls and trees sprayed at full
bloom, was reduced by the 2 000 mg )- 1 fullbloom spray (20.8) and slightly, though not significantly, reduced by the 1000 mg J- 1 spray
(27.0) when compared with the unsprayed controls (32.0).
The effects of treatments on the percentage
of initial fruit set retained at four dates during
fruit development are shown in Figure 2. By 2
June both full-bloom treatments had stimulated fruitlet abscission; the higher concentration spray applied on 12 May had also
slightly stimulated fruit abscission by this date.
Considerable fruit drop occurred on both
unsprayed control and treated trees during the
period between 2 and 23 June; all treatments
except the lower concentration spray on the 12
May stimulated fruit abscission in this period.
By 13 July the final relative effects of the treatments on fruitlet abscission and yields had been
established and these effects were unchanged
on the 3 August sampling date, and at harvest.
Numbers of fruit harvested (6 September),
expressed as a percentage of initial fruit set on
sample branches, showed large treatment
effects; treatments on 12 May and 2 June and
the 2 000 mg 1- 1 spray at full bloom all reduced
final set (Table II). The main effect of spray
timing proved statistically significant whereas
196
Plum fruit thinning with paclobutrazol
I
100
LSD 5%
~ 7.9
r-
Treatment
80
60
PP333
Cone
lmgl-11
Date of Sprays
1n 1983
Control
-
Unsprayed
Ia
2a
lb
2b
lc
2c
ld
2d
1000
2000
1000
2000
1000
2000
1000
2000
21 Apnl
21 Apul
12 May
12 May
2 June
2 June
22 June
22 June
t-
I
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LSD5%=85
40
I
LSD 5% = 6.3
20
r-
C
Ia 2a lb 2b lc 2c ld 2d
C
I
LSD 5%• 5.5
,'
Ia 2a 2b lb lc 2c ld 2d
22 June
2 June
Date
FIG.
2
The mHuence of PP333 treatments on the percentage of the initial fruit set retained on Victoria/St. Julien A trees in 1983:
Experiment 2
TABLE II
The influence of llmmg and concentratwn of paclbutrazol sprays on the yzeld and fruit size of Vzctoria plums m /983·
Experzment 2
Spray
cone.
(mg 1-1)
1000
2000
1000
2000
1000
2000
1000
2000
Unsprayed (Control)
SED
Df
Spray
timing
21
21
12
12
2
2
23
23
April
April
May
May
June
June
June
June
Y1eld/tree
(kg)
FrUits numbers
harvested
(as% initial set)
14.4
12.6*
8.4***
5.1 ...
3.0***
0.8***
13.4
15.4
18.6
2.7
40
139.0
130.7
106.1***
34.1***
28.5***
15.4***
139.0
148.2
164.4
15.7
40
Levels of significance of difference between control and treated values: • P
= 0.05;
•• P
Fruit size
(g)
40.8
44.3*
41.5
49.1***
49.1***
46.0***
34.4
32.4
35 8
3.0
40
= 0.01; ••• P = 0 001
Downloaded by [UNSW Library] at 23:10 28 October 2017
A. D. WEBSTER and LINDA ANDREWS
that for spray concentration did not; no significant concentration:timing interactions were
recorded for this parameter.
Fruit size at harvest was increased by both
treatments applied on 2 June and by the higher
concentration sprays applied at either full
bloom or 12 May; smaller but non-significant
size increases followed the 1 000 mg 1- 1 spray
applied either at full bloom or 12 May. Sprays
applied on 23 June had no effect on fruit size.
Statistical analysis showed the main effect of
spray timing, but not spray concentration, to be
significant.
Fruit colour and percentage soluble solids at
harvest were increased by the higher concentration spray on 12 May and by both sprays
on 2 June (Table III). Statistically significant
concentration and timing main effects and
interactions were recorded for both colour and
soluble solids.
The proportion of fruits developing internal
gumming around the endocarp was increased
by treatments on both 12 May and 2 June and
by the 2 000 mg 1-= 1 spray at full bloom; statistical analysis revealed the main effect to be one
of spray timing rather than concentration.
Branch breakage, as a result of the heavy
crop, was considerable on the unsprayed control and on trees sprayed with I 000 mg J- 1
either at full bloom or 22 June.
Mean shoot extension growth on branch
leaders was unaffected by the treatments in
1983; neither spray timing nor concentration
had any significant main effect upon mean
shoot length.
197
Experiment 3
Yields per tree were reduced by all
paclobutrazol treatments except the 1 000
mg I- 1 spray at full bloom (Table IV). Reductions were most severe (> 77%) where the
higher concentration spray was applied on 13
May or where either concentration was applied
on 7 June. The main effects of both concentration and timing were statistically significant and a significant interaction was also
recorded.
Fruit size was increased by all treatments,
although the response following 2 000 mg 1- 1 on
13 May was not significant. The largest fruits
were harvested from trees sprayed at full bloom
with 2 000 mg 1- 1 paclobutrazol. Although a significant timing:concentration interaction was
detected, only the main effect of spray timing
proved statistically significant.
Measurements of new extension growth
showed no treatment effects upon either the
estimated total growth or number of shoots
produced per tree. Although mean shoot
length was slightly reduced by all treatments,
only the effects induced by the full-bloom treatments proved statistically significant.
DISCUSSION
Paclobutrazol sprays effectively thinned Victoria plums by stimulating fruitlet abscission in
all three experiments. Like other triazoles and
the growth retardant ancymidol, paclobutrazol
has been shown to be an effective inhibitor of
TABLE Ill
Tire mjluence of pac/oblllrazol spray concentrallon and liming on fnut colour, soluble sobds and mternal gumming of Vtctona
fruits: Expenment 2
Spray
cone
(mg 1- 1)
1000
2000
1000
2000
1000
2000
1000
2000
Un;prayed
(Control)
SED
Df
Spray
t1m1ng
21
21
12
12
2
2
23
23
Apnl
Apnl
Mav
May
June
June
June
June
Frmts fully
red
(%)
Soluble
sohds
(%)
Internal
gummmg
(%)
27.4
27.9
54.0
89.8***
86. 7***
91 3***
41 7
37.5
11.1
12.1
13.3
16.3***
16.7***
17.9***
12.1
12.5
49.4
70.4**
79.7***
85.8***
86.5***
82.8***
45.0
26.7
44.5
8.82
40
12.3
0.6
40
38.4
10.9
Levels of >lgntficance of dtfference between control and treated values: •• P
40
= 0.01;
••• P
= 0.001
Mean shoot
length
(em)
15.6
16.0
15.4
15 8
14.4
13.2
15.3
14.9
14.9
NS (1.7)
40
198
Plum fruit thinning with paclobutrazol
TABLE IV
Yteld, fruit stze and shoot growth on Victona plums sprayed with pac/obutrazolthmning treatments m /983. Experiment 3
Spray
cone.
(mg I-')
1000
2000
1000
2000
1000
2000
Unsprayed (Control)
SED
Of
Spray
timing
26
26
13
13
7
7
Yield/tree
(kg)
April
April
May
May
June
June
59.2
37.8*
39.7*
12.1***
9.1 * * *
6.8***
52.5
5.85
36
Downloaded by [UNSW Library] at 23:10 28 October 2017
Levels of sigmficance of difference between control and treated values: • P
the biosynthesis of gibberellins (Rademacher
and Jung, 1981; Hedden et a/., 1983). Gibberellins are thought to have a P"rticularly
important role in preventing the abscission of
stone fruits, and sprays containing gibberellic
acid (GA_1) have been used to prevent the
abscission and increase the yields of Victoria
plum (Webster and Goldwin, 1978 and 1981).It
is not surprising, therefore, that inhibitors of
gibberellin biosynthesis such as paclobutrazol
may stimulate fruitlet abscission.
Although sprays of 2 000 mg 1-I stimulated
significantly more thinning than the 1 000 mg 1-I
sprays on the heavily cropping trees in both
Experiments 2 and 3, spray concentration had
little effect upon the thinning response of the
lightly cropping trees in the first experiment.
The reasons for these differences in response
are not understood and may be associated with
differences in rootstock, the internal hormone
levels or other factqrs influenced by the level of
cropping or alternatively climate in the two
seasons.
Most fruitlet abscission was initiated by
sprays applied in late May or early June; previous research on the cv Victoria has shown that
this timing approximates to the onset of
cytokinesis in the seed endosperm. Less severe
thinning resulted from sprays applied earlier
than this sensitive late May/early June period.
Results from Experiment 2 suggest that
paclobutrazol sprays applied at full bloom or in
early May may take from four to six weeks to
induce plum abscission on trees bearing an
overset of fruits. Sprays to trees carrying only a
poor fruit set, applied in late May (Experiment
1), similarly stimulated abscission of fruits in
Fruit weight
(g)
44. 7* *
50.2***
44 2**
42.3
43.8*
45.0**
38.6
2.00
36
Mean
shoot length
(em)
15.8
13.7*
19.3
18.4
14.6
16.3
21.4
3.69
36
= 0.05: •• P = 0.01: ••• P = 0.001
the period from four to six weeks following
spraying.
In 1983 fruit size was increased by all those
treatments which increased fruitlet abscission.
Sprays at full bloom induced significant size
increases with the least reduction in total yield.
It has long been established that the earlier fruit
thinning is practised then the earlier the reduction in inter-fruit competition for water, minerals and assimilates and the greater the
benefits in increased fruit size. Unfortunately,
however, the yields were insufficiently reduced
by some of the full-bloom treatments in 1983
and branch breakage on some trees was severe.
Treatments in 1982 (Experiment 1) reduced
yields but failed to increase fruit size. The crops
on the unsprayed control trees were very poor
in this experiment, however, and fruit size was
unlikely to benefit from any reduction in interfruit competition.
The paclobutrazol sprays in both May and
early June (Experiment 2) advanced ripening
and increased the percentage soluble solids in
fruits. Previous experiments have shown that
any thinning treatment which substantially
reduces the number of fruits on a tree is likely
also to advance fruit ripening. The very minor
effect of the full bloom and late June sprays on
fruit ripening, soluble solids and crop yields
suggest that effects upon fruit ripening are not
directly the effect of paclobutrazol treatment
but rather the result of fruit thinning.
Sprays applied in late May and early June
and the 2 000 mg 1-I spray at full bloom all
increased the proportion of fruits developing
internal gumming. This may be partly attributable to the treatment effects upon ripening;
A. D. WEBSTER and LINDA ANDREWS
Downloaded by [UNSW Library] at 23:10 28 October 2017
previous observations have shown that the
severity of gumming increases as fruit approach
full ripeness. The 2 000 mg 1- 1 spray at full
bloom had only a minimal effect on ripening,
however, and yet significantly increased fruit
gumming.
Fruit gumming is frequently a serious problem on the cv Victoria, reducing the value of
harvested fruits, particularly those sold for processing. Any thinning treatment which significantly increases fruit gumming will, therefore,
have only a limited commercial value and
further experimental work is needed with
paclobutrazol to ascertain if this undesirable
side-effect can be alleviated.
Treatment effects on the growth of new
shoots were minimal; only in Experiment 3 did
paclobutrazol sprays, when applied at full
mean
shoot
length.
bloom,
reduce
Paclobutrazol sprays in early May, at a higher
199
concentration than those tested here (3 000
mg 1- 1), have been shown to reduce both total
and mean shoot length formed on three plum
cvs (Webster and Quinlan, 1984), and on one cv
the effects persisted into the second season following spraying. Further research is obviously
needed to evaluate fully the long-term effects of
annual paclobutrazol thinning sprays on plum
shoot growth.
These preliminary experiments suggest that
paclobutrazol sprays may prove very effective
for thinning the plum cv Victoria. Further
experiments
are
planned
in
which
paclobutrazol will be tested as a thinner on a
range of plum cultivars and its long-term effects
on tree growth and cropping monitored.
We thank Mrs Celia Beckham, Mr P. Goodright, Mr B. Homer and Miss Maria Teresa
Aguado Abril for their valuable help with these
experiments.
REFERENCES
DODD, B. C. (1967). Thinning Wilson plums with chemicals. Queensland Agricultura!Journal, 93,
476-80.
HEDDEN, P., WARD, D. A., ANDREWS, P., WARD, GILLIAN C. and JAMES, S. (1983). Gibberellin
metabolism and the mode of action of plant growth regulators. Report of East Malting
Research Station for 1982, 147-8.
MARTIN, G. C., FITCH, L. B., SIBBETT, G. S., CARNILL, G. L. and RAMOS, D. E. (1975). Thinning
French prune (Prunus domestica L.) with (2-chloroethyl) phosphonic acid. Journal of the
American Society for Horticultural Science, 100, 90-3.
RADEMACHER, W. and JUNG, J. (1981). Comparative potency of various synthetic plant growth
retardants in the elongation of rice seedlings. Zeitschrift fur Acker-und Pflanzenbau, 150,
363-71.
WAY, D. W. and SKENE, D. S. (1973). Fruit thinning, Plum. Report of East Mailing Research Station
for 1972, 52.
WEBSTER, A. D. (1980). Flower and fruitlet thinning of the plum (Prunus domestica L.) cv Victoria.
Journal of Horticultural Science, 55, 19-26.
WEBSTER, A. D. and GoLDWIN, G. K. (1978). The use of hormone mixtures to increase the set of
plums, Prunus domestica L. cv Victoria. Journal of Horticultural Science, 53, 123-9.
WEBSTER, A. D. and GOLDWIN, G. K. (1981). The hormonal requirements for improved fruit setting
of plum, Prunus domestica L. cv Victoria. Journal of Horticultural Science, 56, 27-40.
WEBSTER, A. D. and QUINLAN, J. D. (1984). Chemical control of tree growth of plum (Prunus
domestica L.): I. Preliminary studies with the growth retardant paclobutrazol. Journal of
Horticultural Science, 59, 367-75.
(Accepted 29 September 1984)
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