SYNTHETIC CYTOKININS INCREASE FRUIT SIZE OF
‘ROYAL GALA’ (MALUS DOMESTICA) APPLE IN ISREAL
Raphael A. Stern1 and Moshe A. Flaishman2
1
MIGAL, Galilee Technology Center, P.O. Box 90000, Rosh Pina 12100 Israel
2
Department of Fruit Trees, Institute of Horticulture, ARO, The Volcani Center, P.O.
Box 6, Bet Dagan 50250, Israel.
Introduction
In the hot and dry summer of Israel, ‘Royal Gala’ trees produce relatively small fruits,
with an average size of 65-70 mm, which command a low price. Fruit size is
increased commercially, primarily by crop load reduction with chemical thinners.
Thus, although the total yield is reduced, the remaining fruit are of higher value
because of their increased size. The synthetic cytokinin benzyladenine (BA) is known
to be thinner in apple fruits that increases fruit size, but it also increases fruit size
independently of this effect by stimulating additional cell division within the fruit
which has a major influence on final fruit. Studies with the synthetic cytokinin CPPU
[N(2-chloro-4-pyridyl);N’-phenylurea] have indicated that the endogenous cytokinin
level is one of the major factors limiting growth and final fruit size.
The present study reports findings in field trials in Israel to examine the effects of BA
and CPPU on ‘Royal Gala’ apple fruit size, quality and shape in relation to the
concentration of each cytokinin applied.
Material and methods
In all experiments reported here, CPPU (SKW, Germany) containing 1% a.i. and BA
as the commercial product Bongrow (Safe-Pack Product Ltd, Israel) containing 4%
a.i. were applied as a foliar spray with the surfactant Triton X-100 (0.025%).
Application time was always at full bloom + 14 d (FB + 14) when the fruitlets
reached a diameter of ca. 10 mm. The experiments were conducted on whole trees;
two to three liters of spray per tree were applied with a motorized gun sprayer
1
(Holder), used at high pressure (20 atm.). Control trees were not sprayed. The
experimental design consisted of eight randomized blocks, with one tree per
treatment. At harvest, the yield of each tree was determined, and all the fruits were
sorted for size.
Results
CPPU –
In 2000, CPPU significantly increased the yield of large fruit by 72% (19 tonne ha-1
versus 11 tonne ha-1 in the control) with no negative effect on the total yield (Figure
1). The fruit size distribution was much more favorable in the CPPU treatment than in
the control trees (Figure 2).
In 2001, the effects of lower concentrations were determined (Table 1). The lowest
concentration of 2.5 ppm CPPU did not have any effect. At 5 ppm CPPU a positive
effect on fruit size was noticed (30% increase in yield of large fruit), but it was
accompanied by fruit thinning, which resulted in total yield reduction. Application of
10 ppm CPPU produced the highest effects on total yield (72 tonne ha-1 versus 65
tonne ha-1 in the control) and fruit size (30 tonne large fruit ha-1 versus 20 tonne ha-1
in the control), with no thinning effect (532 and 525 fruit per tree in the CPPU and
control treatment, respectively). The highest concentration of 20 ppm CPPU caused
fruit thinning (325 fruits per tree). Therefore, although the remaining fruits grew
faster and reached 24 tonne ha-1 versus 20 tonne ha-1 in the control trees, this did not
fully compensate for the massive fruit drop. Thus, the total yield was lower than in the
control.
BA –
In 2000, the effect of three concentrations was examined. The higher concentrations
of 100 and 150 ppm caused fruit thinning which led to a total yield reduction. The
total increase in the yield of large fruit was 55%: 17 tonne ha-1 in the BA treatments
versus 11 tonne ha-1 in the control trees (Figure 1). The lowest concentration of 50
ppm BA caused slight fruit thinning but a considerable and significant increase of
91% in the yield of large fruits (Figure 1). All the BA treatments, as well as CPPU 20
ppm, induced a significant shift to larger fruit sizes, and a reduction in the percentage
of small fruits (Figure 2). In 2001, the lower concentration of 50 ppm was again much
better than the higher one (Table 1).
2
Conclusions
During our three year study of the effects of CPPU and BA on ‘Royal Gala’ apple, a
significant improvement in fruit size was found, with no negative effect on fruit shape
or tree performance. Our findings demonstrate the effectiveness of both cytokinins
and show that they can elicit a significant improvement in crop loads.
Fig. 1
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Fig. 2
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CAPTION FOR FIGURES
Fig. 1
Effect of CPPU and BA on yield of large (≥75 mm) fruits, Merom-Golan, 2000.
Fig. 2
Effect of CPPU and BA on fruit size distribution (%), Merom-Golan, 2000.
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Table 1
Effect of CPPU and BA on fruit size and yield, El-Rom 2001.
Synthetic
Concentration Total yield
Cytokinin
Yield of
Increase in large fruit
large (≥75)
yield
fruit
CPPU
BA
(ppm)
(tonne ha-1)
(tonne ha-1)
(tonne ha-1)
%
0.0
65 ab
20 b
2.5
65 ab
19 b
-1
-5
5.0
52 b
26 ab
6
30
10.0
72 a
30 a
10
50
20.0
49 b
24 ab
4
20
50.0
70 a
29 a
9
45
100.0
61 ab
23 ab
3
15
Results within a column followed by different letters differ significantly according to
Duncan’s multiple range test, P ≤ 0.05.
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