Synthetic Cytokinins Increase Fruit Size of Royal Gala (Malus Domestica) Apple in Isreal

SYNTHETIC CYTOKININS INCREASE FRUIT SIZE OF ‘ROYAL GALA’ (MALUS DOMESTICA) APPLE IN ISREAL

Raphael A. Stern1 andMoshe A. Flaishman2

1MIGAL, Galilee Technology Center, P.O. Box 90000, Rosh Pina 12100 Israel

2Department 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 (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-1versus 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-1versus 65 tonne ha-1 in the control) and fruit size (30 tonne large fruit ha-1versus 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-1versus 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).

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

Fig. 2

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.

Table 1

Effect of CPPU and BA on fruit size and yield, El-Rom 2001.

Synthetic Cytokinin / Concentration /

Total yield

/ Yield of large (≥75) fruit / Increase in large fruit yield
(ppm) / (tonne ha-1) / (tonne ha-1) / (tonne ha-1) / %
CPPU / 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
BA / 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.

1