Cucurbit Genetics Cooperative Report 7:10-11 (article 5) 1984

Chlorflurenol-induced Seed Coat Development in Parthenocarpic Pickling Cucumbers

A.P.M. den Nijs and G. de Wolf

Institute for Horticultural Plant Breeding, P.O. Box 16, 6700 AA Wageningen, The Netherlands

Application of chlorflurenol to pickling cucumbers to accelerate and concentrate yield for once-over mechanical harvest by inducing parthenocarpic fruit set has met with difficulties related to unpredictable weather conditions. In the meantime, genetically parthenocarpic cultivars were developed which hold promise for reliable and increased field production if early fruit set due to the presence of staminate flowers can be prevented. For optimal harvest planning, a combination of genetically parthenocarpic cultivars and application of chlorflurenol has been advocated. The effect of the spraying is enhanced by the genetic parthenocarpy of the plants. However, the processing quality of the fruits must also be considered. Here we present some results on fruit quality, which were obtained during a separate study of the competition between pollinated and parthenocarpic fruits on the plant.

In two temperature-controlled glasshouses (23/17°C D/N and 20/23°C D/N), 18 plants of each of three parthenocarpic pickling cucumber cultivars (Aida, Andrea and Belia) were grown in pots which were randomly placed on trolleys. Chlorflurenol treatments started September 1 on half of the plants. Individual flowers were dipped in a 100 ppm solution (Curbiset, courtesy of Asepta N.V., Leiden, the Netherlands). Fruit length (L) and diameter (D) were measured and the inside of the fruits was observed at the end of the experiment.

Chlorflurenol hardly affected the mean relative growth rates of the fruits, regardless of cultivar. The ultimate volume of the fruits, however, was significantly increased by the treatment. The effect of chlorflurenol on the length/diameter (L/D) ratio for the three cultivars is presented in Table 1. The significantly lower ratio of treated fruits was exclusively due to increased diameter, whereas length remained the same. The reason for this becomes clear from Table 2, which shows the percentage of fruits with developed seed coats. Almost all chlorflurenol-induced fruits contained such seed coats in high numbers, whereas only few of the genetically parthenocarpic fruits did. Only a fair percentage of fruits of 'Belia' set at 23/17°C without treatment contained seed coats. All examined seed coats were completely empty.

Table 1. Effect of chlorflurenol application on the length/diameter ratio of parthenocarpic pickling cucumbers grown in 2 different temperature regimes (°C day/night).z
Cultivar	Untreated		Chlorflurenol
	23/17	20/23	23/17	20/23
Alda	3.45	3.62	2.68	2.80
Andrea	3.30	3.10	2.57	2.83
Belia	2.94	2.92	2.65	2.71
zData are means of 24 fruits per treatment.

Table 2. Percentage of parthenocarpic fruits with seed coats as affected by chlorflurenol treatment of cucumbers grown in 2 different temperature regimes (°C day/night).z
Cultivar	Untreated		Chlorflurenol
	23/17	20/23	23/17	20/23
Alda	6	5	95	79
Andrea	16	5	89	84
Belia	60	14	86	60
zData are means of 18 to 21 fruits per treatment.

Dutch slicing cucumbers contain barely-visible ovules in their parthenocarpic fruits. Dutch slicers were the source of parthenocarpy in the pickling cucumber cultivars used in this experiment (2), so the absence of seed coat development is to be expected. In breeding lines and exotic accessions, we have observed a wide range of seed coat development in parthenocarpic fruits set under insect-free glasshouse conditions. The number and size of such seed coats bears no relationship to the parthenocarpy of the genotypes.

Processing quality of parthenocarpic fruits has been disputed. Pasteurized fruits of Dutch genetically parthenocarpic cultivars proved to be too soft for consumer acceptance. Pollinating parthenocarpic cultivars resulted in even softer fruits, presumably caused by a larger seed cell. Chlorflurenol likewise increases fruit softness while decreasing the L/D ratio. Fruits often became egg-shaped (3). The decrease in L/D ratio was also observed in American non-parthenocarpic slicing and pickling cucumbers treated with chlorflurenol (1). The results presented here demonstrate that the induction of ovule development into seed coats by chlorflurenol may be a major factor in the loss of external and internal fruit quality of parthenocarpic pickling cultivars. Caution is therefore advised with harvest planning schemes involving genetically parthenocarpic cultivars and chlorflurenol application.

The effect of chlorflurenol on the ovules suggests that the mechanism of action of this chemical in inducing parthenocarpy differs from the genetically determined one.

Literature cited:

1. Cantliffe, D.J. 1977. The induction of fruit set in cucumbers by chlorflurenol with and without pollination. HortScience 12:58.
2. Ponti, O.M.B. de and F. Garretsen. 1976. Inheritance of parthenocarpy in pickling cucumbers (Cucumis sativus L.) and linkage with other characters. Euphytica 25:633-642.
3. Rol, W. 1981. Conservenkwaliteit van parthenocarpe augurkenrassen. Groente en Fruit 36(27):46-47.