CGC4-27

Cucurbit Genetics Cooperative Report 4:50-53 (article 27) 1981

Reciprocal Crosses Between Cucumis africanus L.f. and C. metuliferus Naud. III. Effects of Pollination Aids, Physiological Condition and Genetic Constitution of the Maternal Parent on Crossability

J. B. M. Custers, A. P. M. den Nijs and A. W. Riepma

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

The positive effect of mentor pollen and amino-ethoxy-vinyl-glycine (AVG) on the success of reciprocal crosses between Cucumis africanus L.f. and C. metuliferus Naud. was recently reported (2). Because of the preliminary nature of those results, we have repeated the crosses in a series of experiments especially designed to evaluate the efficacy of the different pollination aids. We reports here on the results of these experiments.

Four experiments were carried out spread over the summer season. In all of them only one accession of C. africanus [Gene bank no. (Gbn) 0181] and one accession of C. metuliferus (GBn 1734) were used. The first two experiments each comprised 20 plants per species, grown from seed. They were cultivated in a glasshouse (minimum temperature 22°C D/20°C N) in 10 l. plastic containers with Trio peat soil, standing on dishes. This constraint on the root volume limits vegetative growth and promotes female flowering. It also saves glasshouse space, since the plants can be rearranged after the pollinations have ended. The treatments are listed in Table 1. On every plant up to eight flowers were pollinated: one without any pollination aid ( control), one with lanolin paste applied at pollination time, four with AVG in lanolin paste, one with mentor pollen and one with both mentor pollen and AVG. Preparation and application of the mentor pollen and the AVG were as described earlier (2, 3). The larger number of AVG treatments derives from the low fruit set in last year's crosses (2). All pollinations were made during four weeks starting with the opening of the first pistillate flower. The last two experiments were similar to the above except for the following: half of the plants of C. metuliferus were planted in full soil and only control and AVG pollinations were made. In the last experiment the C. africanus group consisted of six clones of five plants each, which derived from two exceptionally successful and four mediocre individual plants out of the first experiment.

Results of the first two experiments are in Table 1. Cucumis metuliferus x C. africanus yielded more fruits than the reciprocal cross, the mentor pollen treatments excepted. Fruit set in both crosses responded similarly to the pollination aids; it was generally higher in the control than in the AVG treatment. Lanolin itself proved detrimental. The two treatments with mentor pollen gave a high fruit set. All fruits in the control, the lanolin and the AVG treatment contained embryos. The number of these embryos per fruit was generally high. By contrast, in the two treatments with mentor pollen only part of the fruits contained embryos (especially in the cross C. africanus x C. metuliferus), and the number of embryos in those fruits was low or very low.

The percentage fruit set in both reciprocal crosses in the control and AVG treatment are summarized for all four experiments in Table 2. Only data from seed grown plants cultivated in containers are included. It is evident that these percentages depend not only on the direction of the cross but also quite heavily on the pollination period. In the last two experiments, C. metuliferus plants that were grown in containers set fruit (results in Table 2), but those in full soil did not.

So the constraint on the root volume promotes not only flowering but also fruit set. The two C. africanus clones in the fourth experiment, which were derived from the successful plants of the first experiments, yielded 10 and 47% fruit set in the control and o and 50% respectively in the AVG treatment. The four other clones set no fruits, so there appears to be variation for crossability.

The embryo growth was slowed down by cultivation of the maternal plants in containers. Mature seeds of C. metuliferus x C. africanus germinated in soil. The embryos of C. africanus x C. metuliferus remained smaller than last year (1), and embryo culture of them was unsuccessful.

The results of the experiments on pollination aids were very different from the preliminary ones in 1979 (2). We did not discern a positive effect of AVG on crossability, probably because of the high fruit set in the control and the negative effect of the carrier of AVG, lanolin paste. An explanation for this high fruit set in the control may be the cultivation of the plants in the restricted amount of soil, as illustrated by the results of the last two experiments. There was, however, also no positive effect of AVG on fruit set in full soil grown plants. The fruit set in the control was very different in the four experiments (Table 2). The main variable factor among these experiments was the weather, which changed from cloudy and relatively cool during the pollination period of the first experiment to bright and warm during that of the third one (temperature over 30°C in the glasshouse). Temperature and light intensity were again lower during the fourth experiment. The genetic variation for crossability as was found in the fourth experiment seems worthy of exploitation in our future crossing program.

Table 1. Effects of pollination aids on the results of reciprocal crosses between Cucumis africanus and C. metuliferus.

	*Cucumis africanus x C. metuliferus*					*C. metuliferus x C. africanus*
Treatment	No. of pollinated flowers	Fruit set %	No. of analyzed fruits	No. of fruits with embryos	% ovules with embryos	No. of pollinated flowers	Fruits set %	No. of analyzed fruit	No. of fruits with embryos	% ovules with embryos*
Exp.1 Control	17	12	2	2	96	20	70	10	10	87
Lanolin	16	0	-	-	-	19	21	4	4	87
AVG	68	7	4	4	97	77	45	24	24	91
Mentor pollen	17	100	9	3	6	20	80	16	14	29
Mentor pollen plus AVG	16	100	10	2	5	14	79	10	6	33
Exp. 2 Control	20	5	1	1	100	20	65	5	5	91
Lanolin	19	5	1	1	3	15	7	1	1	94
AVG	80	8	4	4	98	73	26	10	10	90
Mentor pollen	17	94	7	1	2	11	100	10	10	15
Mentor pollen plus AVG	12	92	8	1	12	9	100	9	9	29

*Only fruit with embryos; percentage calculated on the basis of the total number of enlarged ovules, ranging from 25 to 125 per fruit.

Table 2. Percent fruit set in the reciprocal crosses between Cucumis africanus and C. metuliferus in the control and AVG treatment during four pollination periods.

		*C. africanus x C. metuliferus*		*C. metuliferus x C. africanus*
Experiment	Pollination period	Control	AVG	Control	AVG
Exp. 1	June 6 - July 4	12%^z	7%	70%	45%
Exp. 2	June 30 - July 25	5	8	65	26
Exp. 3	August 11 - September 5	0	0	18	13
Exp. 4	September 22 - October 17	- ^y	-	41	10

^z Numbers of pollinated flowers, on which the percentages are based; for the first experiments see Table 1; C. africanus x C. metuliferus in experiment 3: 65 pollinations in the control and 54 in the AVG treatment; C. metuliferus x C. africanus in experiment 3: 11 and 8 pollinations respectively; in experiment 4: 17 and 10 respectively.
^y For cloned maternal plants, see text.

Literature Cited

Custers, J. B. M. and G. van Ee. 1980. Reciprocal crosses between Cucumis africanus L.f. and C. metuliferus Naud. II. Embryo development in vivo and in vitro. Cucurbit Genetics Coop. Rpt. 3:50-51.
Nijs, A. P. M. den, J. B. M. Custers and A. J. Kooistra. 1980. Reciprocal crosses between Cucumis africanus L. f. and C. metuliferus Naud. I. Overcoming barriers to fertilization by mentor pollen and AVG. Cucurbit Genetics Coop. Rpt. 3: 60-62.
Nijs, A. P. M. den and E. H. Oost. 1980. Effect of mentor pollen on pistil-pollen incongruities among species of Cucumis L. Euphytica 29: 267-271.