Cucurbit Genetics Cooperative Report 7:21-22 (article 10) 1984

Effect of Inbreeding on Horticultural Performance of Cucumber Families Developed from a Variable Population

D.B. Rubino and T.C. Wehner

Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609

Species of the Cucurbitaceae have been reported to exhibit little inbreeding depression (1). However, Ghaderi and Lower (3) reported that in 5 of 6 crosses of cucumber, heterozygosis had significant positive linear effects for all yield characters measured. They also suggested that since hybrid vigor and inbreeding depressing are both aspects of the same phenomenon, inbreeding depression might also be expected to occur in cucumber (2). Therefore, self pollination should be expected to result in deterioration of cucumber lines as it does in maize (Zea mays L.), where a reduction in vigor accompanying inbreeding was reported as early as 1908 (4). The objective of this experiment was to determine if inbreeding depression occurs in cucumber families derived from an open-pollinated population.

Methods. Twenty-four cucumber families were developed by self-pollination of randomly-selected plants from the open- pollinated pickling cucumber population NCMBP, for 6 generations. Seeds from all generations were planted on July 13, 1983 using a split-plot in a randomized complete block design with 2 replications. Whole plots were the 24 families and subplots were the 6 generations. Check plots of 'Calypso' and NCMBP (S₀ plants) were included for reference points. A stand of 15 plants per 1.5m plot was maintained with standard cultural practices. All plots were harvested when 'Calypso' check plots reached 10% oversize fruit (>51mm in diameter).

At harvest, the total and marketable fruit number per plot were counted. Fruit shape and color, and the size of the seed cavity were rated visually on a scale of 1 (poor) to 9 (excellent). Earliness was measured as the number of oversize fruit per plot on the day of harvest. The regressions of yield, earliness, and quality on F, the coefficient of inbreeding, were used to measure the effect of the level of homozygosity on performance.

Results. The differences among generation means were small for all traits measured (Table 1). Inbreeding apparently had no deleterious effect on these traits. The regressions for all traits resulted in slopes that were not significantly different from 0 (Table 2). This indicated that no significant inbreeding depression occurred, since a reduction in vigor or quality with inbreeding would result in negative slopes.

Heterozygosis was reported to have a positive effect on yield in 5 of 6 cucumber crosses studied by Ghaderi and Lower (3). However, most of the genetic variance for yield in a population studied by Smith et al. (5) was found to be additive. Results from this study support previous observations (1) that inbreeding depression is not important in controlling the fruit yield, earliness, or quality of inbreds developed at random from the pickling cucumber population NCMBP.

 

 

 

Table 1. Yield, earliness, and fruit quality for 6 generations of inbreeding from the NCMBP population (generation 0).z
Generations of inbreeding	Yield (Fruit no./plot)			Fruit quality scorex
	Total	Marketable	Earlinessy	Shape	Seedcell	Color
0	26.0	23.0	23.0	5.1	4.3	5.5
1	24.3	21.2	15.9	5.2	4.9	6.0
2	24.1	20.9	17.4	5.5	5.5	6.2
3	29.7	26.4	22.6	5.3	4.9	5.9
4	27.1	23.8	18.9	5.4	4.9	6.1
5	26.1	22.4	19.1	5.3	5.3	6.1
6	25.9	21.4	17.5	5.4	5.2	6.1
LSD (5%)	3.1	2.8	2.7	0.4	0.4	0.4
CV (%)	26.6	27.6	32.8	17.1	17.0	14.6
zData are means over 24 families and 2 replications.
yEarliness is the number of oversize fruit per plot at harvest.
xQuality scored 1 to 9 (1=poor, 5=average, 9=excellent).

Table 2. Relationship of the level of inbreeding (F) to performance of 24 families for yield, earliness, and fruit quality.
Trait		Regression of trait on F (b)z
Yield
	Total (Fruit no.)	5.57
	Marketable (Fruit no.)	3.90
Earlinessy		6.35
Qualityx
	Shape	0.36
	Seedcell	0.32
	Color	0.08
zNone of the regression slopes were significant at the 5% level.
yEarliness is the number of oversize fruit per plot at harvest.
xQuality scored 1 to 9 (I=poor, 5=average, 9=excellent).

Literature Cited

1. Allard, R.W. 1960. Principles of Plant Breeding. John Wiley and Sons, Inc., New York.
2. Ghaderi, A. and R.L. Lower. 1978. Heterosis and phenotypic stability of F₁ hybrids in cucumber under controlled environment. J. Amer. Soc. Hort. Sci. 103:275-278.
3. Ghaderi, A. and R.L. Lower. 1979. Heterosis and inbreeding depression for yield in populations derived from six crosses of cucumber. J. Amer. Soc. Hort. Sci. 104:564-567.
4. Shull, G.H. 1908. The composition of a field of maize. Amer. Breeders' Assoc. Rep. 4:296-301.
5. Smith, O.S., R.L. Lower and R.H. Moll. 1978. Estimates of heritability and variance components in pickling cucumber. J. Amer. Soc. Hort. Sci. 103:222-225.