Early Generation Testing in Cucumber

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Cucurbit Genetics Cooperative Report 7:19-20 (article 9) 1984

D.B. Rubino and T.C. Wehner
Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609

Early generation testing would benefit cucumber breeders by allowing poor lines to be discarded early in the inbreeding process. Often, lines are self pollinated six generations before testing. That method wastes time, space, and other resources on lines that will eventually be discarded. With early testing, lines are tested and evaluated after only one or two generations of self pollination.

Early testing was first suggested by Jenkins who found that maize (Zea mays L.) lines obtained their individuality in crosses in the S1 or S2 generation (1). Others have also found early testing to be successful (2,4). The efficiency of early testing for a given trait should depend greatly on the magnitude of the heritability for that trait. Heritabilities of 0.17 for fruit number, 0.25 for fruit color and 0.49 for carpel wall thickness were reported for cucumber (3). The objective of this experiment was to determine whether early testing was of value in cucumber.

Methods. Twenty-four S0 (open-pollinated) plants from the North Carolina Medium Base Pickle (NCMBP) population were self-pollinated for six generations. Generations S1 and S6 of each family were crossed with 2 testers, NCMBP, the medium base pickle population from which the 24 families were derived (referred to as testcrosses), and Gy14A, a gynoecious inbred line (referred to as hybrids). The testcross and hybrid seeds along with remnant seeds from the 6 inbred generations were planted on July 13, 1983, in 1.5 m plots using a split-split plot design with 2 replications. Whole plots were the 3 testers, subplots were the 24 families, and sub-subplots were the 3 generations. A stand of 15 plants per plot was maintained with standard cultural practices. All plots were harvested when ‘Calypso’ check plots reached 10% oversize fruit (>51 mm in diameter).

In order to measure the value of early testing, the S6 testcrosses were regressed on the S1 testcrosses, the S6 hybrids were regressed on the S1 hybrids, and the S6 inbreds were regressed on the S1 through S5 inbreds. The advantages of S1 line testing (early testing) versus S6 line testing were calculated using the equation for predicting gain from selection, assuming additive variance and phenotypic variance were the same for all generations. Thus, advantage = (b6.1)(k1/k6)(2.5) where b6.1 = the regression coefficient, and k1 and k6 the selection intensity in units of standard deviations for S1 line testing and S6 line testing, respectively. It was estimated that the number of lines that a cucumber breeder could handle were 300 for S1 line testing and 100 for S6 line testing. The equation was multiplied by 2.5, since 1 year is required for S1 line testing versus 2.5 years required for S6 line testing.

Results. The regression coefficients,and thus the prediction value for inbred yield,increased as the S6 generation was approached (Table 1). This trend was not apparent for earliness and quality.

Table 1. Coefficients (b) for the regressions of S6 on S1 through S5 inbred performance for yield, earliness, and quality.

  Yield (Fruit no./plot)   Fruit quality scorez

Regression

 Total Marketable Earlinessy Shape Seedcell Color
b6.1 0.52* 0.36* 0.76* 0.26* 0.17 0.21
b6.2 0.26 0.21 0.44 0.45* 0.33* 0.22
b6.3 0.69* 0.41* 0.60* 0.45* 0.00 0.09
b6.4 0.70* 0.48* 0.62* 0.38* 0.12 0.14
b6.5 0.82* 0.70* 0.74* 0.95* 0.00 0.26*
zQuality scored 1 to 9 (1 = poor, 5 = average, 9 = excellent).
yEarliness is the number of oversize fruit per plot at harvest.
*b greater than standard error.

The calculated advantages indicated that early testing for specific combining ability (hybrid performance) and early testing for inbred performance should be used for yield and earliness but not for quality traits. However, early testing for general combining ability (testcross performance) was not as efficient for detecting superior lines.

Literature Cited

  1. Jenkins, M.T. 1935. The effect of inbreeding and of selection within inbred lines of maize upon hybrids made after successive generations of selfing. Iowa State Col. J. Sci. 9:429-450.
  2. Lonnquist, J.H. 1950. The effect of selection for combining ability within segregating lines of corn. Agron. J. 42:503-508.
  3. 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.
  4. Sprague, G.F. 1946. Early testing of inbred lines of corn. J. Amer. Soc. Agron. 38:108-117.