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Abstract
Variance components for low-temperature germination ability in cucumber (Cucumus sativus) were estimated for 3 germination variables (actual days to germination, days to 50% germination, and percentage of germination) at 17°C using a North Carolina Design I analysis. The estimates were made using the North Carolina Medium Base Pickle (NCMBP) population, which was developed by intercrossing adapted pickling cucumber cultivars with diverse lines for 3 cycles without selection. Estimates of additive and dominance variance for the percentage of germination and days to 50% germination were equal, but additive variance was predominant for actual days to germination. Heritabilities based on half-sib families tested in 2 replications ranged from 0.44 to 0.61 for the 3 germination variables. There were large significant genetic correlations among the 3 germination variables, ranging from 0.61 to 1.03 in absolute value. Selection for either the percentage of germination or for actual days to germination at 17° should result in significant progress in improving low-temperature germination ability of cucumbers in the NCMBP population.
Cucumber (Cucumis sativus L.) populations often are intercrossed after each selection cycle using bees in isolation blocks. Previous research showed the rate of natural outcrossing in monoecious cucumber inbreds was 36%. The objective of this experiment was to determine whether the rate of natural outcrossing could be increased using hormones, plot size, and node of fruit for seed harvest. The experiment was run with 2 years (1997, 1998), two hormones (treated, none), two plot sizes (hills, small plots), two nodes of harvest (2, 8), and four replications. Each treatment combination consisted of four plots or hills planted in 1.5-m rows in one isolation block. Plots or hills were planted to white-spined `Sumter', and were surrounded by rows of black-spined `Wis. SMR 18'. Treatment combinations receiving hormones were sprayed at the cotyledon stage and 1 week later. Plots or hills received ethrel to make them gynoecious, and surrounding rows received silver nitrate to make them androecious. Unsprayed isolation blocks remained monoecious. At maturity, fruit were harvested from nodes 2 or 8 from the white-spined plants in each isolation block. Node of harvest had no effect on outcrossing rate. However, hormones and plot size had a significant effect. Sprayed plots, sprayed hills, and unsprayed hills had high outcrossing rates relative to unsprayed plots. Therefore, if families are to be intercrossed in isolation blocks, they should be sprayed with hormones for maximum outcrossing among families.
Several major traits (yield, earliness, quality) of interest to cucumber (Cucumis sativus L.) breeders are quantitatively inherited. The objective of this study was to determine the progress made on such traits using recurrent selection in 4 fresh-market cucumber populations (NCWBS, NCMBS, NCES1, NCBA1). During population improvement, 1 to 2 replications of 200 to 335 half-sib families were evaluated for 5 traits: total, early and marketable fruits per plot, a quality rating, and a simple weighted index (=.2Total/2 + .3Early + .2%Marketable/10 + .3Quality). Families from each population were intercrossed in an isolation block during each summer using remnant seeds of the best 10% selected using the index. Progress was evaluated using a split-plot treatment arrangement in a randomized complete block design with 32 replications in each of 2 seasons (spring and summer). Whole plots were the 4 populations, and subplots were the 11 cycles (cycles 0-9 plus checks). Greatest gains were made for the NCBA1 population, with an average of 45% gain from cycle 0 to 9 over the 5 traits, and for early yield, with an average of 58% gain from cycle 0 to 9 over the 4 populations. Populations were improved for performance in a selected (spring season) as well as a non-selected environment (summer season).
Cucumber (Cucumis sativus L.) breeders have provided growers with many useful cultivars since production began in the U. S. in the 1600s. The objective of this study was to determine how much breeding progress has been made for yield, earliness, quality, and anthracnose resistance. The experiment was a split-split-split plot in a randomized complete block with 3 replications. Treatments were 2 years (1989, 1990), 2 seasons (spring, summer), 2 production systems (stress, elite), and 14 cultivars (2 important ones from each of 7 time periods, from 1786 to 1982). Plants were grown at Clinton, N. C. using recommended cultural practices, except for the stress treatment, which received half the recommended amount of fertilizer, irrigation, and pesticides. Total yield over 8 harvests increased from approximately 20 Mg/ha for the old cultivars to 30 Mg/ha for the new cultivars. Similar increases were measured for marketable and early yield. Fruit quality (rated 1 to 9) also was improved by breeding, with shape improved 2, and fruit color improved 3 rating points. Part of the improvement in yield was probably due to improved anthracnose resistance. However, improved yield also was obtained in the spring season where anthracnose was absent. In conclusion, the relatively small cucumber breeding effort produced large gains for most traits measured.
Cucumber (Cucumis sativus L.) is one of the more chilling sensitive crops. Chilling resistance could provide growers with protection against late spring frost. Significant differences for chilling resistance were observed among a set of 9 diverse cucumber cultigens when grown at 22 C to 1st true leaf stage, then given a chilling treatment of 4 C for 7 hours in full light (PPFD 500 μmol.m-2.s-1). Two populations, NCWBP and NCES1, were used to measure narrow-sense heritability (estimated as twice the parent-offspring regression coefficient) for chilling resistance. Sets (256/population) of parents and offspring were evaluated in separate tests for seedling resistance. Plants were rated for damage 0 (none) to 9 (dead) on the cotyledons and 1st true leaf, 3 and 5 days after chilling. Ratings were corrected for position in the Phytotron chamber, and log transformations used to normalize the data. Generally, correction reduced the heritability estimates and transformation improved them. Heritability was highest for cotyledon ratings made 5 days after chilling, ranging from 0.35 for NCWBP to 0.70 for NCES1. Ratings of the 1st true leaf were more difficult to make, and produced lower estimates of heritability. Breeders should be able to make good progress in selecting for chilling resistance using this seedling test.
Most gynoecious hybrid cucumbers (Cucumis sativus L.) grown in the U.S. require pollination for proper fruit set. Early flowering pollenizers may help yield, earliness, or quality. Two experiments were run to measure the value of early pollenizers using fields isolated from other cucumbers by at least 1 km. The first experiment used `Armstrong Early Cluster' and `Sumter' as the early and normal pollenizer, with 30 and 35 days to flower, respectively. Gy 2, Gy 3, Gy 4, and Gy 14 were used as the gynoecious pickling cucumbers. The experiment was run in 2 years (1994, 1995) and seven locations in North Carolina with two pollenizers and the four gynoecious inbreds. There were four replications of plots within each whole plot to help control variability inherent in an experiment where treatments are in separate fields. The second experiment had only 1 year (1996), but the same seven locations, four replications, and four gynoecious inbreds, but only one pollenizer (`Sumter') planted at the same time, or 2 weeks earlier than the gynoecious lines. Plots were harvested once when 30% of the fruits were >50 mm diameter. None of the differences in either experiment were significant (F-ratio test, 10% level). Therefore, it does not appear that use of early flowering pollenizers in blends with gynoecious pickling cucumbers will have a large effect on the yield, earliness, or internal quality of the crop.