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David B. Rubino and David W. Davis

This study was conducted to investigate the effects of mild mass selection for adaptation on the performance, genotypic variance, combining ability, S1 family-testcross correlation, and midparent heterosis of S1 families derived from a sweet corn (su) × tropical maize (Zea mays L.) composite (Composite 1R). Four cycles of random mating followed by 10 cycles of 10% stratified mass selection were conducted for earliness, plant and ear type, and freedom from pests. Selection significantly (P < 0.01) decreased plant height, ear height, percentage barrenness, and ear length, and significantly (P < 0.01) increased stalk breakage, earliness (Celsius heat units to 50% anthesis and silking), and kernel row number of both S1 families and their testcrosses. Juvenile plant height at 45 days after planting increased in testcrosses only. Percentage tip blanking and pericarp thickness did not change. For most traits, the greatest response occurred during the first five of 10 selection cycles. Cycle 10 testcrosses performed at least as well as elite check testcrosses for eight of 10 traits. The tropical parents improved combining ability for increased juvenile plant height and kernel row number, and decreased percentage of stalk breakage. As a result of selection, genotypic variance among S families decreased by >40% for heat units to 50% anthesis and silking, ear height, and percentage of barrenness, although for all traits measured, significant genotypic variation persisted following 10 cycles of mass selection for adaptation. S1-testcross correlations and percentage midparent heterosis tended to be consistent across selection cycles. Five cycles of mild stratified mass selection increased the adaptation of a temperate sweet corn × tropical maize composite to the temperate zone of the United States while maintaining significant genotypic variation.

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David W. Davis and Karl J. Sauter

Attention has been given in recent literature to crop breeding for heat tolerance, but, as with certain other physiological traits, such as photosynthetic efficiency, practical gain has lagged. The question remains as to whether heat tolerance can be improved, and, if so, if it can most efficiently be improved by a holistic approach, as in breeding for yield following timely high temperature levels in the field environment, or whether the breeding for heat (and drought) tolerance components in the laboratory would be feasible. At issue is the identification and repeatability of key plant responses, such as cell membrane damage, heat shock protein formation, increased ethylene output and other responses, and the relevance, effectiveness and cost of screening for such traits. Results from our laboratory, and the work of others, will be reviewed.

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Mary S. Joyce and David W. Davis

Concern over insecticide usage for control of European corn borer (Ostrinia nubilalis Hübner) in sweet corn (Zea mays L.) in recent years has increased the need for genetic control. Our objectives were to determine the degree of ear feeding resistance transmitted by resistant breeding lines to testcrosses and to investigate the relationship between resistance and both ear silk channel length, and infestation level. Testcrosses averaged 35% acceptable ears compared to 7%. for two commercial control hybrids and 45% for the lines per se when artificially infested at the ear tip at mid-silk at two locations. Generally, resistance in the testcrosses was closer to the resistance level of the resistant parent, indicating good combining ability for resistance. Heterosis above the resistant parent was found in 8 of 12 testcrosses. Across entries, ears having relatively longer silk channel length tended to have less damage but the relationship was not strong (r 2 = 0.24, P < 0.01). When silk channels were artificially shortened at infestation, resistance levels were lower, but five of seven lines had a higher proportion of acceptable ears than did the control hybrid. Across increasing levels of infestation from 50 to 200 neonate larvae per ear eight of nine lines had higher resistance (P < 0.05) than the control.

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Daniel F. Warnock and David W. Davis

Some scales combine quantitative and qualitative components that inadvertently may skew damage estimates and eliminate potentially useful germplasm. Two visual evaluation scales to estimate European corn borer feeding damage were compared for their effectiveness in classifying sweet corn germplasm. Both the traditional 1 to 9 scale, combining ear feeding damage and damage location, and the alternative 1 to 5 scale, based solely on ear feeding damage, consistently separated sweet corn genotypes into resistant, marginally resistant, and susceptible classes. Inbred MN 3002, Hybrid MN 3004, `Apache', and `More' were classified as having marginally acceptable resistance levels. Inbred Mn 3003, Inbred W182E, and `Jubilee' were susceptible to European corn borer. Individual genotype rankings varied by scale, but genotype classifications were consistent with regard to the degree of commercial acceptability. The combination of quantitative and qualitative components did not compromise genotypic characterization, as the previously untested hybrid, MN 3004, was placed in the marginally acceptable class by both scales. Plant breeders should carefully evaluate the efficacy of individual visual scales before incorporating them into a selection program.

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Daniel F. Warnock and David W. Davis

European corn borer, Ostrinia nubilalis Hübner, is an economic pest of sweet corn. Consumer demand for high-quality, insect-free produce with minimal pesticide residue necessitates exploitation of various control options. Ear feeding resistance could reduce insecticide inputs. The inheritance of ear feeding resistance and silk channel length in the F1 derived from a diallel cross (Griffing's model I, method 2) of eight breeding stocks describing a wide range of feeding resistance was investigated in field experiments. Feeding damage, based on a 1 (no damage) to 9 (>10% ear damage) visual rating scale, and silk channel length of ears that had been manually infested at the ear tip with O. nubilalis were recorded. A significant (P ≤ 0.05) year by location interaction was found for ear feeding damage and silk channel length. Genotype ear feeding damage and silk channel length differences were significant (P ≤ 0.01) beyond genotype by environment (year and location) interactions. Mean feeding damage ranged from 2.5 (parents 1 × 7) to 8.8 (parent 2) and mean silk channel length ranged from 1.9 cm (parents 2 × 7) to 9.0 cm (parent 3). Ten of the 28 possible crosses (reciprocals combined) and 1 parent were classed as resistant (damage rating < 3.0). Eleven crosses, including all 7 involving parent 2, and 2 parents were susceptible (damage rating > 4.0). Pearson's correlation analysis indicated lower damage levels were weakly to moderately associated with increased silk channel length for both parents (r = –0.18) and progeny (r = –0.44). The general combining ability (GCA) component was significant (P ≤ 0.01) for ear feeding damage, suggesting additive effects control ear feeding damage. GCA and specific combining ability (SCA) effects did not account for silk channel length variability, suggesting strong environmental influences. Improved ear feeding resistance should be possible via recurrent selection with recombination.

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David W. Davis and Karl J. Sauter

Attention has been given in recent literature to crop breeding for heat tolerance, but, as with certain other physiological traits, such as photosynthetic efficiency, practical gain has lagged. The question remains as to whether heat tolerance can be improved, and, if so, if it can most efficiently be improved by a holistic approach, as in breeding for yield following timely high temperature levels in the field environment, or whether the breeding for heat (and drought) tolerance components in the laboratory would be feasible. At issue is the identification and repeatability of key plant responses, such as cell membrane damage, heat shock protein formation, increased ethylene output and other responses, and the relevance, effectiveness and cost of screening for such traits. Results from our laboratory, and the work of others, will be reviewed.

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Daniel F. Warnock and David W. Davis

The germplasm evaluation techniques in resistance breeding programs may improperly characterize insect damage. For example, the relationship between economic damage levels and biological damage levels may not be linear as some techniques assume. Most commercial sweet corn hybrids are highly susceptible to European corn borer (ECB), Ostrinia nubilalis Hübner, ear feeding. Genotype variation for ECB damage in our breeding program traditionally has been identified by using a 1 (no damage) to 9 visual rating scale that combines damage levels, damage site on the ear, and the economic consequences of ECB feeding for the processing industry. An alternative 1 to 5 scale based solely on a visual percentage assessment of ear feeding damage was developed and compared to the traditional scale. Seven entries, including moderately resistant and susceptible hybrids and inbred lines of the ECB ear resistance breeding program, were evaluated with both scales in 1994 and 1995 at two locations. Inbred MN3002, Hybrid MN3004, `Apache', and `More' had lower mean damage ratings (3.4, 3.4, 3.6, 3.8, traditional vs. 2.4, 2.2, 2.2, 2.3, alternative, respectively) than `Jubilee', Inbred W182E, and Inbred MN3003 (5.3, 5.6, 7.3, traditional vs. 3.3, 3.0, 4.2, alternative, respectively). Thus, four entries were classified as moderately resistant (3.0 to 4.0 traditional vs. 2.0 to 3.0 alternative) and three entries were classified as susceptible (>4.0 traditional vs. >3.0 alternative). Individual entry ranks varied by scale, but this did not alter resistance classifications. Although the nine traditional ratings were based on economic consequences and the five alternative ratings were based strictly on feeding levels excluding damage location, both scales effectively identified genotypes historically classified as moderately resistant and susceptible. The value of scales is often questionable for many situations and should be considered prior to evaluation.

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Jing-fen Chen, Paul H. Li, and David W. Davis

Exposure of young pepper plants to chilling temperatures delays the development of terminal flower buds to flowering during post-stress growth. Degree of adverse influence depends on chilling intensity, exposure duration and varietal sensitivity. `Ma Belle' pepper plants were grown in a greenhouse (GH) during winter months on the St. Paul campus, No supplemental lighting was provided. When plants were at the 2- to 3-leaf stage, they were foliar sprayed with mefluidide (Technical grade) at 0, 5, 10 and 15 ppm. One day after treatment, some plants were transferred from GH to a cold room (3° ∼4°C day/night) with 12-h photoperiod. Treatad plants remaining in the GH served as the control. Plants were chilled for 1, 2, 4 and 6 days and then brought back to the GH for post-stress growth and development observation. Treated and untreated plants grown in the GH showed no difference in days to flowering, and reached 50% flowering at about 62 days after treatment. When untreated plants were chilled for 1,2,4 and 6 days, they showed a delay of 8, 18, 30 and 34 days, respectively, to flowering, If not killed, as compared to the control The long delay to flowering was due to the injury of the terminal flower buds. After 4 and 6 days of chilling, most terminal flower buds were killed. However, when plants were treated with mefluidide and subsequently chilled days to flowering were significantly shortened. A difference of 10-12 days was observed between chilled untreated plants and chilled treated plants. Concentrations of 5 to 15 ppm were equally effective in protection against chilling.