Cuphea viscosissima Jacq. is being developed as a commercial source of caprylic, capric, lauric, and myristic acids. Germplasm resources for characterizing the genetic diversity of this species became available following explorations by the United States Department of Agriculture (USDA) in 1986 and 1987. In this paper, we describe the fatty acid and oil percentage diversity of forty‐ two populations ofC. viscosissima collected from seven states within the United States. Caprylic (18.0%) and capric acid (69.9%) were the major fatty acids of these populations. The fatty acid percentage ranges were narrow for every fatty acid,e.g., 16.4 to 20.4% for caprylic acid and 66.6 to 71.3% for capric acid. The maximum lauric acid percentage was 3.4%. Oil ranged from 27.3 to 33.4%. Although the populations surveyed cover a fairly wide geographic range, they display limited fatty acid diversity. Surveys of germplasm from other parts of the range are needed to further characterize the fatty acid diversity of this species.

Differential plant populations occurring among plots in yield trials can be critical in evaluating potential worth of sunflower (Helianthus annuus L.) hybrids or breeding lines. The objective of this study was to determine degree of loss in seed yield when stand is reduced at various growth stages of plant development and to identify the nature of plant compensation for stand loss in sunflower. No significant reductions in seed yield occurred when plant populations were reduced 25% from the check (50,000 plants/ha) at the 4, 8, and 16‐leaf growth stages. Compensation was due to significant increases in head diameter. Stand loss of 25% at later growth stages resulted in significant yield loss. Compensation occurred in seed weight as well as head diameter when stands were reduced 50% at early growth stages. Seed yields were reduced 18 to 23%. Sunflower plants were able to significantly compensate even when 75% stand reduction was applied at bud and flowering stages, with seed weight accounting for most recovery in yield. A 38 to 42% and 56 to 58% loss occurred when stands were reduced 75% at the 4 to 16‐leaf and bud to flowering growth stages, respectively. Oil percentages of seeds from plants grown in plots with high stand reductions were reduced 6% and height was decreased 25 cm. Significant negative correlations were noted between oil percentage and height with yield, head diameter, and seed weight at the lowest plant population. Plant population would be critical to control in studies investigating inheritance of plant height or oil percentage of sunflower.

The objectives of this study were to determine relative effectiveness of inbred versus single‐cross testers in identifying combining ability of inbred sunflower (Helianthus annuus L.) lines; to determine whether epistasis was significant in the genetic systems controlling selected traits; and to evaluate the importance of additive and dominance variance for traits not influenced by epistasis. The hybrids evaluated in this study were formed by crossing the inbred lines RHA 274 and RHA 278, and their single‐cross, RHA 274/RHA 278, to 10 randomly selected female inbred lines. The design used was a variation of the triple testcross, an extension of the Experiment III design.The interaction of female ✕ restorer parent was significant, indicating that the three testers did not rank the female lines identically for yield. However, the three testers did identify three of the top four lines when a 40% selection criterion was applied. For oil percentage, the interaction of female ✕ male parents was nonsignificant indicating that the three testers gave very similar rankings. Utilizing any one of the three testers will differentiate or identify high‐yielding potential female lines for yield and oil percentage in a population improvement or inbred development program.Epistasis was a minor factor in the overall genetic variation for all traits studied in these sunflower lines. Additive variances were most important, with dominance variances important only for yield. The estimate for degree of dominance for yield (0.62) indicated that partial dominance predominated, an observation with significant implications for breeders in countries that primarily use open‐pollinated cultivars. Improvement by development of high‐yielding synthetics or open‐pollinated cultivars should be continued. The results also indicate that breeders should develop population improvement schemes that utilize the large additive portion of genetic variance detected in this study.

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