Some factors that affect pollination and seed formation in alfalfa, Medicago sativa L Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/jd4730300

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  • The objectives of this study were (a) to measure the amount of cross-pollination caused by three bee species at 10, 20 and 40 rods distance from a foreign pollen source, (b) to measure the effects of bee species, planting pattern and season on broad sense heritability estimates for seed yield in alfalfa, (c) to determine the incidence of selfing caused by the bees, (d) to investigate the usefulness of bees as a breeding tool in alfalfa and (e) to determine which of the three planting patterns used in this investigation is the best in effecting the maximum amount of cross-pollination. Three bee species, the honey bee, Apis mellifera L.; the leaf-cutter bee, Megachile rotundata F. and the alkali bee, Nomia melanderi Ckll. were chosen for this investigation because of their importance as pollinators in alfalfa. Studies under conditions of good isolation for bee species and competitive bloom were necessary to obtain reliable results. In the center of a three mile square isolated area, seven plots of recessive white-flowered clonally established plants were planted 10, 20, 40, 80, 160, 240 and 320 rods north of a colored-flowered population, and seven plots were planted at the same distance east of the colored-flowered population. Seven planting patterns were used in the east series. Three of these were utilized by the bees described. The number of florets tripped, pods developed and seeds developed in each pod were recorded for each raceme visited by pollen collecting bees. Seed samples from the white-flowered clones were grown in a greenhouse to ascertain whether the seed resulted from cross- or self-pollination. A record of flower color in 1963 demonstrated that at 10, 20 and 40 rods from a contaminant source honey bees caused 15.7, 11.2 and 5.8 percent cross-pollination; leaf-cutter bees caused 13.1, 4.8 and 8.1 percent cross-pollination while endemic pollinators caused 42.9, 30.9 and 10.0 percent cross-pollination in 1962. An important fact in this regard was that endemic pollinators caused 6.52 percent cross-pollination at a distance of a mile. Bee species, planting pattern and season caused large fluctuations in broad sense heritability estimates for seed yield in alfalfa. The ratio environmental variance to phenotypic variance gave a good indication which environmental factor caused the most fluctuation in the estimate. When endemic pollinators were used 94.7, 88.3, 36.3 and 23.1 percent selfed seeds were recorded as occurring in pods with one, two, three and four or more seeds per pod. One seven seeded pod was recorded as possessing three selfed seeds. Data from leaf-cutter bees showed that all of the pod types had over 67 percent selfed seeds. Distance from the contaminant source affected the percentage selfed seeds. At 40 rods only the four or more seeded pods contained any crossed seeds (20 percent) Leaf-cutter bees trip 27-50 percent more florets per raceme and cause less cross-pollination at 10, 20 and 40 rods than honey bees. Honey bees cause approximately the same amount of cross-pollination at 10 and 20 rods while the value obtained at 20 rods for the leaf-cutter bee was close to one-third of the value at 10 rods. Leaf-cutter bees also showed less preference between colored- and white-flowered alfalfa flowers. Of several planting patterns considered, maximum cross-pollination was obtained in a plot planted with alternating rows of alfalfa.
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