Doctoral Dissertations

Date of Award

5-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Botany

Major Professor

Patricia L. Walne, Raymond W. Holton

Committee Members

Karen Hughes, Don Williams

Abstract

Various aspects of the reproductive biology of Cornus florida L. (Cornaceae) have been investigated using light, fluorescence and electron microscopy, controlled cross-pollinations, and field observations. Five floral stages were characterized on the basis of the appearance of the petals, anthers, nectaries, and stigmas. In 1995, 1996, and 1997, fertility among the floral stages following controlled cross-pollinations ranged from 14-90%. Fruit set was highest in newly opened flowers (stage 3) in 19 95 (90%) and 1996 (80%) and in flowers 1-2 days after flower opening (stage 4) in 1997 (79%). The duration of the flower opening period was 10 days in 1996 and 2 3 days in 1997. The mean temperatures of the flower opening periods were 16.9° C and 11.9° C in 1996 and 1997, respectively. The natural fruit set was 6.1% and 2.5% in 1996 and 1997, respectively; the mean of the two years was 4.8%. The fruit-to-flower ratio was 1:21. Viability of pollen was evaluated 1 hr after pollen was placed on a 10% sucrose-1% agar germination medium, pH 6.0, and incubated in a humid, sealed container at 30° C. Pollen germination was 47-51%. The minimum dimensions of the polar axis and the equatorial diameter of dehydrated pollen grains were 47 µm and 2 5 µm, respectively. The bilaterally symmetrical pistil consisted of one stigma, one style, and a bilocular, inferior ovary. The unicellular, club-shaped papillae occupied ca. half of the surface of the stigma and extended into a groove. The style consisted of an uniseriate epidermis, multicellular cortex, numerous vascular traces, and a centrally located stylar canal. The stylar canal was shaped like an inverted, flattened funnel, and was filled with a densely staining, amorphous substance. The canal cells were composed of cylindrical-to-cubical secretory cells with prominent nuclei, small vacuoles, thickened primary cell walls, and numerous organelles. The megagametophyte was deeply embedded in the unitegmic ovule. Floral initiation occurred in July in 1994-1996, and flowers were morphologically mature 6-7 weeks later. Floral development (floral initiation to anthesis) required ca. 9 months, fruit development required ca. 6.5 months, and the entire process from floral initiation to fruit drop required ca. 15.5 months. Thus, portions of the reproductive cycle of one year coincided with portions of the next year’s cycle. Mature pollen grains were found only in anthers >1.0 mm long and in inflorescence buds >5.3 mm wide. Softening and clearing of fixed pistils with NaOH and overnight staining of pistils with aniline blue followed by dilute toluidine blue allowed the removal of obscuring pistil tissues prior to observations of the location of the self-incompatibility reaction with fluorescence microscopy. Self- and cross-pollen adhered to the stigma, germinated, and pollen tubes were visible in the stylar canal 2 days post-pollination. Cross-pollen tubes reached the upper region of the ovary 4 days post-pollination and entered the micropyle 8 days post-pollination. No self-pollen tubes were observed in the distal portion of the stylar canal or in the ovary. These results suggest that the self-incompatible reactions were restricted to the stylar canal and that the self-incompatibility mechanism was gametophytic. By the second day of the inflorescence flower opening period in 1997, the center-most flowers of the third, fifth, and seventh rows were open in 21 of 24 monitored inflorescences. These studies contribute basic information on the reproductive biology of Cornus florida that should aid investigators in future breeding experiments.

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