An investigation of sexual reproduction in Picea (Pinaceae) including prezygotic stages, fertilization, and proembryo formation

Date

2017-05-31

Authors

Runions, Clifton John

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Abstract

Sexual reproduction of interior spruce was investigated. In a preliminary study, seeds were produced from 50.5±20.0% of ovules in the most productive seed orchard trees. Seed production in most trees was far lower. Development between the time of pollination and early embryo formation was studied and reasons for low seed set are described. Low levels of pollination, as judged by dissection of ovules, was a problem in the seed orchard so the pollination mechanism was studied. The seed orchard is in a hot, dry location and there was a suspicion that pollination drops might not be produced under these conditions. Pollination drops were secreted very late by ovules as seed cones began to close at the end of the receptivity period. Micropylar arms withered and no longer functioned in pollen capture by the time pollination drops were secreted. Pollen move into inverted ovules by floating upwards within the pollination drop. Sacci, the 'wings' of conifer pollen, function as floatation devices in the pollination mechanism. Pollination drops are large and nearly fill the space that the micropyle occupies within the closed seed cone. Pollen adhering to the micropylar arms float within the pollination drop into the micropyle but, as well, pollen adhering to the cone axis near the micropylar arms are scavenged by the large drop. Rain may function in the pollination mechanism of interior spruce and an evolutionary scenario for the pollination drop mechanism is proposed. Picea orientalis has saccate pollen that sink into upright ovules. This is different from all other spruce so saccate morphology was investigated. TEM revealed that the exine layer of the sacci is porous when compared to interior spruce. Hydration of the pollen body forces air out of the sacci as observed by confocal microscopy. Ovule orientation and pollen floatation are described as correlated characters. An anatomical study was made of ovule development from pollination through fertilization and early embryo formation. Many of the observations confirm reports in the literature but examination of resin embedded specimens has allowed for a more detailed description of development. Many abnormalities which prevent fertilization were observed. Because prezygotic failure other than lack of pollen is not generally considered to be a constraint in conifer seed production, abnormal developments are described. Abnormal ventral canal cells and nucelli account for most of the observed prezygotic seed losses. Losses due to lack of pollen, and abnormal development were estimated at 15% each. Self-pollination results in low seed set in conifers so prezygotic development in self-pollinated ovules is described. Two types of prezygotic abnormality which prevent fertilization and that occur only after self-pollination were described and the possibility that some type of self-incompatibility mechanism might occur in conifers is discussed. Failure at prezygotic stages is common in interior spruce but it is generally only possible to observe these stages by doing careful study of the fertilization period. When seeds without developing embryos are examined after the time of fertilization it is usually impossible to tell if failure was pre-, or postzygotic. Seed losses in conifers are usually described to result from genetic load. The degree of prezygotic failure observed in this study suggests that conifers may not have as many embryonic lethal recessive alleles as estimated. Recommendations for increasing seed production in the seed orchard through effective use of supplemental pollen and for further studies of a botanical nature are made.

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Rhododendrons

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