Graduate Thesis Or Dissertation


The Cosmetic Stay-Green Trait in Snap Bean and the Event Cascade that Reduces Seed Germination and Emergence Public Deposited

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  • The persistent color (pc) trait in snap bean (Phaseolus vulgaris L.) belongs to a member of the cosmetic stay-green gene family. It is considered very desirable by snap bean breeders for its impacts on pod quality. Persistent color imparts a uniform dark-green color to pods and expresses in seeds as a light green color. Commercially, pc genotypes are reported on approximately 40% of the total snap bean acreage in the USA. However, cultivars from pc lines have a lower field germination and emergence rate compared to white- and colored-seeded genotypes. Therefore, examination of the pc genotypes comparing the other seed genotypes either by anatomical, physical or chemical structure will give insight into why pc types have a lower germination and emergence rates. Soil borne pathogens may also be involved because fungicide treatment helps mitigate germination and emergence problems in the field. The research began with production of seeds in the same environment to obtain uniform samples for the future trials. The seeds involving pc, white-, and colored-seeded lines were grown first in greenhouse and then increased in the field. Then we tested seed germinability in both the lab and the field. In the process of producing the seeds, from greenhouse, we discovered mixtures in lines that allowed us to develop paired genotypes that differed only in their seed type (pc or white). Subsequent analyses were modified to exploit these genotype pairs and allowed us to make inferences about the effects of pc without the complicating factor of genotype. This body of research consists of a series of experiments that include 1. Germination in a laboratory setting without the presence of soil borne pathogens; 2. The field emergence percentage determined by planting a diverse set of cultivars of fungicide treated and nontreated seed grown in a randomized complete block design; 3. Evaluation of a molecular marker for pc in pc lines that have individuals with both white and green cotyledon expression; 4. Comparison of leaf colors among pc and white-seeded genotypes; 5. Evaluation of seed viability by tetrazolium assay; 6. Comparison by isogenic or sister lines for seed water uptake; 7. Evaluation of seed cracking after water-uptake; 8. Determination of different genotypic response to mechanical injuries of seed by the drop test; 9. Verification of seed moisture content; 10. Measurement of electrical conductivity rates of isogenic pairs to quantify solute leakage from seeds; 11. Examination of seed coat thickness by studying anatomical structure and lastly; 12. Comparison of amounts of sugar compounds in different genotypes. The lab germination tests exhibited no significant differences in germination among seed types whereas in the field we observed higher infection rates on seedlings of pc genotypes. Some pc lines had a few individual plants that produced green cotyledons rather than the bleached white cotyledons normally observed for this seed type. In most cases, this effect was transmissible to the progeny. The pc lines that expressed green cotyledon color were verified to be pc by molecular marker analysis. These green cotyledon types are an interesting variant that should be investigated further to determine what role this trait may play in seed physiology. Evaluation of leaf color revealed differences across individual plants with upper leaves lighter than older, more mature leaves, but no consistent differences among genotype pairs were observed. Therefore, leaf color cannot be used to predict whether a line has the pc gene or not. There was more rapid water uptake in seeds of pc lines, and this was associated with higher rates of the imbibitional cracking. All snap bean types showed increases in water uptake after being mechanically injured in the drop test, but seeds of pc bean lines were particularly sensitive to mechanical injury. Even uninjured pc seeds leaked more solutes as shown by the electrical conductivity test. Anatomically, pc seeds exhibited thinner seed coats with all three outer layers of the testa showing reduced thickness, but the osteosclereid layer in particular was significantly thinner. The sugar compounds were not statistically significant different among seed type for soil infection, only fructose was higher in pc seeds, but not raffinose or sucrose. Thus, it appears that in beans the seed coat structure and integrity is important for soil borne pathogen defense. Fungicide treatment is used in commercial production to maintain germination and emergence rates of pc cultivars, but finding ways to reduce seed cracking could be beneficial.
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  • Turkish Government provided support for the graduate assistantship.
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