| Common beans (Phaseolus vulgaris L.) are a nutritious food that provides quality protein, dietary fiber, iron, and zinc, and it is an important crop in many parts of the world, especially Central America, East Africa, and South America. Drought stress is one of the greatest limits to common bean production, for not only is drought common in areas that rely the most on beans, but many common bean cultivars in use are also sensitive to drought stress. Furthermore, under field conditions, heat stress often coincides with and exacerbates the effects of drought stress in beans. As a result, one of the major goals of bean breeding efforts is to improve drought and heat tolerance within available germplasm. To support these efforts, the research described in this dissertation examined the physiology of drought and heat stress in a selection of bean genotypes with varying degrees of stress tolerance. These genotypes included tepary bean (Phaseolus acutifolius A. Gray), a particularly stress tolerant species closely related to common bean. The response of different metabolites to drought stress was a major focus. Beans exposed to drought stress had no differences in free proline concentration in their leaves, either between treatments or among genotypes. For soluble carbohydrates, no differences among genotypes were found under control conditions, but the concentration of malic acid, glucose, fructose, inositol, and raffinose all increased in the leaf tissues of plants exposed to drought stress. Glucose, fructose, and inositol were all found in higher concentrations in more tolerant genotypes, so it is likely that their accumulation is correlated with drought tolerance. These compounds accumulated in sufficient quantities to osmotically adjust bean leaf tissues, and those genotypes that accumulated more soluble carbohydrates under drought stress also had lower leaf water potentials while no differences among genotypes existed for leaf water potentials under control conditions. Abscisic acid was responsive to drought stress in beans, but differences in its concentration among genotypes did not seem directly related to drought tolerance. Grafting experiments revealed that it is shoot identity that controls the concentration of ABA in root tissues under drought stress. Drought stress also affects a number of photosynthesis related traits in beans. Photosynthesis vs. intercellular CO2 concentration curves revealed that none of the photosynthetic parameters derived were related to drought tolerance, but the maximum carboxylation rate of rubisco and the rate of electron transport could be related to general productivity. Based on measurements of gas exchange on control and drought stressed beans, lower stomatal conductances are associated with drought tolerant genotypes regardless of water treatment. Lower stomatal conductances would allow a plant to conserve more water during periods of drought stress. Grafting experiments showed that stomatal conductance is controlled mainly by factors located in the shoot tissue and not the root tissue. However, these factors are unrelated to leaf density or the density of stomata on leaf surfaces. Bean plants exposed to temperatures of 45 °C for two days showed measurable signs of heat stress. Measures of gas exchange, chlorophyll fluorescence, and oxidative stress were for the most part only affected by this high temperature and not by any temperatures below 45 °C. These measures also correlated well with visual signs of damage on leaf tissue caused by heat stress. The method was useful for screening a large group of germplasm for heat tolerance, but this heat tolerance only partially related to drought tolerance observed in the field. Plant breeders can utilize some of the methods described in this dissertation to supplement field data and further characterize the stress tolerance of later generation bean lines. |
