Chilling effects on antioxidant systems of maize (Zea mays L.).

When chilled, plants produce greater amounts of toxic oxygen compounds than they do under non-stressed conditions. These toxic oxygen compounds have the potential to cause severe damage to plants. Plants have evolved antioxidant systems which can scavenge these toxic oxygen compounds and thus allow for the avoidance of their detrimental effects. The purpose of this thesis was to characterize antioxidant capacities of differentially chilling sensitive lines of maize (Zea mays L.) in order to test the hypothesis that the most chilling sensitive lines would have less antioxidant capacity and, hence, less ability to scavenge damaging toxic oxygen compounds, than the more tolerant lines. Three objectives were set to test this hypothesis. The first objective was to select out relatively chilling sensitive and tolerant inbred maize based on their physiological responses to chilling. This first objective was successfully met by subjecting the inbred lines to laboratory chilling tests at the germination stage and early growth stages. Field trials which assessed physiological parameters at both the emergence and early growth phases were then carried out which confirmed laboratory results. The second objective of this thesis was to test if the selected chilling sensitive inbred maize lines had less antioxidant capacities than the tolerant. To this end, activities of the antioxidant enzymes catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (ASPX; EC 1.11.1.11), superoxide dismutase (SOD; EC 1.15.1.1), monodehydroascorbate reductase (MDHAR; EC 1.6.5.4), and glutathione reductase (GR; EC 1.6.4.2), along with concentrations of the antioxidant compounds ascorbate, glutathione, $\beta$-carotene, and $\alpha$-tocopherol and levels of the general metabolic indicators of chilling stress (carbohydrates, chlorophyll, and soluble proteins) were assessed. These parameters were assessed at three developmental stages (first, third and fifth leaf stages) and under control (25$\sp\circ$C) and both short- and long-term chilling ($11\sp\circ$C) regimes. This second objective was successfully met. Significantly lower percent of control activities of the antioxidant enzymes ASPX, MDKAR, and CAT, were observed in the most chilling sensitive inbred line as compared to the tolerant at the first leaf stage of development. The third objective of this thesis was designed to determine if there were similar results for antioxidant capacities and metabolic indicator concentrations for the chilled hybrids as there were for the inbred maize lines. Thus, a complete diallel cross between the above selected inbreds was made, and the resulting hybrids then classified as relatively chilling sensitive or tolerant depending upon how they performed in the laboratory germination and early growth stage screening tests with confirmation in the field. The hybrids were grown until the third leaf stage under short-term chilling (11$\sp\circ$C) and assessed for activities of the antioxidant enzymes and concentrations of antioxidant compounds and metabolic stress indicators. This third objective was successfully met. The chilling sensitive maize hybrids were found to have lower, although not significantly so, percent of control activities of CAT, MDHAR, and ASPX than the chilling tolerant hybrids, paralleling the results of the parental inbreds. (Abstract shortened by UMI.