Ecophysiology of antioxidation and the xanthophyll cycle in plants

The research described in this dissertation examines the ecophysiology of two key defense mechanisms against oxidative stress, i.e. xanthophyll cycle-dependent energy dissipation and the detoxification of reactive oxygen species. I conducted a series of experiments wherein a range of plant species acclimated to environmental stresses in the field, or in the greenhouse, were characterized.; Of the constituent carotenoids, the xanthophyll cycle exhibited the most pronounced positive response to growth light intensity in nineteen plant species acclimated to different irradiances in a subtropical rainforest. High light-acclimated leaves also possessed several-fold higher levels of ascorbate than low light-acclimated leaves.; The response of photosynthetic electron transport and energy dissipation to sunflecks in Alocasia brisbanensis (F. M. Bailey) Domin beneath the canopy of a subtropical rainforest were monitored. Antheraxanthin and zeaxanthin were formed during the first sunfleck and retained throughout the day while the level of energy dissipation appeared to be modulated by the magnitude of the trans-thylakoid membrane proton gradient.; Patterns of acclimation of photosynthesis, energy dissipation, and antioxidation in Cucurbita pepo L. and Vinca major L. acclimated to four irradiances in the field indicated that, like energy dissipation, Mehler-peroxidase pathway capacity (a system of antioxidant enzymes and metabolites that detoxifies superoxide that results from oxygen photoreduction) appeared to respond to the degree of excess light absorption.; Seasonal acclimation of energy dissipation and antioxidation was studied in three populations of Mahonia repens (Lindley) Don growing in the foothills of the Rocky Mountains. In the summer plants acclimated to increased irradiance with increased energy dissipation as well increased Mehler-peroxidase pathway capacity. A strong interaction between the light environment and seasonally colder temperatures complicated the winter-acclimation of these processes.; Limiting nitrogen availability in Spinacia oleracea L. led to a strong and coordinated decrease in photosynthetic capacity, leaf pigment content, and Mehler-peroxidase pathway capacity and resulted in enhanced levels of energy dissipation.; Though enhanced levels of energy dissipation were consistently observed in plants acclimated to environmental stresses, no single response of all photoprotective processes to environmental stress emerged from these studies. I postulate that this is so because stresses such as high irradiance, low temperature, and limiting nitrogen availability can differ in magnitude and the timing of their occurrence, and because environmental stresses perturb whole plant source/sink relationships in different ways.