Photoprotection During Winter Stress

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9631064 Adams The proposed studies will examine the low temperature acclimation of a photoprotective process facilitating the harmless dissipation of excess excitation energy in plants that retain leaves during the winter. Excess excitation energy can be removed directly within the chlorophyll/carotenoid-binding pigment complexes in a process requiring the carotenoid zeaxanthin and a low pH within the chloroplast membrane. On warm days, under otherwise favorable conditions, zeaxanthin accumulates during midday when light is excessive and is reconverted to its precursor by the end of the day as light levels become nonexcessive. During the winter large amounts of zeaxanthin can be retained throughout cold nights, ready to become engaged in energy dissipation the instant chlorophyll absorbs light in the morning. In the proposed studies the mechanisms underlying the nocturnal retention of this pigment, and its continued engagement for energy dissipation, will be examined. The first mechanism involves the hypothesized maintenance of a pH gradient across the photosynthetic membrane of leaves exposed to winter conditions which results in the sustained engagement of zeaxanthin in energy dissipation, primarily in the minor, proximal chlorophyll-binding proteins. The second mechanism to be tested involves the hypothesized synthesis of zeaxanthin from beta-carotene in the core antenna proteins that may accompany acclimation to the conditions of excess light during the winter. To test this hypothesis the carotenoid composition of specific chloroplast proteins will be determined from leaves acclimated to favorable temperatures in the summer as well as low temperatures in the winter. Ultimately the information gleaned through these studies may prove useful in improving the tolerance of agricultural species planted in areas th at experience lower temperatures than those experienced by the plants during their evolutionary history.