PHOTOSYNTHETIC LIGHT-HARVESTING DURING LEAF SENESCENCE IN PANICUM-MILIACEUM

During plant senescence, carotenoid pigments are lost from the leaves slower than chlorophyll pigments. To determine if the decreasing chlorophyll to carotenoid content ratio leads to a relative increase in photosynthetic light-harvesting by carotenoids, the excitation spectrum of chlorophyll fluorescence at 25 degrees C in thylakoid preparations from prose millet (Panicum miliaceum L. var Minco) was measured. During the time-course of either drought-accelerated senescence of intact plants or induced senescence of excised leaves, the observed changes in the shape of the excitation spectrum were usually small (< 15%) and mostly reflective of changes in the chlorophyll a to chlorophyll b ratio. A small increase in excitation around 535 nm occurred during drought-accelerated senescence. The near constancy of the shape of the excitation spectrum shows that the relative contribution by carotenoid pigments to photosynthetic light-harvesting does not increase significantly during natural senescence and suggests that loss of light-harvesting capacity occurs in a coordinated manner.