Abstract
Forest-grown American ginseng (Panax quinquefolius L.) is exposed to daily and seasonal light variations. Our goal was to determine the effect of understory light changes on the maximum quantum yield of photosystem II, expressed as <TEX>$F_v/F_m$</TEX>, and photosynthetic pigment composition of two-year-old plants. Understory light photon flux density and sunfleck durations were characterized using hemispherical canopy photography. Our results showed that understory light significantly affected the <TEX>$F_v/F_m$</TEX> of American ginseng, especially during the initial development of the plants when light levels were the highest, averaging 28 mol <TEX>$m^{-2}d^{-1}$</TEX>. Associated with low <TEX>$F_v/F_m$</TEX> during its initial development, American ginseng had the lowest levels of epoxidation state of the xanthophyll cycle of the season, suggesting an active dissipation of excess light energy absorbed by the chlorophyll pigments. As photon flux density decreased after the deployment of the forest canopy to less than 10 mol <TEX>$m^{-2}d^{-1}$</TEX>, chlorophyll a/b decreased suggesting a greater investment in light harvesting pigments to reaction centers in order to absorb the fleeting light energy.
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