The shrub Elaeagnus umbellata is a drought-resistant invasive species, with multiple adaptive mechanisms which contribute to development of extensive, dense-canopied thickets. E. umbellata readily colonizes open spaces and edge habitat, but is also successful within forests, and research suggests that it possesses physiological and morphological features typically associated with shade-adapted species. Field and laboratory photosynthesis, chlorophyll fluorescence, and hyperspectral reflectance measurements were used to evaluate diurnal changes of E. umbellata and to quantify physiological responses of plants through periods of stress and recovery. Leaf trichome structure and density also were quantified for both sun and shade leaves, and evaluated as mechanisms of improved light capture and photosynthetic carbon gain in shaded environments as well as during drought conditions.
Daily measurements of laboratory plants and monthly field measurements revealed decreasing ΔF/ F′m values in response to drought stress, with little corresponding decrease in photochemical reflectance index values. This disparity indicates that xanthophyll cycle dissipation is not the dominant photoprotective mechanism at work for Elaeagnus species experiencing water stress. Field studies revealed an increasing chlorophyll a/b ratio with onset of water stress, a possible result of chlorophyll protein re-absorption in response to drought conditions. Trichomes were present on both leaf surfaces; abaxial surface trichome density was at least five times higher than adaxial density. The relatively sparse adaxial trichomes appear to reduce the intensity of incoming light, while densely-packed abaxial trichomes reflect light back towards the mesophyll, enhancing absorption in low light environments. These characteristics appear to enhance the invasive success of E. umbellata, by allowing the shrub to maintain photosynthetic carbon assimilation under drought conditions, and by enabling its expansion from edge habitat into forest understory.
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