Elevation Differentiation in Espeletia Schultzii ( Compositae ), A Giant Rosette Plant of the Venezuelan Paramos

  • Baruch Z
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Carbon dioxide exchanges, plant-water relations, and some morpho-anatomical characters were measured and compared between 3 elevational populations of Espeletia schultzii in the Venezuelan Andes. The paramo climate is characterized by large oscillations in temperature on a daily basis. Precipitation is strongly seasonal with rainy summers and dry winters. The elevational gradient chosen in the Sierra Nevada de Merida extends from 3450 to 4200 m and is characterized by an increase of solar radiation, wind velocity, evaporative forces, and a decrease of temperature and water availability from low to high elevation. Contrasting with temperature alpine areas, the growing season in the paramos extends throughout the entire year. Thus, the maintenance of adequate leaf temperatures on a daily basis and an adequate moisture balance on a seasonal basis are more critical than accumulating resources during a shortened growing season. Morpho-anatomical characters of E. schultzii such as thick leaves, dense pubescence, and water-storing parenchyma play important roles in regulating water loss by increasing water vapor diffusion resistance and buffering the periodic water stress during the dry season. The caulescent life form of the plant elevates the rosette of leaves above the soil surface avoiding extreme ground temperatures. Pubescence and thickness of the leaves may help to control leaf temperature by increasing albedo and heat capacity. The importance of such characteristics is evidenced by their increased prominence in plants of higher elevations. Photosynthetic rates, obtained in the field, using ^1^4CO"2 on mature leaves, were relatively low (2-4 mg Co"2@?dm^-^2@?h^-^1) but correlate well with the slow growth rates and generation times reported. These low rates are not so disadvantageous in the paramos due to the extended growing season. Pubescence and density of the leaves probably have negative effects on photosynthesis by decreasing the photosynthetic active radiation (PhAR) reaching the chloroplasts and increasing resistance to CO"2 diffusion. Controlled conditions in the laboratory produced different responses of the populations regarding photosynthesis. Under cold acclimation, high elevation populations showed highest rates. Under warm acclimation, low elevation populations showed highest rates. Also, different degrees of homeostasis were found among the populations which were correlated with their elevation of origin. The populations of E. schultzii are further differentiated by the degree of drought resistance; the higher elevation population being the most resistant. Among the populations, differences in density-thickness and leaf pubescence are maintained even under uniform laboratory conditions. The maintenance of differences among the populations of E. schultzi even under uniform conditions indicates that they represent genetically based ecotypes.

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  • Zdravko Baruch

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