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Articles

A Comparison of Photosynthetic Characteristics of Encelia Species Possessing Glabrous and Pubescent Leaves ¹

Department of Plant Biology, Carnegie Institution of Washington, and Department of Biological Sciences, Stanford University, Stanford, California 94305

Measurements of the dependence of photosynthesis on light, CO₂, and temperature are reported for two species of Encelia (Compositae) which differ in leaf pubescence and in geographical distribution. Encelia californica is glabrous and occurs in relatively mild, but arid habitats and Encelia farinosa is heavily pubescent and occurs in hot, arid habitats. Both species possess the C₃ photosynthetic pathway. Under high irradiances and normal atmospheric conditions the two species have high photosynthetic rates, exceeding 3 nanomoles of CO₂ per square centimeter per second (48 milligrams of CO₂ per square decimeter per hour) and complete light saturation does not occur by full noon sunlight. The high photosynthetic capacity is related to a high efficiency of utilization of intercellular CO₂ combined with high stomatal conductance. Leaf estimates of total soluble protein and fraction I protein are higher in these species than in most plants, although the proportion of fraction I protein is not higher. Both E. californica and E. farinosa attain a maximum rate of photosynthesis between 25 and 30 C, despite the fact that the two species grow in very different thermal habitats. Neither E. californica nor E. farinosa shows significant acclimation in the temperature dependence of photosynthesis when grown under different temperature regimes. The presence of leaf hairs which reduce leaf absorptance and consequently leaf temperature plays an important part in the ability of E. farinosa to survive in its native high temperature environment. When the effects of pubescence are taken into account, there are few if any significant differences in the photosynthetic characteristics of the two species.

¹ Supported by National Science Foundation Grant BMS 75-03933 and a Carnegie Institution Predoctoral Fellowship to J. E. C. I. W.-D. P. B. Publication No. 614.