Photosynthesis by flowers in Encelia farinosa and Encelia californica (Asteraceae).

Photosynthetic rates by flowers of the shrubs Encelia farinosa and E. californica were studied during three phenological stages of flower development. Both gross photosynthesis and dark respiration rates in the flowers were of similar magnitude and decreased with floral development. Floral photosynthetic rates were saturated by an irradiance equivalent to one half full noon sunlight. Net photosynthesis of flowers was rarely positive and decreased substantially with increased flower temperature. Gas exchange by the flower was unaffected by the water vapor pressure gradient. These results are analyzed in terms of the microclimate experienced by the flowers and the relative contribution of floral photosynthesis to the economy of the flower.