The trichome dimorphism of Datura wrightii is intriguing because glandular trichome production has a high fitness cost. Plants producing glandular trichomes (‘sticky’ plants) are resistant to many insect herbivores that attack plants producing non-glandular trichomes (‘velvety’ plants). When protected from herbivores, sticky plants initially produce fewer seeds than velvety plants but grow to a larger size. In a three-year field experiment, we tested the hypothesis that sticky plants acquire a competitive advantage through greater vegetative growth. In the absence of herbivores, sticky and velvety plants grew to similar sizes in their first year but sticky plants grew larger in the second and third years. Seed production of sticky plants was 46 – 60% less than velvety plants in their first year and this caused a 13% reduction in their finite rate of increase overall, even though sticky plants produced more seeds than velvety plants in later years. The impact of herbivory varied with plant density and herbivores reduced plant fitness more at low plant density than at high. The differences in growth associated with trichome morphology occurred too late to provide a competitive advantage to sticky plants and probably contribute little to the maintenance of D. wrightii’s trichome dimorphism.

Plant trichomes commonly serve a role in mechanical and chemical defence against herbivores, but may also have the potential to alter physiology by reducing the amount of light absorbed by leaves, lowering temperatures, and reducing water loss. Populations of Datura wrightii Regel in southern California produce 'sticky' plants with glandular trichomes and 'velvety' plants bearing non-glandular trichomes. Because stickiness is inherited as a dominant Mendelian trait, and the proportions of sticky plants vary among populations with the moisture availability of their environment, there may be some ecophysiological differences between trichome types that contribute to their ability to survive in a particular geographic location. To examine the possible physiological significance of trichome variation, we measured the spectral properties, midday gas-exchange rates, and water potentials of D. wrightii leaves from sticky and velvety plants growing in a field experiment. The differences in leaf reflectance (0.9%) and absorptance (1.3%) of photosynthetically active radiation (PAR) between trichome types are too small to have any direct physiologically significant effect. Simulations of leaf temperatures based on the difference in leaf absorptances reveal that leaf temperature would be no more than 1degreesC lower in velvety compared to sticky plants. Gas-exchange measurements revealed no significant difference between types in their transpiration rates or stomatal conductances. In this case, trichome variation may be more important to plant defenses than to physiology.