Natural populations of Datura wrightii in southern California are dimorphic for trichome type. Some plants within populations produce greater than 85% glandular trichomes, whereas other produce mainly nonglandular trichomes. Glandular trichome exudates in D. wrightii consist of glucose esterified with straight chain C6–C9 acids. These exudates, and similar exudates in other species, confer resistance to several insect herbivore species. We tested the hypothesis that water was limiting sugar ester production and examined the extent to which trichome density was determined by environmental factors by measuring the concentrations of sugar esters and the densities of trichomes on leaves of plants grown under different irrigation treatments. Water did not limit sugar ester production, as unwatered plants produced 36% more millimoles of glucose esters per square centimeter of leaf surface than did watered plants. Although the addition of water increased leaf size, densities of both nonglandular and glandular trichomes did not change with leaf length or area, suggesting that plants having larger leaves initiated more trichomes in order to maintain nearly constant densities. Millimoles of sugar esters produced did not correlate with densities of glandular trichomes, suggesting that other factors in addition to glandular trichome number govern the production of sugar esters for plant defense.
