Variation in leaf trichomes of Wigandia urens: environmental factors and physiological consequences.

Seasonal and environmentally induced variation in the type and frequency of leaf trichomes of Wigandia urens (Ruiz & Pavón) Kunth (Hydrophyllaceae) was studied. Depending on the microsite, W. urens plants had smooth leaves with glandular trichomes or bristly leaves with both glandular trichomes and urticant trichomes (stinging hairs). Trichome density (number of urticant trichomes per unit leaf area) was higher in the dry season than in the wet season, and was significantly correlated with both temperature (r = 0.353, P < 0.05) and photosynthetic active radiation (r = 0.313, P < 0.05). Plants established in sun-exposed areas had trichome densities three times higher than those of plants established in shaded areas during the dry season, and 28 times higher during the wet season. At both exposed and shaded sites, trichome densities of the youngest leaves of young plants were higher than those of the youngest leaves of mature plants. In smooth and bristly leaves, transpiration rates decreased with increasing temperature during the day. However, smooth leaves had higher transpiration rates than bristly leaves at both exposed and shaded sites. In laboratory studies, trichome density was significantly (P < 0.01) reduced when small sun-grown plants (0.20-0.30 m tall) were either shaded or irrigated. In larger plants, also, irrigation significantly (P < 0.01) reduced trichome density relative to that of unirrigated controls.