The Stinging Nettle, Urtica Dioica, Increases Trichome Density after Herbivore and Mechanical Damage

Abstract

It is argued that stinging trichomes are a more reliable measure of defence investment than secondary compounds which often have proposed alternative functions. We hypothesise that grazing pressure should lead to higher stinging trichome densities in grazed than in ungrazed areas as a result of selection for more heavily defended plants and/or because damaged plants regrow with increased trichome densities. Urtica dioica is used as a model species and the hypotheses tested by comparing trichome densities on plants from grazed and ungrazed areas, and before and after cutting in the field and in the greenhouse. Plants from grazed fields have significantly higher trichome densitites than plants from ungrazed fields, but the latter showed more variation in density. Regrowth plants have higher trichome densities than initial growth and shoots from the centre of patches had lower trichome densities than edge shoots more exposed to grazing. In greenhouse experiments regrowth again has higher trichome densities than initial in both high and low nutrient conditions, but when plants are recut densities fall most markedly in low nutrient conditions. The results are discussed in relation to plant defence strategy, induced defence and resource allocation.