The influence of leaf ontogenetic stage and plant reproductive phenology on trichome density and constitutive resistance in six tomato varieties

Abstract

Among other functions, trichomes defend plants against herbivores. Based on resource allocation theory, we hypothesized that foliar trichome density is determined by the balance between the fitness benefits and costs of trichome production, including possible resource allocation trade-offs with other functions, all of which may change with ontogeny and reproductive phenology. To assess the benefits and possible allocation trade-offs of trichome production, we examined the degree to which trichomes confer resistance against herbivores, and tested whether trichome density and resistance change with leaf ontogeny and with fruit production. Using six tomato varieties, we determined trichome density and tested for constitutive resistance of unexpanded and expanded leaves sampled from plants at their vegetative and reproductive stages by means of choice bioassays with a generalist caterpillar. We found a positive association between trichome density and resistance, albeit the relation was saturating rather than simple linear. Unexpanded leaves had greater trichome density and resistance than expanded leaves in vegetative plants. No such ontogenetic pattern was seen in reproductive plants. Trichome density of expanded leaves of reproductive plants did not differ significantly from that of unexpanded leaves of either vegetative or reproductive plants. Lastly, varieties with greater seed production had lower resistance. Such a clear trade-off suggests a high cost of resistance, which may have important implications for crop improvement programs.