Developmental plasticity in birch leaves: defoliation causes a shift from glandular to nonglandular trichomes

The structures on leaf surfaces, e.g. trichomes, can act as effective antiherbivory mechanisms as chemical repellents. Structural defences usually represent constitutive resistance, but there are also a few cases of inducible morphological defences. We tested whether defoliation may induce changes in trichome production in white birch (Betula pubescens). The studied birches were either 0, 50 or 100% defoliated during the previous or current summer, and we measured the alterations in the production of glandular vs. nonglandular leaf trichomes, developmental instability (fluctuating asymmetry, FA) and leaf and shoot growth. We detected a clear shift from glandular to nonglandular leaf trichomes following previous-year defoliation but not after current-year defoliation. Furthermore, the density of nonglandular trichomes around the mid-vein of leaves increased following previous-year defoliation but decreased after current-year defoliation. While leaf and shoot growth showed a distinct decrease in response to defoliation, FA turned out to be less sensitive. Consequently, previous-year defoliation can induce the production of nonglandular trichomes in birch leaves. Because this response was accompanied by a reduction in glandular trichomes, the present results may suggest a trade-off between the different trichome types of birch leaves.