'Kill them with your hogweed hairs' sang Peter Gabriel in 1971 in the Genesis song 'The Return of the Giant Hogweed'. It remains doubtful whether Genesis were trying to emphasize the meaning of trichomes in H. mantegazzianum. Nevertheless, the plant's hairs do make a significant contribution to its defence. Root hairs are the sole exception. They are not involved in plant protection and will not be discussed here. Ochsmann (1996) investigated the morphology of the above-ground trichomes in H. mantegazzianum in detail. He described five different kinds of trichomes: simple hairs, hairs on pedestals, hairs with globular-shaped tops, triangular-shaped hairs and spines. All types have in common that they are unicellular and non-glandular trichomes. The simple hairs are usually shorter than 0.5 mm. They occur predominantly along the veins on the lower surface of the leaves (see also Hansen et al., Chapter 11, this volume). The
second hair type (Fig. 13.3A) comprises a unicellular hair located on a reddish pedestal-like emergence. They are not trichomes in the strict sense, because the pedestal is probably derived from epidermal and subepidermal cell layers. However, to avoid confusion we will also use the term trichome in this case. This type is by far the longest, with a length of up to 5 mm. Their occurrence is restricted to the leaf petioles and flower stems. They can exhibit remarkable densities, especially on young petioles (Fig. 13.4). Their sticky surface impedes the movement of herbivores and may trap small species. The hairs with globular-shaped tops (Fig. 13.3C) emerge on the pedicles of the inflorescences and on the ovaries. They are approximately 1 mm long and possess a small conus at the end of their head. The hairs on the ovaries degenerate during fruit development. Small triangular-shaped hairs (Fig. 13.3B) occur on the pedicles. Spines (Fig. 13.3D) are restricted to the fruit margins in H. mantegazzianum.
Trichomes may complement the chemical defence of a plant by secreting or storing toxic and repellent compounds. Despite extensive work on fura-nocoumarins, surprisingly few studies have examined them in combination with trichomes. In the only study we are aware of, Zobel and March (1993) observed furanocoumarins on the surface of trichomes on the seeds of Daucus carota L.
The shape of the trichomes with globular tops resembles glandular trichomes or the stinging hairs in other plant species. Therefore they might contain secondary defence compounds. On the other hand, the sizeable hairs on pedestals are situated on the plant organs with the lowest furanocoumarin contents.
Besides defence, trichomes may serve other functions as well. For instance, they can protect from UV radiation (via reflection, absorption or shading), reduce water loss (via an increased water diffusion pathway or by trapping moisture), or alter the impact of wind (Press, 1999). However, these attributes are usually related to leaf pubescence. Given that plant hairs in H. mantegazzianum are absent from the upper side of leaves and are not located near stomata on the lower leaf surface, it is unlikely that they contribute significantly to the plant's energy or water budget. In addition, protection against UV light would simultaneously reduce the effectiveness of the phototoxic chemical defence compounds. The spines on the fruits are an exception. They are presumably involved in the dispersal of seed via biotic vectors (see Moravcova et al., Chapter 5, this volume).
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