Trichomes – the small, hair-like structures on plant leaves and stems – are more common and more dense on the leaves of plants in arid or sun-exposed environments. This is thought to be due to trichomes’ ability to reflect sunlight and reduce water loss. However, trichomes can also act as a deterrent to herbivores and egg-laying insects. This latter function may indirectly play a role in decreasing water stress as well: certain insects cause galls to form on the leaf, increasing the surface area of the leaves and potentially increasing evaporative water loss. So trichomes that block gall-forming insects may be doubly adaptive under arid conditions.
In a recent paper published in Annals of Botany, Gaku Amada and colleagues tested this possibility by quantifying the relationship between gall formation and water loss in the leaves of Metrosideros polymorpha. This endemic Hawaiian tree occurs along a range of habitat conditions and varies greatly in both the length and density of trichomes on the lower leaf surface. The tree is parasitized by several species of psyllids that cause three distinct types of gall – cone (large), flat (medium), and pit (small).
The researchers found that water loss from the leaf increased sharply as the number of cone and flat galls increased, indicating that these larger gall types do indeed cause water stress. The smaller pit galls had no such effect. Field survey results showing a negative relationship between trichomes and the larger galls suggest that leaf trichomes in M. polymorpha may impede the insects creating cone and flat galls, but not the makers of pit galls. This can be explained by the location of the trichomes, which are only found on the lower surface of the leaf. Cone and flat gall-forming insects attack the leaf from the bottom, while pit gall insects attack from the upper surface.
These results should be considered preliminary, the authors note, because “the thickness of the trichome layer often seems to be too great for the ovipositor lengths of psyllids (up to several-fold). That leaves with the largest amount of trichomes are common in the alpine zone, where psyllids are not so abundant, also suggests that other selective forces are involved in the adaptive development of leaf trichomes.” The results of the study may be helpful, however, in understanding the coevolution that has taken place between this tree and the various species of psyllids that depend on it.