Do phytoliths play an antiherbivory role in southwest Asian Asteraceae species and to what extent?

Phytoliths (silica bodies) occur in Poaceae species in large numbers and have been shown to have antiherbivory roles. However, phytoliths occur also in many other taxa in much smaller numbers, which raises the question of the extent of both their potential and actual antiherbivory role in these taxa. In order to address the question of their potential antiherbivory role, we sampled 20 wild-growing southwest Asian species of the family Asteraceae, species of which have a much lower phytolith concentrations than Poaceae taxa. We studied the potential positive effect of grazing on phytolith formation and the possible tendency of plants to have higher concentrations of such defence structures in their reproductive organs. We sampled plants from populations of 12 non-spiny and eight spiny species growing in un-grazed and grazed plots in seven sites along a large rainfall gradient (80–900 mm mean annual) in Israel, a region known for its long and intensive grazing history. The study included 21 pairs of un-grazed and grazed plants from 16 of these 20 species. In addition, ten populations of eight species were sampled in order to examine whether phytolith concentrations in the reproductive organs are higher than in vegetative organs. We did not find consistently higher phytolith concentrations in grazed plants compared to un-grazed plants of the same species and habitat (15 species), and in 12 out of 21 pairs of un-grazed and grazed plants (from 15 species) we even found higher phytolith concentrations in un-grazed plants, a phenomenon which was more common in the more arid sites. Phytolith concentrations in inflorescences are commonly (6 out of the 8 species) lower than in the rest of the shoot. We conclude that the antiherbivory potential of phytoliths in the southwest Asian Asteraceae as a group is much smaller than in the Poaceae.