130 — Chewing sandpaper: grit, plant apparency, and plant defense in sand-entrapping plants

If you look closely at a desert plant, you will often find a coating of sand particles that have stuck to the stem and leaves as a result of sticky saps or exudates on the surface. This paper explores two rationales for the presence of sand: Either the plants are releasing this exudate and sand sticks to it almost as a coincidence, or the plants have evolved this sand-sticking trait as a defense against herbivores.

If indeed this is a defense mechanism, then LoPresti and Karban propose two mechanisms: The sand either acts to induce “resistance” — it is less convenient or pleasant for an herbivore to consume — or “crypsis” — the plant is less visible to herbivores due to the similar coloration of foreground and background. Or, of course, these two mechanisms act in parallel.

To test these hypotheses, the researcheres placed caterpillars in three vessels; one with uncoated plants, another with sand the same color as the background, and a third with sand that contrasted with the background.

They found that caterpillars favored the uncoated plants — confirming the “resistance” hypothesis — and did not favor either sand color over the other — rejecting the “crypsis” hypothesis.

This means that plants use sand as a defense mechanism against herbivory, at the expense of (slight) reductions in photosynthetic exposure to sunlight. It’s also possible, the authors suggest, that this presence of sand reduces the abrasive effects of other sand, blowing in the wind (although they note that the natural habitats of the plants in question rarely experience sandstorms).