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Impenetrable sand armour provides plants with an edge up in survival

No one likes getting covered in sand; it's coarse, irritating, and gets everywhere (including your shorts). But for plants, having sandy shorts could mean the difference between life and death.

Sometimes, desert life can suck. Organisms from all walks of life must survive through intense heat, scarce water, and a lack of resources. On top of that, plants must also spend energy defending themselves against attack - being one of the few tasty green things in these habitats can put a target on one's back. However, these plants have a secret for survival - sand.

Covering oneself in sand, termed psammophory, is surprisingly common in plants. This "sand armour" is practiced by over 200 plant species in hot, arid dune communities and is observed in areas as far as the Namib and Middle Eastern deserts. Plants entrap sand on their stems, petioles, and leaf surfaces by either excreting a sticky substance or growing dense hair-like trichomes, which are outgrowths of the epithelium. Sand is scattered over the plants via wind or substrate disturbances and sticks to the excretions/trichomes. Imagine scattering rainbow sprinkles over a decadently iced chocolate cake, except the sprinkles are not something you actually want to eat.

Sand armour is hypothesized to provide a myriad of benefits to a plant, such as protection from further abrasion during sandstorms, reducing water loss and exposure to solar radiation. Sand is also thought to deter herbivores by reducing palatability. But is psammophory really an effective herbivore defence?

Dr. Eric LoPresti and Dr. Richard Karban (2016) of the University of California decided to get the dirt on this plant adaptation. Using plants from our very own backyard (or at least closer to our backyard than the Namib desert), Dr. LoPresti and Dr. Karban experimentally tested whether psammophory acted as an anti-herbivore defence strategy. They accomplished this through sand addition and removal manipulation on two plant species, Abronia latifolia (Nyctaginaceae) and Navarettia mellita (Polemoniaceae), and quantified the resulting herbivore damage.

A. latifolia, also known as sand verbena, is a common dune plant along the California coast. It's short, glandular trichomes facilitate sand entrapment over most of its surface, except for the tops of the leaves, which are only sparsely sandy. A. latifolia is typically grazed on by snails, deer mice, and leaf-mining caterpillars. The second plant tested, N. mellita, is a weedy species endemic to California and can be seen in all its sand-covered glory along roadsides, ditches, and eroded areas. It has glandular inflorescences, or flower bunches, that are eaten by small mammals, such as black-tailed jackrabbits.

Experimental sand removal performed on A. latifolia showed that sand presence significantly deterred herbivory. Conversely, sand reduction roughly doubled the amount of chewing herbivory. Experimental addition of substrate to N. mellita showed similar results, as sand-covered inflorescences had significantly less herbivore damage and a higher degree of inflorescence maturity. Overall, sand-covered tissue was less preferred by the majority of externally feeding mammalian, gastropod, and arthropod herbivores.

In a follow-up study, Dr. LoPresti and colleagues (2018) examined how sand armour affected herbivores' preference and performance, namely the white-lined sphynx caterpillar (Hyles lineata) (Sphingidae). Sphynx caterpillars feed on the entire leaf, starting from the outside in, which results in a large amount of sand ingestion. The impenetrable sand armour effectively protected plants against the sphynx caterpillar; furthermore, the sand continued to mess them up for the rest of their little buggy lives. As a consequence of increased sand ingestion, sphynx caterpillars had extensive wear on their mandibles, lower pupal weight, longer development time, and slower growth rate. These caterpillars also avoided sand-covered plants if given a choice.

While humans may detest getting covered in sand, it's annoying properties can serve as a valuable physical defence to dune-dwelling plants.

Did you know? Seeds can have a mucilaginous coating which becomes sticky when wet, enabling the attachment of sand and dirt. This sand/dirt coating remains after the seeds are dry and can prevent granivory by ants. (LoPresti et al., 2019)


. Impenetrable sand armour Reference

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