Roots of lupine plants exude a gel that facilitates water uptake from the deeper, wetter soil layers, while preventing water loss from older roots closer to the surface, a new study reports.
Plants can respond to water shortage by reducing transpiration or by growing deeper roots, but also by modifying the soil. For example, roots exude mucilage, a gel that increases the water holding capacity ("wettability") of the soil.
However, when roots have taken up most available water, the mucilage dries out, making the contact zone between roots and soil ("the rhizosphere") water-repellent – precisely when water is needed most. But Andrea Carminati here shows that this decrease in wettability with increasing root age is actually an adaptive strategy.
Studying the rhizosphere of lupine with neutron radiography (a technique similar to X-rays), he found that only the mucilage around upper, older roots becomes water repellent, while fresh mucilage ensures that the tips of younger, deeper roots always remain wet.
Carminati concludes that the resulting isolation of older roots from the dry top soil increases the flow of water into the youngest roots.