Rock-eating plants need special roots

Anna Abrahão, Patricia de Britto Costa, Grazielle S. Teodoro, Hans Lambers, Diego L. Nascimento, Sara A. L. Andrade, Megan H. Ryan, Rafael S. Oliveira

Plants that grow on rock outcrops in a very nutrient-poor ecosystem on mountaintops of Brazil called campos rupestres have incredible roots that allow them to dissolve rock and mine phosphorus. Root penetration into the rocks accelerates soil formation and influences the shape of the landscape and future plant establishment. In this study, we wanted to know if these special roots, or any other root or leaf functions, were essential for plants to grow on rocks. We therefore compared 27 rock- and soil-dwelling species to understand which plant functions and structures were associated with the ability of the species to grow on such a phosphorus-impoverished substrate. We found that these special roots are employed by plants that grow on rocks and are rarely present in soil-dwelling species. These special roots are likely very expensive to build, and are only advantageous where the gains in nutrient acquisition outweigh their costs, as likely occurs when growing on rocks. A similar, but likely less costly, strategy is found in soil-dwelling species, where there is no need to dissolve the substrate to access the phosphorus needed for growth. Nutrient-acquisition from phosphorus-poor soils is usually expected to depend on an association with mycorrhizal fungi, but in the substrates that we investigated, the proportion of roots colonized by mycorrhizal fungi and other fungal root endophytes was negligible. Our study clearly confirms that in extremely phosphorus-impoverished substrates, formation of these symbiotic associations is inefficient, while the root specializations are very effective. The leaf functions we investigated were similar for soil and rock-dwelling species, indicating that it is the roots, and more specifically the type of roots, which are key to determining where each species can grow.

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