Shrubby legumes are key species in natural ecosystems worldwide due to their ability to fix atmospheric nitrogen and their high tolerance to drought, salinity and alkalinity. Nitrogen fixation occurs in root nodules by symbiotic rhizobia bacteria, however both nodulation and rhizobial symbioses are influenced by the availability of nutrients and water. A lack of phosphorus in particular can severely reduce root nodulation and in turn impair the growth of the plant. Drought has been found to have a similar effect. Yet it remains unclear how shrubby legumes respond to a scarcity of both water and phosphorus.
In a new study published in AoBP, Míguez-Montero et al. investigated the regulatory effects of phosphorus and nitrogen on nodulation and plant performance of legume shrubs during drought. They studied four species of the genus Cytisus, either native to or commonly represented in the Iberian Peninsula. The results of the study show that these species are able to shift their nitrogen use depending on soil P availability and on the interactions established with their Bradyrhizobum symbionts. In the four species, inoculation with rhizobia clearly enhanced plant nitrogen use efficiency and drought tolerance, whereas phosphorus use efficiency was greater in non-inoculated plants in the irrigated treatment. Differences were found across species in their response to water scarcity, due to differences in their root morphology, and plant ecology. The authors conclude by relating the ability of plants to cope with P and water scarcity to their natural distributions in nature. They highlight that species from environments where both water and nutrients are scarce have to expand their root system to access deep water and nutrients, whilst plants without these limitations do not need to. These modifications are particularly evident in Cytisus multiflora, which is now a serious noxious weed in certain areas, including California and Australia.