The assimilation of inorganic nitrogen by cluster and proteoid roots of Aspalathus linearis (Burm. f.) R. Dahlgren and Protea cynaroides (L.) L. in nutrient-poor ecosystems

Certain plant families have evolved cluster (or proteoid) roots, which facilitate their survival in nutrient-poor ecosystems, specifically related to phosphorus impoverished environments, such as in South Africa, South Western Australia and Chile. Most cluster (or proteoid) rooted studies have focused on their capacity for phosphate acquisition, while in nutrient-poor ecosystems along with phosphate, nitrogen is the most limiting for plant growth. The role of cluster (or proteoid) roots in nitrogen nutrition is poorly understood. Therefore, in a field based experiments two cluster/proteoid rooted species, Protea cynaroides (L.) L. and Aspalathus linearis (Burm. f.) R. Dahlgren, the cluster/proteoid root capacity for inorganic nitrogen assimilation and organic nitrogen recycling utilising was assessed utilising an enzymatic approach. It was shown that cluster/proteoid roots are able to assimilate both NH₄⁺ and NO₃^- through the enzyme activities of Glutamine synthase (GS) (EC 6.3.1.2) and Nitrate reductase (NR) (EC 1.7.1.1). Additionally, cluster/proteoid roots were also able to recycle amino acids into other useable forms. The assimilation and recycling of inorganic - and organic nitrogen by cluster/proteoid roots along with their capacity for phosphorus mobilisation, provides insight into how cluster/proteoid roots form part of a larger system in which belowground organs are integrated to acquire scarce resources.