Catalyzes the hydrolysis of L-arginine to urea and L-ornithine (Probable). The latter can be utilized in the urea cycle or as a precursor for the synthesis of both polyamines and proline (Probable). Possesses agmatinase activity. Catalyzes the formation of putrescine from agmatine (PubMed:28716421). H2O + L-arginine = L-ornithine + urea agmatine + H2O = putrescine + urea Binds 2 manganese ions per subunit. Nitrogen metabolism; urea cycle; L-ornithine and urea from L-arginine: step 1/1. Amine and polyamine biosynthesis; putrescine biosynthesis via agmatine pathway; putrescine from agmatine: step 1/1. Forms homohexamers. Expressed in vasculature of roots, root tips, cotyledons, leaves, cauline leaves, stems, sepals and pollen. Increased formation of lateral and adventitious roots and increased production of NO in roots. Belongs to the arginase family.