Metabolism and exudation of canavanine during development of alfalfa (Medicago sativa L. cv. verko)

Abstract

The structural analog of amino acidl-arginine,l-canavanine (2-amino-4-guanidinooxybutyric acid), is found in 26 cultivars of alfalfa. Its concentration ranges from 6 to 16 mg/g of dry seeds. Canavanine represented more than 70% of the total soluble nitrogen in seeds. Practically all of the canavanine was stored in the cotyledons. Comparison is made of the canavanine content in the cultivars Verko and Europa harvested in different years. During sprouting, 29% of the guanidinooxy compound was translocated into the hypocotyl and radicle in 24 hr. In this early stage of seedling development, the level of the nonprotein amino acid, canavanine, increased threefold whereas the protein amino acid, arginine, as well as asparagine increased 11- and 35-fold, respectively. Two-day-old seedlings are capable of synthesizing canavanine derived from canaline up to 25%. Contrary to this finding in seedlings grown in the time range of 24 days, the guanidino compounds canavanine and arginine were metabolized rapidly, whereas asparagine increased. Furthermore, the toxic canavanine got into the environment of swelled seeds or into the rhizosphere of young seedlings and increased in the milieu to concentrations at 3–57μM. In a biotest, this inhibited the growth of a tomato cell suspension culture as well as the growth of cabbage radicle.