Quantification of cyanogenic glycosides in seedlings of three Macadamia (Proteaceae) species

Members of the genus Macadamia contain cyanogenic compounds, which release hydrogen cyanide (HCN) on hydrolysis. Concentrations of releasable cyanide were measured in tissues of mature nuts and seedlings of Macadamia integrifolia Maiden & Betche, M. tetraphylla L.A.S. Johnson and M. ternifolia F. Muell. Root, cotyledon and leaf samples were assayed at several developmental stages from germination to maturation of the first leaves. All samples contained detectable levels of cyanide. Concentrations were low (0.15 µmol g fresh weight) in cotyledons of mature M. integrifolia and M. tetraphylla seeds, corresponding to the edibility of the seeds of these commercial species, and much higher (9.6 µmol g) in the inedible M. ternifolia seeds. Levels in cotyledons of all three species rose dramatically during germination. Root cyanide concentrations of 6-23 µmol g were measured. The immature first leaf of the commercial species contained the highest concentrations (38-77 µmol g). Levels decreased with leaf maturity, correlating with toughening of the leaf and possibly a consequent diminished requirement for cyanide as a herbivory deterrent. The significance of the results with respect to plant-insect interactions is discussed