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-1 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-1) 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-1 were measured. The immature first leaf of the commercial species contained the highest concentrations (38-77 μmol g-1). 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.
Macadamia is an important horticultural crop of South Africa, and the major cultivars grown have a wide range of attributes. These cultivars originated from diverse backgrounds, but the genetic relatio'nships between them are unclear. Here we describe new insights into the genetic identity, relationships, and species composition of 38 varieties of macadamia representing the diversity currently available within the macadamia industry of South Africa. The varieties were surveyed with the DNA marker system, RAF (randomly amplified polymorphic DNA). Varieties ranged from pure Macadamia integrifolia, through hybrids of varying species proportions, to pure M. tetraphylla, and fall into at least seven major germplasm groups. Local hybrid selections were genetically distinct from those of other selection o'rigins. The cultivar 791 was unusual, identified as a tri-species hybrid containing a significant proportion of the wild species M. temifolia. It was also shown that 741 u is the true 741 cultivar and 741 s is actually the cul!ivar 800.
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