Investigating morphological and biological reproductive traits in self-fertile and -infertile macadamia cultivars

Self-fertility is a commercially valuable trait in crop species, enabling fruit set without reliance on pollinators or external pollinisers. In macadamia, most cultivars are self-infertile, though some can produce nuts with self-pollen. The mechanisms underlying this variation remain unclear. This study investigates herkogamy, dichogamy and in vivo pollen tube growth to investigate self-fertility. The traits were measured on cultivars from self-fertile and self-infertile groups. Herkogamy was assessed by pistil length (PL), stamen length (SmL), and stigma-anther distance (SAD). Significant interactions were observed between fertility groups and cultivars (nested within groups), with PL ranging from 11.0 to 14.5 mm, SmL from 6.5 to 9.0 mm, and SAD from 6.6 to 7.9 mm. The non-significant differences in SAD, together with the presence of approach herkogamy (stigma positioned above anthers) in both the self-fertile and self-infertile groups, demonstrated that spatial separation does not explain fertility differences. Temporal separation was assessed via pollen viability and stigma receptivity across six floral stages, where all cultivars exhibited protandry (dichogamy). The overlap in male and female reproductive maturity in all cultivars confirmed that temporal differences do not account for self-fertility. Fluorescence microscopy revealed inhibited pollen tube growth within styles of incompatible pollinations, indicated stylar self-incompatibility. Pollen tube progression ranged from 0 to 73.3% pistils where pollen tube reached the lower style, with significant general, specific, and reciprocal effects. We concluded that self-fertility in macadamia is not governed by spatial or temporal reproductive differences, but instead reflects a homomorphic stylar self-incompatibility mechanism, providing a foundation for future molecular and genetic investigations.