ASHS 2025 Conference

Macadamia is a commercially valuable nut crop, but the global industry is predominantly based on self-infertile cultivars. Pollinator scarcity can result in reduced nut set and lower yields. The development of self-fertile cultivars provides an opportunity to produce consistent yield in the absence of pollinisers and pollinators and may enhance orchard profitability. While previous studies identified varying degrees of self-fertility in macadamia, a comprehensive understanding of genetic diversity within a broad range of germplasm and the mechanisms of this trait remain unexplored. This study involved 11 commercial cultivars to identify genetic variation and mechanisms associated with self-fertility. An outcross pollen exclusion method was employed to assess self fertility. The percentage ratio of self- to open-pollinated nuts were calculated as a measure of self-fertility. To investigate the mechanisms underlying self-fertility, spatial separation of male and female reproductive organs as well as site of pollen tube growth inhibition have been examined. Pistil length, stamen length and stigma-anther distance were measured over two flowering seasons. Fluorescence microscopy was used to observe the pollen tube growth after self-pollinations. The self-fertility ranged between 17%-65% for six cultivars (‘HAES 741’, ‘HAES 791’, ‘A38’, ‘UQM40’, ‘K’, ‘Q’), which were identified as self-fertile. Five cultivars (‘HAES 344’, ‘A16’. ‘Daddow’, ‘D4’, ‘A268’) were identified as self-infertile. Pistil length of self-fertile cultivars ranged from 12.5 to 14 mm. Except for ‘A268’, which had significantly shorter pistils (8.5 mm), the pistil length of the self-infertile cultivars ranged from 11.5 mm to 14.5 mm. Stigma-anther distance was not associated with self-fertility. It was found that all cultivars exhibited similar stigma-anther distance, ranging from of 6 to 8 mm. There was non significant difference between the self-fertile and self-infertile groups for morphological floral traits. Pollen tube was inhibited in the upper third of style in self-incompatible pollinations. This inhibition indicated gametophytic self-incompatibility in macadamia. Further study will use a diverse collection of macadamia germplasm, including commercial cultivars, breeding lines and wild accessions to estimate heritability of self-fertility. Additionally, differential expressed genes in the pistils of self-fertile and self-infertile cultivars will be examined to understand the molecular mechanism regulating the trait. This research will assist in breeding of high-yielding, self-fertile cultivars for future plantations, ultimately supporting the sustainability of macadamia industry.