Bruce Topp

Japanese plums (Prunus salicina Lindl.) and phytochemicals – breeding, horticultural practice, postharvest storage, processing and bioactivity

Previous reviews of plum phytochemical content and health benefits have concentrated on the European plum, Prunus domestica L. However, the potential bioactivity of red- and dark red-fleshed Japanese plums, Prunus salicina Lindl., so-called blood plums, appears to warrant a significant increase in exposure, as indicated in a recent review of the whole Prunus genus. Furthermore, Japanese plums are the predominant plum produced on an international basis. In this review the nutrient and phytochemical content, breeding, horticultural practice, postharvest treatment and processing as well as bioactivity (emphasising in vivo studies) of Japanese plum are considered, with a focus on the anthocyanin content that distinguishes the blood plums.

Prospects for increasing yield in macadamia using component traits and genomics

Selection of candidate cultivars in macadamia requires extensive phenotypic measurements over many years and trials. In particular, yield traits such as nut-in-shell yield and kernel yield are economically vital characteristics and therefore guide the selection process for new cultivars. However, these traits can only be measured in mature trees, resulting in long generation intervals and slow rates of genetic gain. In addition, these traits are expensive to measure. Strategies to reduce the generation interval and increase the intensity of selection include using yield component traits, identification of markers associated with component traits, and genomic selection for yield. Yield component traits that contribute to resource availability for fruit formation include floral and nut characteristics. In this review, these traits will be investigated to estimate their relative importance in macadamia breeding and their heritability and correlations with yield. Furthermore, the usefulness of genome-wide association studies regarding yield component traits will be reviewed. Genetic-based breeding techniques could exploit this information to increase yield gains per breeding cycle and estimate the quantitative nature of yield traits. Genomic selection uses genome-wide molecular markers to predict the phenotype of individuals at an early age before maturity, thereby reducing the cycle time and increasing gain per unit time in plant breeding programmes. This review evaluates the potential for measurement of yield component traits, genome-wide association studies, and genomic selection to be employed in the Australian macadamia breeding programme to accelerate gains for nut yield.

Characterization of accessions and species of Macadamia to stem infection by Phytophthora cinnamomi

Phytophthora cinnamomi is a major pathogen in most macadamia plantations worldwide. Due to stem lesions, stem cankers and leaf defoliation it results in loss of productivity and tree death. In this study we examined accessions of the four Macadamia species and their hybrids, produced via rooted stem cuttings or germinated seeds, for susceptibility to stem canker and necrotic lesion caused by P. cinnamomi. Plants were wound-inoculated with agar containing P. cinnamomi. The symptoms produced in inoculated plants were used to characterize host susceptibility variation within and among the population. Lesion lengths and severity of stem canker were recorded. The four species and hybrids differed significantly in stem canker severity (P < 0.001) and lesion length (P = 0.04). M. integrifolia and M. tetraphylla hybrids were the most susceptible. M. integrifolia had the greatest stem canker severity and the most extensive lesions above and below the site of inoculation. Restricted lesion sizes were observed in M. ternifolia and M. jansenii. The effects of basal stem diameter and the method of propagation either from cuttings or seed were not significant. The genetic variation in the reactions of macadamia accessions to stem infection by P. cinnamomi is discussed.

Ultra-high-throughput DArTseq-based silicoDArT and SNP markers for genomic studies in macadamia

Macadamia (Macadamia integrifolia, M. tetraphylla and hybrids) is an Australian native nut crop and has a significant economic value in the food industries worldwide. Long juvenility along with traditional breeding strategies impede quick genetic improvement of this crop. The existing cultivars constitute only second to fourth generation of the wild germplasm in the rainforest. The utilisation of molecular markers for genomic selection and genome-wide association studies may accelerate genetic gains. Identification of a robust, reproducible, and cost-effective marker system is instrumental in increasing the efficiency of genomic studies. This study is the first to report the potential of two ultra-high-throughput diversity array technology (DArT) markers (silicoDArT and SNP) in macadamia. Both markers were used to identify the genetic diversity and population structure in 80 macadamia cultivars. Parentage analysis of 25 scions in a rootstock trial was conducted to confirm plant identity where recorded identities did not corroborate with phenotypic field observations. A total of 22,280 silicoDArT and 7,332 SNP markers were reported, of which 11,526 silicoDArT and 3,956 SNP markers were used for analyses after screening with quality control parameters including >95% call rate, >95% reproducibility, and >0.05 one ratio. The average polymorphic information content (PIC) values of silicoDArT and SNP markers were 0.29 and 0.21, respectively. Genetic variance among the cultivars ranged from 0.003 to 0.738 in silicoDArT and 0.004 to 0.412 in SNP markers. Four distinct population groups were identified from SNP data analysis. Most of the accessions used in this study were descended from two or more populations. Cluster analysis clearly separated genotypes of distinct origins, such as the Hawaii Agricultural Experiment Station and Hidden Valley Plantation accessions. Two wild accessions of Macadamia jansenii and M. ternifolia were found to be distantly related to the cultivars. Wild germplasm individuals and their hybrids with cv. ‘660’ formed separate clusters, suggesting that crossing between wild and cultivated genepools can extend genetic diversity. DArTseq-based SNP markers were successfully utilized to confirm the genetic identity of 25 scions in a rootstock trial. Our study suggests that DArT platforms are a robust system for the facilitation of genomic studies with regard to macadamia.

‘MPM1’: a macadamia hybrid showing breakdown of biological rhythm in morphogenesis

Macadamia is an Australian native genus, comprising of four species. Commercial cultivars for the production of macadamia nuts are derived from two species, M. tetraphylla and M. integrifolia. The different species of Macadamia have unique leaf and flowering patterns, due to inherent biological rhythms. Usually, M. tetraphylla has four leaves in a whorl, while the other species have three leaves per whorl. All species produce 1-2 florets at each raceme node. We identified a multi-pistillate macadamia genotype (‘MPM1’) with irregular leaf pattern and floral architecture. The genotype is a progeny of cultivars ‘HAES 705’ and ‘NG18’. In this article, we described the characteristics of this plant and irregularities of its biological rhythm, including the number of leaves per whorl, ranging from 1 to 9, and variation in the number of pistils per floret. This irregular biological rhythm occurred during both vegetative and reproductive growth phases. Future molecular or epigenetic studies could determine the origin and mechanism of these unusual characteristics.

Broad-sense heritability and inter-trait relationships in young macadamia architecture, flowering and yield

Macadamia in the orchard environment is relatively unaltered from its natural form, and there is considerable scope to change vegetative and reproductive architecture for improving yield and related traits through breeding. An understanding of genetic and environmental control mechanisms and the dynamics between vegetative and reproductive characteristics are useful to identify the most important traits for improvement. In breeding programs, considerations of the heritability of vegetative traits are usually limited to tree scale measurements such as canopy volume. Here we studied the broad-sense heritability (H) of characteristics that constitute the macadamia canopy and yield at multiple architectural scales, and also documented relationships between vegetative and reproductive traits. Cutting-grown clones of 15 macadamia genotypes were subsampled from a breeding trial planted in 2011 in South East Queensland, Australia. At the tree scale, canopy volume had very low H (0), suggesting a strong environmental influence. Detailed measurements of individual canopy components give insight to more complex interactions within the canopy. Some vegetative architectural characteristics, such as branch number, average internode length and growth unit (GU) length had medium H (0.31-0.39), and node number per GU had high H (0.57). Other canopy components such as length of primary branch axes and primary branch cross-sectional area (BCA) displayed very low heritability (0-0.05), and are likely to be mainly controlled by environmental conditions. H values were low for yield and nut counts (0.16-0.21), except nuts per raceme (0.43). Heritability was medium to high for other reproductive characteristics (0.34-0.70) that may indirectly relate to yield. Understanding the relationships within and between vegetative and yield traits at different architectural scales informs the choice of multiple useful traits, and will aid the breeding of future elite macadamia cultivars. Genetic correlations and principal component analysis provided an insight to genetic relationships between limb angle and nodes per GU; nodes per GU and GU length; GU length and yield traits; and raceme length and yield.