V. Vithanage

Effect of temperature on pollen germination and pollen tube growth of four cultivars of mango (Mangifera indica L.).

Summary The effect of a constant (10, 15, 20 or 25°C) or a diurnal maximum/minimum (15/5, 20/10, 25/15 or 30/20°C) incubation temperature on in vitro pollen germination and pollen tube growth in the pistils of two poly-embryonic (‘Kensington’ and ‘Nam Dok Mai’), and two mono-embryonic (‘Irwin’ and ‘Sensation’) mango cultivars was studied. In in vitro experiments where pollen was incubated in a liquid germination medium for 24 h in darkness, little difference was found between pollen germination of mono- and poly-embryonic cultivars. Averaged over the four cultivars, 53.9% of pollen germinated at 10°C, this increased to 76.2% when the incubation temperature was increased to 15°C, thereafter up to 25°C the percentage germination remained stable but germination decreased slightly to 68.2% at 30°C. Similarly, there was no difference in percent germination between cultivars when pollen was incubated under diurnal temperature regimes. Mean pollen germination of all four cultivars was 52.3% at 15/5°C and pollen germination increased by 10% when the temperature was raised to 30/20°C. When self-pollinated flowers were incubated for 24 h on a semi-solid agar medium at 10°C, pollen tube growth of the four cultivars was retarded and no pollen tubes reached the ovaries. As the temperature was increased from 15 to 25°C, the mean number of pollen tubes in ovaries increased from 0.04 to 0.25. At 30°C, the mean number of pollen tubes that entered ovaries decreased to 0.04. After incubation under diurnal temperature regimes, the mean number of pollen tubes in ovaries of all four cultivars at 15/5°C was 0.23 and increased to 0.42 when the temperature increased to 30/20°C. At each incubation temperature, there were significant differences in pollen tube growth between cultivars, but there were no differences between the temperature response of pollen from mono- and poly-embryonic cultivars.

A genetic map of macadamia based on randomly amplified DNA fingerprinting (RAF) markers

The first genetic linkage map of macadamia (Macadamia integrifolia and M. tetraphylla) is presented. The map is based on 56 F1 progeny of cultivars ‘Keauhou’ and ‘A16’. Eighty-four percent of the 382 markers analysed segregated as Mendelian loci. The two-way pseudo-testcross mapping strategy allowed construction of separate parental cultivar maps. Ninety bridging loci enabled merging of these maps to produce a detailed genetic map of macadamia, 1100 cm in length and spanning 70–80% of the genome. The combined map comprised 24 linkage groups with 265 framework markers: 259 markers from randomly amplified DNA fingerprinting (RAF), five random amplified polymorphic DNA (RAPD), and one sequence-tagged microsatellite site (STMS). The RAF marker system unexpectedly revealed 16 codominant markers, one of them a putative microsatellite locus and exhibiting four distinct alleles in the cross. This molecular study is the most comprehensive examination to date of genetic loci of macadamia, and is a major step towards developing marker-assisted selection for this crop.

Identification of intracellular calcium oxalate crystals in Chamelaucium uncinatum (Myrtaceae)

Intracellular inclusions in the pedicel and calyx-tube tissues of Chamelaucium uncinatum Schauer (Myrtaceae) flowers are irregular in shape. They were shown, by polarised light and scanning electron microscopy, to be birefringent 8.9–29.5 μm druse (i.e. aggregate) crystals. Energy-dispersive X-ray spectroscopy showed that these crystals were predominantly composed of calcium. Histochemical and acid-solubility tests indicated that the crystals were calcium oxalate. Raman microprobe spectroscopy was used to confirm this chemical identity. The calcium oxalate crystals were located in xylem-vessel lumens and also in parenchyma cells adjacent to vascular tissues. Thus, the crystals may function to regulate soluble calcium concentrations in C. uncinatum tissues near sites where calcium is unloaded from the xylem.

Genetic relationships amongst macadamia varieties grown in South Africa as assessed by RAF markers.

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 relationships 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 origins. The cultivar 791 was unusual, identified as a tri-species hybrid containing a significant proportion of the wild species M. ternifolia. It was also shown that 741 u is the true 741 cultivar and 741s is actually the cultivar 800.

Isozymes as genetic markers for Macadamia

Analytical conditions for resolving nine isozyme systems of Macadamia were investigated and established. Seven of these with nine loci were used to discriminate 73 Macadamia genotypes of the two major species, Macadamia integrifolia Maiden and Betche and Macadamia tetraphylla L.A.S. Johnson. Another species, Macadamia ternifolia F. Muell. was also used in the analysis. No records on the presence or absence of bands were made but enough variation in the distribution of the bands was present to enable identification of each cultivar. The isozyme system that showed the most polymorphism was phosphoglucoisomerase (PGI). By using a combination of systems reported here all cultivars were unequivocally identified. Results of the PGI isozyme analysis confirmed the reported origin of some cultivars and cast doubts on the suggested origin of another.

Effect of different pollen parents on seediness and quality of ‘Ellendale’ tangor

Abstract The role of the pollen parent on pollen tube growth, fruit set, fruit weight and seediness of cultivar ‘Ellendale’ tangor was investigated in two different commercial citrus-growing regions. Cross-pollinations were carried out with pollen of cultivars of ‘Imperial’ mandarin, ‘Silverhill’ satsuma, ‘Emperor’ mandarin, ‘Dancy’ mandarin, ‘Murcott’ mandarin and ‘Valencia’ orange. ‘Ellendale’ tangor was self-compatible, but showed a low fruit set with reduced seed numbers when self-pollinated. Cross-pollinations resulted in a higher fruit set with all except ‘Silverhill’ satsuma pollen which showed high pollen sterility. Fruit weight and seediness of ‘Ellendale’, showed significant differences among pollen parents. ‘Murcott’ and ‘Emperor’ pollen gave the largest fruits with high levels of seeds. ‘Imperial‘ pollen resulted in low seed numbers without significantly affecting fruit size. Fruit quality of ‘Ellendale’, in terms of low seed numbers, may be maintained when it is planted in solid blocks or with ‘Imperial’ mandarin. However, ‘Murcott’ and ‘Emperor’ pollen increased seediness appreciably.

Anatomy of ethylene-induced floral-organ abscission in Chamelaucium uncinatum (Myrtaceae)

Postharvest abscission of Geraldton waxflower (Chamelaucium uncinatum Schauer) flower buds and flowers is ethylene-mediated. Exposure of floral organs to exogenous ethylene (1 mu L L-1) for 6 h at 20 degrees C induced separation at a morphologically and anatomically distinct abscission zone between the pedicel and. oral tube. Flower buds with opening petals and flowers with a nectiferous hypanthium were generally more responsive to exogenous ethylene than were flower buds enclosed in shiny bracteoles and aged (senescing) flowers. The anatomy of abscission-zone cells did not change at sequential stages of floral development from immature buds to aged flowers. The zone comprised a layer of small, laterally elongated-to-rounded, closely packed and highly protoplasmic parenchyma cells. Abscission occurred at a two- to four-cell-wide separation layer within the abscission zone. The process involved degradation of the middle lamella between separation layer cells. Following abscission, cells on both the proximal and distal faces of the separation layer became spherical, loosely packed and contained degenerating protoplasm. Central vascular tissues within the surrounding band of separation layer cells became torn and fractured. For flower buds, bracteoles that enclose the immature floral tube also separated at an abscission zone. However, this secondary abscission zone appeared less sensitive to ethylene than the primary ( central). oral-tube abscission zone as bracteoles generally only completely abscised when exposed to 10 mu L L-1 ethylene for the longer period of 24 h at 20 degrees C. The smooth surfaces of abscised separation-layer cells suggest that hydrolase enzymes degrade the middle lamella between adjacent cell walls.

Genomics of Macadamia, a Recently Domesticated Tree Nut Crop

The tree nut crop known as macadamia includes two cultivated species that readily hybridize. This Australian native from subtropical rainforests was domesticated recently, and cultivated trees are very few generations from their wild progenitors. A genomic understanding of the crop has the potential to deliver massive genetic improvements to a worldwide industry, and reveal the genetic changes that have occurred through the domestication process. The bulk of research efforts in this field have focused on the development of molecular marker technology and its various applications in assessing both cultivated and wild germplasm. Markers were also used as the basis of genetic linkage mapping for macadamia’s chromosomes, but regions controlling important traits have not yet been localized. Gene sequence information for macadamia is very limited, although two genes encoding proteins with antimicrobial properties have been described. Macadamia is the most economically valuable member of the ancient Proteaceae family, and has few cultivated relatives. This crop is the obvious target within the family for developing further genomics resources. Comparing the structure of the macadamia genome and its functional components with those of crops from other plant families, particularly nut, fruit, and tree species, should provide insights for understanding the evolution and genomic regulation of many important biological and agronomic traits.

Sensitivity of Geraldton waxflower to ethylene-induced flower abscission is reduced at low temperature

Summary Exposure to ethylene gas elicits flower abscission from cut stems of Geraldton waxflower (Chamelaucium uncinatum Schauer). Ethylene response rates in plants are mediated by temperature. At 20°C, flower abscission from waxflower ‘Purple Pride’ occurred upon 12 h exposure to 1 µ11–1 ethylene. This ethylene treatment did not cause flower abscission at either 10 or 2°C. Moreover, flowers held at 2°C were insensitive to 48 h exposure to 1, 10 and 100 µ11–1 ethylene. However, increasing the duration of treatment with 1 µ11–1 ethylene at 10 and 2°C to 48 and 144 h, respectively, induced flower abscission. When flowers were held at 20°C in air without exogenous ethylene following continuous exposure to 1 µ11–1 ethylene at 2°C, the duration required to elicit flower abscission was reduced from 144 to 72 h. Collectively, these responses show that maintaining harvested waxflower at low temperature (e.g. 2°C) is an effective means to minimise ethylene-mediated flower abscission.

A comparison of molecular markers for genetic analysis of macadamia

Summary Various marker systems exist for genetic analysis of horticultural species. Isozymes were first applied to the woody perennial nut crop, macadamia, in the early 1990s. The advent of DNA markers saw the development, for macadamia, of STMS (sequence-tagged microsatellite site), RAPD (randomly amplified polymorphic DNA), and RAF (randomly amplified DNA fingerprinting). The RAF technique typically generates dominant markers, but within the dominant marker profiles, certain primers also amplify multi-allelic co-dominant markers that are suspected to be microsatellites. In this paper, we confirm this for one such marker, and describe how RAF primers can be chosen that amplify one or more putative microsatellites. This approach of genotyping anonymous microsatellite markers via RAF is designated RAMiFi (randomly amplified microsatellite fingerprinting). Several marker systems were compared for the type, amount, and cost-efficiency of the information generated, using data from published studies on macadamia. The markers were also compared for the way they clustered a common set of accessions. The RAMiFi approach was identified as the most efficient and economical. The availability of such a versatile tool offers many advantages for the genetic characterisation of horticultural species.