Genetic and environmental influence on foliar carbon isotope composition, nitrogen availability and fruit yield of 5-year-old mango plantation in tropical Australia

The aim of this study was to quantify the effect of different varieties, planting densities, tree training systems and canopy aspect (north and south) on tree water use efficiency and nitrogen (N) availability in relation to mango fruit yield and fruit size as well as soil fertility (particularly total carbon (C) and total N as well as C and N isotope compositions) in a 5-year-old mango plantation of tropical Australia

Mango Breeding Support

The Mango Breeding Support project provided technical, research and development support to the Queensland-based, Australian Mango Breeding Program to develop and evaluate breeding systems and technologies that improve mango breeding efficiency. Adoption of efficient breeding support technologies will allow breeders to identify genes for desirable plant and fruit traits in parent varieties and incorporate those genes into new hybrid varieties more efficiently and rapidly. The project compared traditional hand pollination methods with DNA marker assisted selection (MAS) open-pollinated methods to identify paternity and found both systems had advantages. Marker assisted paternity identification was not practical in all cases and relied on a greater range of technical skills and resources being available to the breeder. It is expected that MAS will become even more efficient when markers are available for production and quality traits in addition to parental identification. Breeding for anthracnose resistance in mango is in its infancy. This project has identified several accessions in related Mangifera species with potential tolerance to postharvest anthracnose and tested the compatibility of these related species with the common mango and if the tolerance is transferable. The project investigated ways of determining a trees postharvest fruit disease resistance status in seedlings to avoid the up to 6 year wait for trees to crop. Identification of genes and gene markers for plant development, stress response, fruit colour and flavour development was another goal of this project. Twenty five new expressed sequence tag (EST) derived single sequence repeat (SSR) DNA markers were identified and tested across a diverse range of germplasm. These markers were shown to be useful in determining genetic relationships, exploring potential pedigrees and estimating the genetic background of cultivated accessions of M. indica. They are the first reported EST-SSR markers suitable to cross-amplify in five wild Mangifera species. The technologies that have been shown to be more efficient have been incorporated in to the Australian Mango Breeding Project. Other technologies being researched that are not yet fully developed to the stage where they can be adopted in a working breeding program are being progressed in other related research projects

Mango germplasm screening for the identification of sources of tolerance to anthracnose

Colletotrichum species are one of the most common causes of postharvest fruit rot in mango in Australia, particularly in the tropical region of north Queensland, and can result in significant losses if not managed. The research aims were to identify sources of anthracnose tolerance and to determine if host material other than fruit could improve or fast track the screening process and result in improved breeding efficiency. Access to the Australian National Mango Genebank (ANMG) collection enabled fruit screening of more than 100 Mangifera indica cultivars or Mangifera species for tolerance to anthracnose by artificial inoculation with Colletotrichum asianum over a period of 14 years. Mean lesion diameters were compared with those on a known susceptible M. indica cultivar Kensington Pride (KP) and a tolerant M. laurina cultivar Lombok. Inoculation of leaf discs and entire leaves was evaluated in the laboratory and the field as alternative assays for tolerance to anthracnose and was assessed by presence/absence of disease. Screening of fruit has shown that anthracnose tolerance within the mango germplasm is highly variable and needs to be assessed over multiple years. None of the alternative laboratory bioassays provided consistent or reliable data. The in-field artificial inoculation of immature leaf flush was successful but was not deemed suitable for adoption due to practical restraints. While resistance to anthracnose in fruit has not yet been identified, some cultivars and Mangifera spp. showed promise for inclusion as parents in future breeding programs

The influence of genetic structure on phenotypic diversity in the Australian mango (Mangifera indica) gene pool

Genomic selection is a promising breeding technique for tree crops to accelerate the development of new cultivars. However, factors such as genetic structure can create spurious associations between genotype and phenotype due to the shared history between populations with different trait values. Genetic structure can therefore reduce the accuracy of the genotype to phenotype map, a fundamental requirement of genomic selection models. Here, we employed 272 single nucleotide polymorphisms from 208 Mangifera indica accessions to explore whether the genetic structure of the Australian mango gene pool explained variation in trunk circumference, fruit blush colour and intensity. Multiple population genetic analyses indicate the presence of four genetic clusters and show that the most genetically differentiated cluster contains accessions imported from Southeast Asia (mainly those from Thailand). We find that genetic structure was strongly associated with three traits: trunk circumference, fruit blush colour and intensity in M. indica. This suggests that the history of these accessions could drive spurious associations between loci and key mango phenotypes in the Australian mango gene pool. Incorporating such genetic structure in associations between genotype and phenotype can improve the accuracy of genomic selection, which can assist the future development of new cultivars. © 2022, The Author(s)

Promotor insertion leads to polyembryony in mango – a case of convergent evolution with citrus

Sexual reproduction in plants is the main pathway for creating new genetic combinations in modern agriculture. In heterozygous plants, after the identification of a plant with desired traits, vegetative propagation (cloning) is the primary path to create genetically uniform plants. Another natural plant mechanism that creates genetically uniform plants (clones) is apomixis. In fruit crops like citrus and mango, sporophytic apomixis results in polyembryony, where seeds contain multiple embryos, one is sexually originated and the others are vegetative clones of the parent mother tree. Utilizing the mango genome and genetic analysis of a diverse germplasm collection, we identified MiRWP as the gene that causes polyembryony in mango. There is a strong correlation between a specific insertion in the gene’s promoter region and altered expression in flowers and developing fruitlets, inducing multiple embryos. The MiRWP gene is an ortholog of CitRWP that causes polyembryony in citrus. Based on the data, we speculate that promoter insertion events, which occurred independently in citrus and mango, induced nucellar embryogenesis. The results suggest convergent evolution of polyembryony in the two species. Further work is required to demonstrate the utility of these genes (mango and citrus) in other biological systems as a tool for the clonal production of other crops

New mango hybrids from Australia

The Australian National Mango Breeding Program has been breeding mangoes since 1994. In recent years, evaluation of the elite selection have identified three high performing hybrids, NMBP1243, NMBP1201 and NMBP4069, which are in the process of commercial release. These hybrids all have 'Kensington Pride' as their paternal parent and are characterised by improved fruit colour and tree productivity over 'Kensington Pride'. NMBP1243 is noted for its early season production, and NMBP1201 and NMBP4046 for their firm fruit. The hybrids were produced using hand pollination breeding and selection techniques. The breeding program is ongoing with the current hybridisation program being supported by a multidiscipline approach, that includes marker assisted screening, disease screening, postharvest evaluation and a genomics gene discovery program

The effect of preharvest nutrition and crop load on fruit quality and postharvest disease in mango (mangifera indica l.)

The aim of this thesis was to investigate how preharvest calcium (Ca) and nitrogen (N) nutrition, and low crop load during fruit development affect fruit quality and postharvest disease of Keitt mangoes (Mangifera indica L). Calcium, N and crop load were chosen because previous research and Gower reports implicate them as major factors in internal disorders, poor colour development, and postharvest disease in mangoes grown in the Mareeba district of Queensland. The symptoms and incidence of the internal fruit disorders seen in Keitt mangoes growing in Queensland were investigated to determine their similarity to other forms of internal disorders, and to develop a severity rating scale to assess fruit in later experiments. While the observed disorder in Keitt shared some symptoms with other named forms of disorders, not all symptoms were similar. The typical symptoms of patches of watery translucent flesh that breakdown and discharge cellular fluids in the more severe cases, are referred to as watery-pulp breakdown throughout this thesis. Watery-pulp breakdown-affected fruit had lower mesocarp Ca concentrations (0.05% dw) than healthy fruit (0.10% dw), but not N, K, P, and Mg concentrations. The severity of symptoms increased as maturity of the fruit increased, and early harvest may be a useful strategy to minimise the disorder. The first nutrition experiment investigated the effects of Ca, N and crop load on fruit quality in 6-year-old Keitt trees, between 1997 and 2000. Trees were fertilised with high (>1 kg Ca tree-1 and >300 g N tree-1) or nil concentrations of Ca and N, and natural or 70%-reduced crop loads. Fruit were assessed for watery-pulp breakdown, fruit colour, postharvest disease, fruit firmness, yield and average fruit weight. Although Ca was applied above the normal industry rates, there were no significant increases in fruit-Ca or leaf- Ca concentrations, or response in any of the fruit quality parameters measured. The factors influencing Ca concentrations in mesocarp were shown to be changes in dry matter accumulation and periodic reductions in Ca supply. Fruit from the high N treatment had significantly (p < 0.05) greater incidence and severity of postharvest side disease lesions (mainly anthracnose) but not stem lesions (mainly stem-end rot). The high N treatments also significantly (p < 0.05) reduced the percentage of yellow skin, and the percentage and intensity of red blush colour in ripe fruit. Watery-pulp breakdown, fruit firmness and average fruit weight were not significantly affected by the high N treatments. Reducing the crop load throughout fruit development significantly (p<0.05) increased the average fruit weights and the incidence of watery-pulp breakdown, but did not significantly affect the other fruit quality parameters measured. Further studies on Ca uptake and distribution were carried out in 3 additional experiments. The effects of Ca supply on fruit quality were investigated in a similar manner to the first experiment, by varying soil-applied Ca. Increased rates (1726 g tree-1 Ca) and more frequent applications (fortnightly) of gypsum were used to overcome leaching. Although soil Ca concentrations at harvest (0 - 30 cm deep) were increased by the Ca treatment, no significant effects were observed on fruit quality. The effect of leaf transpiration on leaf and fruit Ca concentrations was investigated by reducing leaf transpiration with an anti-transpirant throughout flowering and fruit development. Reduced leaf transpiration did not significantly affect mesocarp or leaf Ca, N, P, Mg, K, B concentrations, indicating the balance of transpiration mass flow between leaves and fruit is not a major factor in mango fruit Ca accumulation. Temporal changes in uptake and distribution of Ca during flowering and fruit development were studied using elemental strontium (Sr) as a Ca analogue. Sr and Ca concentrations were not similar during the experiment making prediction of Ca uptake and distribution based on Sr uptake and partitioning unsuitable. The effect of high N in increasing postharvest fruit disease in the first experiment was further studied to confirm the earlier results and to identify the mechanisms involved in this relationship. Fruit with a range of N concentrations were produced from trees fertilised with a range of N rates (0 417 g tree-1). The incidence and severity of postharvest anthracnose (C. gloeosporioides) increased as fruit-N increased, with significant relationships between anthracnose severity and exocarp-N concentration (anthracnose severity in sprung fruit = 163.18x2 151.85x + 36.987, r2 = 0.97**). High N had a similar effect on the incidence of Penicillium sp., Rhizopus sp., Guibnardia sp., or Epicoccum sp., but not Dothiorella sp. (stem-end rot). The antifungal compounds 5-n-pentadecyl resorcinol and 5-n-heptadecenyl resorcinol were influenced by exocarp-N in two ways. Resorcinol concentrations were lower in fruit with high exocarp-N than low-N fruit, and concentrations decreased faster in ripening fruit with high exocarp-N. The differences in resorcinol concentrations between high- and low-N fruit was strongest in the sprung stage of ripeness, and positive exponential relationships were found between exocarp-N and concentrations of 5-n-pentadecyl resorcinol (p=0.006. r2= 0.92) and 5-n-heptadecenyl resorcinol (p=0.016, r2=0.82). There were also positive relationships between anthracnose severity and 5-n-pentadecyl resorcinol (p=0.003, r2=0.86) and 5-n-heptadecenyl resorcinol (p=0.003, r2 =0.87). No significant differences were observed in exocarp morphology to explain the N effect on postharvest disease incidence or severity. The effect of timing of application of N on fruit quality and disease was investigated in a field experiment on 8-year-old Keitt mango trees on which N was applied at flowering, at mid fruit development, 2 weeks pre-harvest, or not applied (control). N applied at flowering and mid fruit development, significantly increased the severity of postharvest Colletotrichum sp. (p<0.05) Penicillium sp., Rhizopus sp., Guibnardia sp., and Epicoccum sp. (p<0.01) in ripe fruit. N applied 2 weeks prior to harvest did not significantly affect postharvest diseases, with fruit having similar anthracnose severity and incidence to the nil-N (control) treatment. Fruit Ca concentrations were not able to be manipulated by imposing field treatments. The inability to easily manipulate fruit Ca concentrations indicates it is unlikely to be an effective tool to manipulate fruit quality in mango. Nitrogen was shown to greatly influence fruit quality, having a major influence on postharvest disease, skin colour and sunburn susceptibility, but not watery-pulp breakdown. Increased N was also shown to reduce concentrations of natural antifungal resorcinol compounds as fruit ripened, making fruit more susceptible to postharvest rots. The sensitivity of fruit quality to N makes management of N a critical tool in managing fruit quality and disease susceptibility. Reduced crop load had a major effect on the incidence and severity of watery-pulp breakdown, indicating tree management practices to maximise crop load will also reduce the severity of watery-pulp breakdown

Changes in the cuticular surface during the development of mango (Mangifera indica L.) cv. Kensington Pride

The surface morphology of mango fruit, cv. Kensington Pride, was studied throughout fruit development to gain an understanding of the development of the cuticular structures. This was done to identify possible weaker points where water and chemicals may penetrate and lead to skin disorders. At anthesis, no epicuticular wax fine structure was evident on the flower ovary and the surface of the cutin layers was smooth and unbroken. By fruit set, a thick layer of flattened polygonal epicuticular wax scales was evident on the surface. When the fruit diameter reached approximately 3 mm the epicuticular wax scales had developed a series of slightly flattened radiating wax rodlets on their base and perimeter. At about 12 mm fruit diameter, the epicuticular wax scales with radiating rodlets began to fragment and the cutin layers began to crack, forming irregular cuticular platelets. At this stage, a third form of epicuticular wax was evident as small erect scales projecting from the cuticle surface, these became the dominant wax form in mature fruit. As the fruit developed, the number of cuticular platelets increased by subdivision and became a more regular pentagonal shape. The thinnest points of the cuticle were in the central cracks of the cuticular platelets. These cracks may be possible entry points for chemicals, leading to skin blemishing

Crop production: Mineral nutrition

This chapter on mango covers the following: soils mineral diagnosis and sampling; tissue mineral diagnosis and sampling; interpretation of soil and leaf analyses; uptake and translocation of nutrients (N, P, K, Mg, S, Zn and Mn); nutrient deficiency and toxicity; and effect of nutrients on crop production and fruit quality