Asad, Habat Ullah (2017) Improving mango breeding efficiency through improved pollen storage, fruit retention and understanding of the heritability of quantitative tree architectural traits. PhD thesis, James Cook University.

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Abstract

Mango is one of the most important tropical/subtropical fruit crops (Chapman, 2000), with a world production of 45.225 million tonnes (FAOSTAT, 2014b). Worldwide mango breeding programs aim to develop novel cultivars that fulfill the needs of modern mango industries (Campbell and Zill, 2009). The conventional breeding technique involves crosses between selected parents and evaluating the resulting progenies for desirable characters. A lack of pollen availability, poor selection, a low rate of fruit set and retention are major problems in conventional breeding for mango crop improvement (Iyer and Dinesh, 1997; Iyer and Degani, 1997; Roizman, 1986). The aim of this research is to address these issues and to improve the efficiency of traditional breeding techniques. This research focusses on higher breeding efficiency by extending pollen storage, and investigates the improved methods of assessing tree vigour via an understanding of the heritability of quantitative traits related to dwarf phenotypes. Furthermore, this work explores enhanced hybridised fruit set and retention through application of plant growth regulators (PGRs) and micronutrients.

Short-term storage and retrieval of mango pollen

Two storage media in combination with four different storage temperatures were evaluated to extend the storage and viability of mango cv. NMBP-1201 pollen. Subsequent evaluations revealed that pollen stored in hexane at room temperature and pollen stored alone at -20ºC and -80ºC retained the highest viability following one week of storage. Pollen viability was significantly reduced following one week of storage in all conditions. Analysis of pollen germination rates showed that, following storage for one and two weeks, the phytotoxic effect of paraffin oil on mango pollen was greater than that of hexane at all temperatures. However, paraffin oil exhibited phytotoxic effects at all storage temperatures. These findings suggest that paraffin oil has higher phytotoxic effects on mango pollen compared to that of hexane; however, pollen stored in hexane retained a significantly higher level of viability for one week at room temperature. Thus, hexane may be a suitable pollen storage option for routine mango breeding work.

Identification and heritability assessment of the most efficient method to assess tree vigour

From twelve analysed tree morphological traits, trunk cross-sectional area (TCA) displayed the highest correlation with tree vigour, and thus was determined to be the most suitable trait for rapid vigour assessment. The heritability of TCA was assessed across several mango breeding populations growing on two research stations. TCA was recorded in 1909 progeny across 41 mango breeding families. Subsequent assessment revealed poor heritability for TCA (h2 = 0.23) among the tested mango breeding population. Poor heritability indicates that environment has a greater influence on TCA than genotype. However, a number of potential low-vigour families were identified that could serve as parental lines in future crosses to develop low-vigour mango plants.

The effects of foliar-applied PGRs on fruit set and retention

The effects of PGRs on fruit set, retention, and quality were determined in the two mango cultivars NMBP-1243 and Keitt during the 2014 and 2015 mango seasons at Walkamin Research Station, Department of Agriculture and Fisheries, Mareeba. Three PGRs, namely NAA (25 and 50 ppm), 2,4-D (25 and 40 ppm), and gibberellic acid (5 and 10 ppm), were sprayed alongside a control treatment (no spray) onto the inflorescences of selected mango trees at full bloom stage. Following treatment, 25 panicles per tree were selected and tagged to observe subsequent fruit set and retention until harvest. Results showed that, in both varieties in both seasons, 2,4-D (40 ppm) treatment either significantly reduced or did not affect fruit set as compared to untreated trees (mean fruit set per panicle for cvs. NMBP-1243 and Keitt is 7.5 and 12.9 respectively) observed 28 days following full bloom stage. However, trees treated with 2,4-D had significantly higher rates of fruit retention at harvest. Foliar spray of 2,4-D reduced fruit size and weight in cv. NMBP-1243, but did not affect cv. Keitt. Thus, it was concluded that foliar-applied PGRs administered at flowering do not increase early fruit set, but may increase fruit retention at harvest to twice that of untreated trees. An increase in fruit retention can improve breeding efficiency as well as mango tree productivity and farm profitability.

The effects of foliar-applied micronutrients on fruit set and retention

Fruit set and retention are important phenomena in commercial fruit production. Low fruit set and retention significantly affect mango breeding efficiency and contribute towards low yields in commercial mango orchards. The process of fruit set and retention is multidimensional and is directly or indirectly affected by nutritional factors. This work aimed to improve fruit set, retention, and quality in the two mango varieties NMBP-1201 and R2E2 via the application of micronutrients. Foliar application of the two micronutrients zinc and boron at two concentrations was performed at the start of bloom. Results showed that these micronutrients did not improve fruit set, retention, or quality in either mango variety.

Summary

In summary, the findings of this research suggest that the tested techniques may have significant effects on classical mango breeding efficiency. Mango pollen was successfully stored for one week in hexane and maintained significantly higher pollen viability as compared to paraffin oil storage. TCA was identified as the most suitable candidate for rapid vigour assessment in large breeding populations; however, subsequent assessment revealed poor TCA heritability which indicates a low influence of genotype on the TCA phenotype. A number of mango breeding families with low TCA were identified as potential parental lines for future crosses to breed low-vigour trees. Foliar application of 2,4-D and NAA led to significantly higher levels of hybridised fruit retention compared to that in control trees, which may increase the amount of progenies available for further growth and assessment in mango breeding. Conversely, foliar application of micronutrients had no significant impact on fruit set and retention. Further research is required to refine these techniques to increase the efficiency of classical mango breeding projects.

Item ID:	51203
Item Type:	Thesis (PhD)
Keywords:	crop improvement, foliar application, fruit retention, fruit set, horticulture, Keitt, Mangifera indica, mango pollen, mango, micronutrients, NMBP-1201, NMBP-1243, PGRs, plant breeding, R2-E2, tropical fruit
Funders:	John Allwright Fellowship (JAF), AusAID
Date Deposited:	18 Dec 2017 02:59
FoR Codes:	07 AGRICULTURAL AND VETERINARY SCIENCES > 0706 Horticultural Production > 070602 Horticultural Crop Improvement (Selection and Breeding) @ 30% 07 AGRICULTURAL AND VETERINARY SCIENCES > 0703 Crop and Pasture Production > 070305 Crop and Pasture Improvement (Selection and Breeding) @ 55% 06 BIOLOGICAL SCIENCES > 0604 Genetics > 060499 Genetics not elsewhere classified @ 15%
SEO Codes:	82 PLANT PRODUCTION AND PLANT PRIMARY PRODUCTS > 8202 Horticultural Crops > 820214 Tropical Fruit @ 100%
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