Comparative studies on cultivated and wild accessions of Vigna vexillata (L.) A. Rich

Damayanti, Farida (2010) Comparative studies on cultivated and wild accessions of Vigna vexillata (L.) A. Rich. Masters (Research) thesis, James Cook University.

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Abstract

Vigna vexillata (L.) A. Rich is an underutilised species, but has attracted international interest because of several attractive multi-purpose features, such as its tuberous roots, its high protein content biomass and its potential as a source of resistance genes. However, despite these useful attributes, V. vexillata remains largely un-researched as a crop. Most studies have focussed on wild accessions from Africa and to a lesser extent Australia, with only very limited numbers of accessions from Southeast Asia. While there are reports of the occasional use of wild plants by indigenous peoples in several countries, there are remarkably few reported studies on cultivated varieties. Further, there are very few cultivated accessions in international germplasm centres. Several taxonomic varieties have been described, all but one of which are wild. The exception is var. macrosperma, a large seeded variety reported to be at least partially domesticated. Recently, a large seeded cultivated variety sharing some of the attributes of var. macrosperma was reported from Bali, Indonesia. The Bali accessions are currently the only accessions in major international germplasm collections that are specifically designated as "cultivated".

To underpin future varietal improvement of V. vexillata, better understanding is required of the difference between the cultivated and wild varieties, their genetic compatibility and the inheritance of potentially useful traits. Several sets of germplasm were available for the present study: several cultivated accessions from Bali, a known accession of var. macrosperma from Africa, and wild accessions from Africa and Austronesia. Three objectives were set up for the study: (i) to examine genotypic diversity of cultivated accessions from Bali and compare them with var. macrosperma and the wild accessions; (ii) to explore the genetic compatibility among the Bali accessions, and with var. macrospenna and the wild accessions; and (iii) to explore the inheritance of qualitative and quantitative traits in hybrid populations. In addressing these objectives, emphasis was placed on cultivated vs. wild traits to gain information potentially relevant to assisting future breeding programs for V. vexillata.

Six cultivated accessions from two villages in Bali, the var. macrosperma accession and 12 wild accessions from Africa or Austronesia were evaluated for selected morphological, agronomic and phenological traits when grown in large pots on benches outdoors. For data analyses, genotypic variation was assessed between the three main classes of accessions, between provenances within classes, and between accessions within provenances. The experiment was conducted at James Cook University from spring 2007 to winter 2008. Weekly mean daily maximum temperatures ranged from a high of about 40°C in midsummer down to a low of about 10°C in winter. Photoperiod increased from 12.47 h at planting to a maximum of 14.10 h in late December, and then back to 11.77 h in June.

Most of the accessions flowered 50-100 days from sowing. The exceptions were the cultivated Bali accessions and a wild accession from Africa, which did not flower until autumn (April-May). It was apparent that flowering in these accessions was most likely delayed by the long photoperiods during midsummer. The cultivated Bali accessions and var. macrosperma exhibited several qualitative attributes, including ovate leaflet shape, broad leaflet width, non black-speckled seed testa and non-dehiscent pods that distinguished them from the wild accessions. Likewise, the Bali accessions and var. macrosperma shared many quantitative traits. For example, leaflet width, peduncle length and pod and seed size were all generally larger than the wild accessions. The Bali accessions exhibited bigger seed size than var. macrosperma (6.9 vs. 4.6 g per 100 seeds, respectively) while the wild types averaged only 1.6 g per 100 seeds. The Bali accessions were unique in having an average of eight flowers per peduncle, whereas all other lines had around four or fewer. Generally, the largest source of variation in quantitative traits was between the three classes of accessions, with the exception of some tuber attributes, where the largest source of variation was between accessions within provenances.

A second study explored the genetic compatibility of the cultivated Bali accessions with the two other accession classes. Seventeen accessions, comprising eight cultivated Bali accessions, one var. macrospenna accession and eight wild accessions from Africa and Austronesia, were grown in pots in shade house facilities. Not all hybrid combinations were attempted because for some accession combinations, suitable matching flowers were not available at the same time. The main aim was to attempt enough crosses between accessions from the respective classes to establish whether the classes were genetically cross-compatible. Hybridisation was conducted by hand pollination in the morning, using newly-open flowers that had been emasculated before sunset on the day before. The cultivated Bali accessions initially flowered under the shorter photoperiods of spring, but then reverted to vegetative growth as photoperiods lengthened. Renewed flowering was successfully stimulated by placing black plastic covers over the plants each day to reduce the photoperiod to 12 hours.

Pods and viable hybrid seed were obtained from the Bali x Bali, var. macrosperma x wild and wild x wild combinations. However, there was difficulty in obtaining viable and/or fertile hybrids between the Bali accessions and the other two classes. Depending on the particular hybrid combination, different genetic breakdown mechanisms were observed with the Bali x var. macrosperma and Bali x wild combinations. In some instances, flowers failed to set pods and/or the young pods abscised before maturity; pods set but seed were shrivelled and/or nonviable; viable seeds were set but the hybrid seedling plants were short-lived; or, in a few instances (Bali x Austronesian hybrids), vigorous hybrid plants were obtained but were sterile.

A laboratory study was therefore undertaken to explore possible reasons for the observed hybrid breakdown. Cytological analyses were conducted for the cultivated Bali x wild Austronesian hybrids to examine chromosome numbers during meiosis and mitosis in the parents and hybrids. There was no difference in chromosome number between the Bali accessions, the Austronesian accessions and those hybrids that were viable but infertile. All exhibited 2n = 22. Pollen viability analyses using Alexander's stain were also conducted to investigate the possible cause of the observed sterility of the Bali x Austronesian hybrids. The percentages of viable pollen and the numbers of pollen grains in the hybrids were significantly lower than both the cultivated and wild parental accessions. Consistent with this observation, a small number of viable seeds was obtained when viable pollen from the parents was backcrossed onto the sterile hybrids.

A final experiment was carried out to explore the inheritance of qualitative and quantitative traits using hybrid combinations combining putative wild and domesticated traits i.e. between a wild parent (PI) and either a Bali cultivated accession or var. macrosperma (Pl ). Three subsets of experiments were established: (i) two backcross populations (P₁, P₂, F₁, BCP₁, BCP₂) from cultivated Bali x wild Austronesian crosses; (ii) two hybrid populations (P₁, P₂, F₁) from var. macrosperma x wild African crosses; and (iii) one 'complete' population set (P₁, P₂, F₁, BCP₁, BCP₂, F₂) from a var. macrosperma x wild Australian cross. The populations within subset (i) were incomplete, because no F₂ generation seed were obtained, and only limited numbers of backcross seeds were available. Subsets (i) and (ii) were grown in pots in the shade house, whilst subset (iii) was grown outdoors in pots on benches.

The backcross plants in subset (i) were either largely infertile, or particularly in the case of the backcrosses to the cultivated parent (BCP₂), were strongly photoperiod-sensitive and like the cultivated parental plants, did not flower during the evaluation. Thus observations were possible on only a limited number of vegetative traits, e.g. leaflet shape, leaflet width and time to flowering. Based on Chi-Square analysis, it was evident that lanceolate leaflet shape and narrow leaflet width were each controlled by a single dominant gene. As the hybrid plants and the backcrosses all flowered later than the wild parent, but most flowered sooner than the cultivated parent, time to flowering was probably a quantitative trait controlled by several genes.

With subset (iii), lanceolate leaflet shape and narrow leaflet width were again each controlled by single dominant genes as were short leaflet hair length, black-speckled seed testa pattern and dehiscent pods. For most of the quantitative traits measured, the F₁ generation exhibited mid-parent values, suggesting largely additive genetic variance. Some of the attributes often associated with domestication, e.g. seed size and stem thickness, exhibited high narrow sense heritability suggesting they would be easy to breed for. Likewise, tuber traits tended to have high narrow sense heritability. However, narrow sense heritability for some important agronomic traits, like seed yield per plant and harvest index, were low to moderate. In general terms, the limited observations possible with the subset (ii) populations were consistent with those observed in the other subsets.

To summarise, the studies here have generated substantial comparative information about the genotypic variation and genetic compatibility among cultivated Bali accessions, var. macrosperma and wild accessions from Africa and Austronesia. The data suggested that genotypic variation within the cultivated Bali accessions was small compared with the wild accessions. Despite some similarities between the Bali accessions and var. macrosperma, those two classes of accessions clearly belong to different gene pools given the genetic barriers between them. Even so, the inheritance of some of the qualitative traits common to both classes was apparently similar. It is concluded that development of improved varieties of V. vexillata using the cultivated Bali type would require access to greater genotypic variation than observed here, although it would be possible (if time-consuming) to backcross in important traits from wild accessions. A more conventional breeding approach could be followed using var. macrosperma, but again, a concerted attempt will be required to identify wider genotypic variation than appears to be available in international collections. Finally, a taxonomic revision is required for the cultivated Bali accessions, given their strong genetic incompatibility with all the other V. vexillata accessions evaluated in these studies.

Item ID: 25890
Item Type: Thesis (Masters (Research))
Keywords: Africa; agricultural crops; Bali; comparative biology; crop improvement; cultivars; cultivated accessions; cultivation; domesticated accessions; domestication; food crops; food production; genetic diversity; germplasm; heritability of traits; hybridisation; Indonesia; inheritance; new crops; phylogenetics; plant breeding; plant genetics; plant varieties; tuberous roots; tubers; V. vexillata; varietal improvement; Vigna vexillata; wild accessions; zombi pea
Date Deposited: 26 Jul 2013 06:17
FoR Codes: 07 AGRICULTURAL AND VETERINARY SCIENCES > 0703 Crop and Pasture Production > 070305 Crop and Pasture Improvement (Selection and Breeding) @ 100%
SEO Codes: 82 PLANT PRODUCTION AND PLANT PRIMARY PRODUCTS > 8202 Horticultural Crops > 820299 Horticultural Crops not elsewhere classified @ 30%
82 PLANT PRODUCTION AND PLANT PRIMARY PRODUCTS > 8205 Winter Grains and Oilseeds > 820503 Grain Legumes @ 70%
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