Theses and Dissertations

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Browsing Theses and Dissertations by Author "Gerrano, A. S."

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    Variation in Drought Tolerance Attributes Among Tepary Bean (Phaseolus acutifolius) Germplasm
    (2023-05-19) Nong, Refilwe Aljareau; Gwata, E. T.; Gerrano, A. S.
    Tepary bean (Phaseolus acutifolius A. Gray) is an important food legume which originated from South America. In South Africa, it is cultivated by smallholder growers mainly in the drought prone Sekhukhune District of Limpopo Province. Currently, there are no significant breeding efforts aimed at cultivar development of this crop and it remains under-utilized despite the potential of the crop. Therefore, this study evaluated drought tolerance and growth attributes of the tepary bean emphasising on the leaf proline content that are associated with drought tolerance directly or indirectly. The study also determined the drought tolerance and growth relationships as well as identified potentially superior genotypes of tepary bean. The germplasm was evaluated before and after the soil moisture stress treatment which was imposed on the trial by withholding water for 21 days. A 6 x 7 rectangular lattice design replicated three times was used for evaluating 42 genotypes. The results showed that prior to soil moisture stress, there were significant (P<0.05) differences among the 42 genotypes for all the six phenotypic parameters that were measured. The highest (1.05 μmol/g dry weight) and lowest (0.32 μmol/g dry weight) leaf proline content (LPC) were observed for genotypes ‘Ac-35’ and ‘Ac-9’, respectively. The trial mean for proline was 0.69 μmol/g dry weight. The genotype ‘Ac-42’ attained the highest (27.85) leaf chlorophyll content (LCC) which was 48.94% higher than the check genotype (‘Ac-34’). The genotype ‘Ac-33’ achieved almost two-fold higher relative water content (RWC) (84.72%) than genotype ‘Ac-11’ which recorded the lowest (43.12%) RWC. The highest (68.70 mmol m-2s-1) stomatal conductance (SC) was three-fold more than for the check genotype (19.90 mmol m-2 s-1). At least four genotypes (‘Ac-6’, ‘Ac-7’, ‘Ac-22’ and ‘Ac-28’) attained significantly (P < 0.05) greater stem height (SH) than the trial mean (28.63 cm). After the soil moisture stress treatment, the results revealed that the LPC ranged from 1.26 to 0.36 μmol/g dry weight that were observed for genotype ‘Ac-35’ and ‘Ac-9’, respectively. The LPC showed a positive but not significant (P > 0.05) correlation with each of the other remaining attributes both before and after the moisture stress treatment. Similarly, after the soil moisture stress, the LCC maintained a highly significant (P < 0.01) positive correlation with the RWC but a negative correlation with the SH. In both soil moisture conditions, there was no discernible correlation between the SD and the SH. In general, the soil moisture stress lead to a variable increment in the LPC among the genotypes. An independent samples t-test which was used to determine the significance of the change in LPC showed that there was a highly significant (P < 0.00019) difference between the measurements of this amino acid before and after soil moisture stress. The results also showed a reduction in LCC during the soil moisture stress period but there was no clear pattern of the influence of the soil moisture stress on both the SC and RWC. The principal component analysis showed that before the soil moisture stress, the first two principal components accounted for 45.49% of the total variation and three traits (SC, LPC and SH) were highly associated with PC1. In addition, SC contributed the most variation for this component. However, PC2 was highly associated with LPC and RWC. In contrast, PC3 was dominated by SH. The results also showed that after the soil moisture stress, the first two principal components accounted for >50.0% of the total variation. The LPC and SH were highly associated with PC2 but PC3 was dominated by both LCC and SD. In the biplot analysis four genotypes (‘Ac-2’, ‘Ac-19’, ‘Ac-30’ and ‘Ac-41’) were clustered around the origin prior to the moisture stress treatment while five genotypes (‘Ac-3’, ‘Ac-9’, ‘Ac-11’, ‘Ac-28’ and ‘Ac-35’) were distinct and positioned far away from the origin. The genotypes in the right top quadrant (including ‘Ac-4’, ‘Ac-6’, ‘Ac-7’ and ‘Ac-28’) were associated and characterized by high leaf proline, high degree of stomatal opening and tall shoots. The tallest shoots were associated with the genotypes that were grouped in the left top quadrant while the remainder of the genotypes were characterized by thick stems and grouped in the left bottom quadrant. The tepary bean genotypes were grouped into three main clusters with the majority of the genotypes (64.28%) grouped in cluster III. Cluster I consisted of only seven genotypes including ‘Ac-40’ (which was associated with high LCC) as well as ‘Ac-2’, ‘Ac-35’, and ‘Ac-37’ (which were characterized by both LPC and RWC). The check (genotype ‘Ac-34’) was grouped in cluster III in a sub-cluster with genotype ‘Ac-20’. This study discusses the implications of the observed variability among the tepary bean genotypes for these phenotypic attributes and growth parameters. There will be merit in validating these results on a field basis together with grain yield evaluation and genotyping over multiple locations and seasons to determine elite germplasm that breeders and growers can utilize.
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    Variation in Root Nodulation Traits among Parental Genotypes and Segregating f 2 Pigeonpea Plant Populations
    (2023-05-19) Mthombeni, Tinyiko; Gwata, E. T.; Gerrano, A. S.
    Pigeonpea (Cajanus cajan L. Millsp.) is an important grain legume that originated in the Indian sub-continent. In South Africa, it is grown either as single plants or as a hedge, mainly in Kwazulu-Natal, Limpopo, and Mpumalanga Provinces. The crop provides highly nutritious food for human consumption and fixes considerable amounts of atmospheric nitrogen (N) thus contributing to the improvement of soil fertility. Root nodulation in pigeonpea is an integral part of the symbiotic process that results in N fixation thus contributing to the productivity of the crop. Currently, there are no reports that determined the genetics of root nodulation in pigeonpea. Therefore, this study was designed to determine the mode of inheritance for selected root nodulation traits. The experiment was conducted in a greenhouse at the Agricultural Research Council­Plant Health and Protection (ARC-PHP). The average day and night temperatures in the greenhouse were 28°C and 15°C- respectively, with 14 hours of daylight. A randomized complete block design with two replications was used to set up the experiment. Six pigeonpea genotypes were used in the study together with thirty-six rhizobia strains originating from soil that was collected from diverse locations across South Africa. The nodulation variables which were measured included leaf chlorophyll content (LCC), shoot dry weight (SDW), root dry weight (RDW) and nodule dry weight (NDW). The data sets for each of these quantitative variables were subjected to the analysis of variance followed by mean separation using the least significant difference at the 5% probability level using statistical software (Statistix 10.0), and subsequently to analysis of goodness of fit test using standard Chi-square procedures for various Mendelian ratios. The results revealed that the method which was employed to phenotype both the parental genotypes and the F2 progenies was effective and enabled a distinction between the phenotypic classes among the treatments hence a rapid, simple technique to identify contrasting parental genotypes for specific nodulation traits for use in the subsequent genetic study. The GGE biplot analysis revealed that the rhizobial strains 'R24', 'R28', 'R31' and 'R34' were clustered around the origin. In contrast, the rhizobial strains 'R7', 'RB', 'R10', 'R27' and 'R29'were positioned far away from the origin. The biplot also indicated that the pigeonpea parental genotypes (coded as environment scores), 'Gen-1' (E1 ), 'Gen-2' (E2), 'Gen-3' (E3) and 'Gen-5' (ES) were separated by acute angles between them and grouped in the same quadrant. The 'which-won where' biplot explained 56.05% total variation of which PC1 and PC2 accounted for 29.40% and 26.65% of the total variation, respectively. The results also revealed that the rhizobial strains (depicted as genotypes) on the vertices of the polygon 'R 1 0', 'R 11 ', 'R27' and 'R3S' performed best with the pigeonpea parental genotypes (depicted as environments) 'Gen-2' (E2), 'Gen-S' (ES), 'Gen-4' (E4) and 'Gen-6' (E6), respectively. The genotype 'Gen-S' (ES) showed the longest vector line, suggesting a high discriminating ability. The frequency distribution curve for the F2 plant population that was derived from the cross Pa x P1 showed approximately a normal distribution curve but with a slight skew to the right suggesting the presence of epistatic gene action for the LCC trait. The segregation ratio of 9 high:7 low chlorophyll content in the cross P4 x P2 (P-04-SST x P-02-DC) suggested duplicate recessive epistasis in which there is complete dominance at both gene pairs; but, when either gene is homozygous recessive, it masks the effect of the other gene. For SDW, the results also confirmed that the 9:7 ratio was the best fit. The segregation pattern, based on the LCC, of the F2 population in the cross P-04-SST x P-02-DC, best fitted the 9:7 ratio. The results showed that the 9:7 ratio was generally predominant for the traits that were studied thus indicating a high probability that more than one gene, with epistasis are involved in their genetic control. The LCC showed a weak negative correlation with each of NDW and SDW in the F2 progenies that were derived from P-04-SST x P-02-DC. However, there was a positive but weak correlation between NDW and SDW in this set of progenies. In contrast, there was a highly significant (P < 0.01) positive correlation between NDW and SDW in 'cross 2'. The LCC was positively correlated to both NDW and SDW in the F2 progenies that were derived from the cross involving Pax P1. It is recommended that future studies should include the determination of heritability values that can be used in breeding programs aimed at the genetic improvement of N fixation in pigeonpea. It may also be necessary to combine classical Mendelian genetics with modern genomics tools to gain a better understanding of the complex nature of N fixation in pigeonpea as well as its genetic manipulation.