Variation in Root Nodulation Traits among Parental Genotypes and Segregating f 2 Pigeonpea Plant Populations

Abstract

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.