Studies on Aspects of Reproductive Biology and Pod Yield in Bambara Groundnut (Vigna subterrenea (L.) Verdc)

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Abstract

Two field experiments were conducted in April and August, 2007 cropping season at the Department of Crop Science research farm, University of Nigeria, Nsukka, to evaluate the pollen germination potentials, rate of pollen tube growth, floral, agronomic and yield attributes of thirteen bambara groundnut cultivars. The first experiment (early planting) was in April and the second (late planting) was in August 2007. The results obtained showed that genotypes had significant effect on the pollen germination only at the late planting. In the early and late planting, pollen grains incubated immediately after harvest had the highest germination percentage, while pollen grains exposed for five minutes prior to in vitro germination showed very poor germination. Pollens exposed beyond five minutes after harvest did not germinate. Genotypes significantly (P<0.05) affected the pollen tube growth at both early and late planting dates. The pollen tube growth decreased drastically with increase in duration of pollen exposure. The curve fitting analysis results showed that exponential, logistic and gomperzt growth models can be used for the computation of the pollen tube growth rates. The PCA and cluster analyses were used to group the genotypes in relation to the levels of pollen survival under ambient conditions. During the early planting, the genotype, Bg-01 had moderate surviving pollen grains while Bg-08, Bg-09, Bg-11 and Bg-10 were found to have poor surviving pollens. At late planting, the genotype, Bg-04 and Bg-07 had high pollen survival while Bg-01 had poor pollen survival. The planting dates had significant effects (p<0.05) on all the floral and agronomic traits measured except for stigma diameter. Significant genotype and genotype x planting date interaction effects were observed for pistil length, stamen length, stigma-anther separation and days to 50% flowering. The principal component analysis of the floral and agronomic traits showed that the first three components accounted for 70.54% and 72.96% of the total variation in the early and late plantings, respectively. The traits representing the genotypes along the first principal axis were anther diameter, number of pods per plant, stigma-anther separation and seed weight per plant for the early planting and number of flowers per plant, number of leaves per plant, number of pods per plant, plant height, stamen length, stigma-anther separation and seed weight per plant in the late planting. Genotypes were differentiated on the basis of anther length and days to 50% flowering in the early planting and anther diameter, anther length and pistil length during the late planting along the second principal axis. The cluster plot revealed that the 13 bambara groundnut genotypes were grouped into three and two clusters during the early and late plantings, respectively. In the early planting, the genotypes in cluster I were associated with large anther diameter, very marginal stigma-anther separation and high potentials for production flowers, pods and high seed yield while cluster II genotypes flowered earlier and had smaller anther diameter, wide stigma-anther separation, good vegetative growth and low seed yield. The cluster III are early flowering genotypes with long pistil and stamen. During the late planting, the cluster I comprised of genotypes with large anthers, very marginal stigma-anther separation, high vegetative growth and high seed yield attributes while cluster II comprised genotypes with long pistil and stamen but performed poorly in pod production and seed yield. The correlation coefficient for seed weight per plant was highly significant and positive with number of leaves per plant, plant height, number of flowers per plant, number of pods per plant and anther diameter indicating that increase in these traits will ultimately increase seed weight per plant. However, stigma-anther separation was negatively correlated with seed weight per plant (r = - 0.61**) and number of pods per plant (r = - 0.45*) implying that the two yield traits decreased with increase in stigma-anther separation.

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