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

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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

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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.

INTRODUCTION

In Africa, bambara groundnut (Vigna subterrenea (L.) Verdc) is the third most important legume after groundnut (Arachis hypogaea) and cowpea (Vigna unguiculata) (Howell, 1994) and a major source of vegetable protein (Amarteifio and Moholo, 1998; Uguru and Akubuo, 1999; Essien and Akaninwor, 2000; Basu et al., 2003). It has several production advantages

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in that it can yield on soils of low fertility and with little rainfall. It is nutritionally superior to other legumes, and is the preferred food crop of many local people (Brough and Azam-Ali, 1992). Bambara groundnut is primarily grown for its seeds. The seeds command a high market price, with demand far outweighing the supply in many areas (Coudert, 1982). Currently, there is a growing awareness on its potential importance as food and a major source of dietary protein among rural and urban dwellers. Despite there potentials, bambara groundnut is mainly cultivated by poor resource farmers at subsistence level. To wean bambara groundnut from the subsistence production and integrate it fully into a commercial/full scale production would require some level of improvements aimed at generating genotypes with good agronomic potentials.

Attempts to improve bambara groundnut through the conventional breeding methods have not been successful. Hybridization has been largely constrained by the failure of the crop to set seeds after artificial crosses. Thus, the available genotypes are selections from the aboriginal landraces. Hybridization and selection of new forms are important crop improvement strategies. Hybridization of selected parental lines allows for creation of new forms through genetic recombinations. The reasons for hybridization failures have not been clearly understood. The cytological status of Bambara groundnut lines have been studied (Uguru and Agwatu, 2006) with no discernable cytological impediments to fertilization and seed set. The architecture of the reproductive structures and longevity of the pollen grains after shedding may provide a clue to the possible causes of hybridization failures. After shedding, pollen grains are exposed to the prevailing environmental conditions and they have to reach a receptive stigma while still viable. The duration of pollen viability after anther dehiscence is very crucial for successful pollination. Pollen viability and longevity are also important physiological attributes that enable the breeder understand species reproductive