STUDIES ON ASPECTS OF REPRODUCTIVE BIOLOGY AND POD YIELD IN BAMBARA GROUNDNUT (VIGNA SUBTERRENEA (L.) VERDC)

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.

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

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

performance and therefore, enhance the successful implementation of breeding programmes. Therefore, the knowledge of the duration of pollen viability of bambara groundnut will be very useful in developing strategies to manage pollen transfer and  increase the chances of successful  pollination  and  fertilization.  There  is  also  lack   of  research  result  on  the relationships among the agronomic, floral and the  yield components in bambara groundnut. This has created an information gap on the yield  related traits that will be relevant in the improvement of the crop. This experiment was therefore, designed to study;

   the reproductive structures of bambara groundnut lines as a prelude to ascertaining the causes of hybridization failures in breeding programmes.

   the agronomic and reproductive biology of bambara groundnut.  the yield potentials of bambara groundnut lines in relation to the reproductive biology, floral and agronomic traits.

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