The Breeding System of Sesbania Sesban (L.) Merr. (Leguminosae)

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Addis Ababa University


The floral biology of Sesbania sesban ( L.) Men'. ( Leguminosae: Papilionoideae) was studied in general, and in particular the level of out-crossing was detennined under conditions of openpollination. Floral biology studies were conducted on one cultivated population (ILRI Accession No. 15022) of S. sesban at the compound of the International Livestock Research Institute (ILRI) in Addis Ababa. For the purpose of estimating out-crossing rate, seeds were collected from S. sesban populations at Addis Ababa (Ethiopa), Debrezeit (Ethiopia) and Zambia. Stigma receptivity and anther dehiscence time vary considerably for different flower bud sizes. Stigma become receptive before shedding of pollen grains and lor maturity of pollen grains. Therefore, the flowers of this species were detennined to be protogynous. There were significant differences between mean bud size (mm) with receptive stigma (x = 14.1±1.46) and those with non-receptive ones (x= 12.1±1.84). Also between mean size of buds with dehiscing anthers (x = 15.39 ± 2.28) and those with non-dehiscing anthers (x =10.68 ± 2.36) at P < 0.001 level of significance. Further more, pollen tubes were found to be abundant in more mature flower buds (x = 16.53 mm) from deliberate tripping. Gennination of self-pollen under natural condition takes place at a very late bud stage i.e. when a flower bud size of 22 rum and above is attained. On the other hand, tripping of flowers indicated that pollen gennination could take place at an early bud stage (stage 2, x = 14 rum). Pollen tube growth was more abundant in those styles, which received hand tripping treatment than with forceps or toothpick tripping. Seed production (no.l pod) was also higher due to hand tripping (x = 29.9 ± 1.27) than due to bagging alone (x = 23.67 ± l.84) or toothpick tripping (x = 24.11± 2.25), although not statistically significant. However, seed production from open-pollination (x = 34.11 ± 2.60) was significantly higher than from the tripping and bagging treatments. Because of difficulties in preparing an optimum medium for maximum pollen grain gernlination in vitro and due to short hydration time (6 hours of incubation), 49.43% gennination was the average score obtained in 20% sucrose. On the other hand, with fluorochromatic reaction test an average of 90.51 % viability was scored from flower buds that fall in the range of 11 to 22 rum. Flower bud size was significantly negatively correlated (r = -0.61, P =0.000) with pollen grain viability and pollen grain viability was also found to correlate (r = - 0.39, P = 0.000) inversely with germination. There was a significant positive correlation (r = 0.87, P = 0.000) between flower bud size and gennination atP < 0.05. Although the floral structure of S. sesban could favour self-pollination, the delay in self-pollen gelmination under natural condition rather seems to favour out -crossing in the presence of appropriate pollinators. Based on the electrophoretic assay of a large number of progenies on one polymorphic locus of Malate dehydrogenase (Mdh), on average, an estimated 64% of the seeds were found to be derived from out-crossing, ranging from 0.395 to 1.105 among populations. The genotype frequencies of the populations of Accession 15022 and Accession 15019 were in line with those of the Hardy-Weinberg expected genotype proportions, whereas genotype frequencies of the Zam-R024 population deviated from the expectations indicating the occurrence of non-random mating in this population. It is possible to conclude that S. sesban can under go complete self-fertilization in the absence pollinators and also complete out-crossing in the presence of suitable pollinators.