Browsing by Author "Dakora, F D"

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    Impact of increased ultraviolet-B radiation stress due to stratospheric ozone depletion on N2 fixation in traditional African commercial legumes
    (2004) Chimphango, S B; Musil, C F; Dakora, F D
    Reports of diminished nodule formation and nitrogenase activity in some Asian tropical legumes exposed to above-ambient levels of ultraviolet-B (UV-B: 280–315nm) radiation have raised concerns as to the impact of stratospheric ozone depletion on generally poorly developed traditional African farming systems confronted by the high cost and limited availability of chemical fertilisers. These rely on N2-fixing legumes as the cheapest source of N for maintaining soil fertility and sustainable yields in the intrinsically infertile and heterogeneous African soils. In view of this, we examined the effects of supplemental UV-B radiation approximating 15% and 25% depletions in the total ozone column on N2 fixation in eight traditional African commercial legume species representing crop, forest, medicinal, ornamental and pasture categories. In all categories examined, except medicinal, supplemental UV-B had no effect on root non-structural carbohydrates, anthocyanins and flavonoids, known to signal Rhizobiaceae micro-symbionts and promote nodule formation, or on nodule mass, activity and quantities of N fixed in different plant organs and whole plants. In contrast, in the medicinal category Cyclopia maculata (Honeybush) a slow growing commercially important herbal beverage with naturally high flavonoid concentrations, displayed decreased nodule activity and quantities of N fixed in different plant organs and whole plants with increased UV-B. This study’s findings conclude negligible impacts of ozone depletion on nitrogen fixation and soil fertility in most traditional African farming systems, these limited to occasional inhibition of nodule induction in some crops.
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    Rhizobial infection of African landraces of sorghum (Sorghum bicolor L.) and finger millet (Eleucine coracana L.) promotes plant growth and alters tissue nutrient concentration under axenic conditions
    (Springer Verlag, 2005) Matiru, V N; Jaffer, M A; Dakora, F D
    Seven strains of infective root-nodule bacteria, ("rhizobia"), namely Rhizobium GHR2,Bradyrhizobium japonicum Tal 110, Sinorhizobium meliloti strain 1, Rhizobium leguminosarum bv. viciae Cn6, R. leguminosarum bv. viciae strain 30, Rhizobium NGR234 and Azorhizobium caulinodans ORS571, were used to study the effects on growth of sorghum and finger millet seedlings cultured aseptically in Leonard jars with 1/2 strength Hoagland nutrient solution containing 1 mM KNO3. The use of scanning electron microscopy on 10-d-old plants revealed the presence of all 6 test strains on root epidermal surfaces as well as inside the tissues of inoculated, but not uninoculated, sorghum and millet roots. Applying root macerate prepared from surface-sterilized inoculated sorghum plant material successfully induced nodule formation and N2 fixation in soyabean seedlings, thus authenticating these internally located root tissue bacteria as rhizobia. Inoculating sorghum seedlings with 4 rhizobial strains (i.e. B. japonicum Tal 110,S. meliloti strain 1, R. l. bv viciae Cn6 and R.l. bv. viciae strain 30) significantly (P<0.05) promoted sorghum shoot growth by 11-51% on fresh weight basis and 8-55% on dry weight basis. There was also 21-32% increase in root length of inoculated sorghum plants compared to uninoculated control. Additionally, root tissue concentrations of P and K were markedly (P<0.05) increased by 17-250% in inoculated sorghum roots relative to uninoculated plants, while in shoots Zn and Cu were significantly (P<0.05) decreased. Bioassays of the test strains for indole acetic acid (IAA) showed that they produced biologically active concentrations of this growth-promoting molecule, which ranged from 0.18 to 2.26 µg IAA per ml culture filtrate. These findings suggest that rhizobial infection of cerals such as sorghum and finger millet can promote an increase in plant growth via improved P and K nutrition and possibly the release of metabolites such as IAA.