Abstract
Reports of diminished nodule formation and nitroge-nase 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 N 2 -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 N 2 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, antho-cyanins 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.
Highlights
In Africa, the intrinsically low fertility and heterogeneous nature of soils is a major limitation of crop plant yields (Munns and Franco 1981)
Increased concentration of phenylpropanoids in UV-B irradiated leaves (Tegelberg and Julkunen-Tiitto 2001, Kolb et al 2001) and their subsequent accumulation in the roots may promote nodule formation (Muofhe and Dakora 1999), since some phenylpropanoids, flavonoids, have been reported to act as plant signals to symbiotic bacteria in the Rhizobiaceae (Dakora and Phillips 1996, Phillips 2000, Cullimore and Dénarié 2003). In view of these conflicting hypotheses and the important role of N2 fixation in maintaining soil fertility and sustainable yields in African agriculture, we examined the effects of increased levels of UV-B radiation on N2 fixation in some traditional African legumes representing five commercial categories
Vigna unguiculata seeds were inoculated with Bradyrhizobium strain CB756, G. max seeds with B. japonicum strain CB 1809, P. vulgaris seeds with Rhizobium leguminosarum bv
Summary
In Africa, the intrinsically low fertility and heterogeneous nature of soils is a major limitation of crop plant yields (Munns and Franco 1981). Recent reports of substantial (up to 70%) reductions in nitrogenase activity and nodule production in some Asian tropical legumes, such as Vigna radiata and Phaseolus mungo, exposed to elevated levels of ultraviolet-B (UV-B: 280–315nm) radiation (Singh 1997) have raised concerns for biological N2 fixation, since sustained depletion of the stratospheric ozone has led to increased levels of solar UVB radiation in the troposphere of both the Southern and Northern Hemispheres (McKenzie et al 1999). Ozone recovery remains uncertain due to increased CFC production in some developing countries (Fraser and Prather 1999)
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