Assessment of symbiotic nitrogen nutrition in marama bean (Tylosema esculentum L.), a tuber-producing underutilized African grain legume

Marama bean (Tylosema esculentum L.) is a legume indigenous to Southern Afr ica and produces protein-rich seed and tubers with potential for use as hum an food and animal fodder. The source of N used by the plant has been asses sed. Field surveys combined with inoculation studies using rhizosphere soil s and compost under different conditions show that marama bean does not hav e nodules or nodulate. Values from analysis of N isotope ratios (N-15/N-14 and delta(15)N values), which indicate whether N is obtained from inorganic sources in the soil or from symbiotic fixation of atmospheric N-2 by Rhizo bium bacteria, ranged from 6.8 to 8.8 parts per thousand (7.9+/-0.6 parts p er thousand) for shoots, 5.9 to 9.8 parts per thousand (7.9+/-1.6 parts per thousand) for tubers and 8.05 to 10.12 parts per thousand (8.96+/-0.87 par ts per thousand) for seeds of plants sampled fi;om the field at Roodeplaat near Pretoria, South Africa. Plants from Sandveld Research Station near Gou babis, Namibia, had mean values of 5.9+/-0.3 parts per thousand for shoots, 7.0+/-0.06 parts per thousand for tubers and 5.3 parts per thousand for se eds. These values are larger than those of three nodulating N-2-fixing Acac ia species, which ranged from -4.0+/-0.16 parts per thousand for roots to - 1.4+/-0.45 parts per thousand for leaves, and also higher than those of see ds of bambara groundnut (-0.17 to 4.5 parts per thousand, mean = 1.9+/-0.5 parts per thousand) with active symbiotic N-2 fixation. The marama bean del ta(15)N values are comparable to those of soil N (5 to 15 parts per thousan d) and to two non-N-2-fixing cereal plants, sorghum (8.3 to 10.3 parts per thousand, mean = 9.5 +/- 1.0 parts per thousand) and pearl millet (7.9 part s per thousand). The delta(15)N values for marama bean plants were similar to those of the related species Tylosema fassoglense and Bauhinia petersian a. Absence of root nodules on field-grown plants and similarity-of delta(15 )N to soil values and non-N-2-fixing cereals and dissimilarity to symbiotic N-2-fixing legumes together show that marama bean uses N from the soil, an d not from symbiotic fixation of atmospheric N-2.