Research on cereal nitrogen fixation at ICRISAT [India]

The acetylene-reduction assay for nitrogenase activity has shown that the roots of sorghum and pearl millet stimulate N2 fixation. A soil-core assay used for measuring nitrogenase activity of field-grown plants showed large plant-to-plant variability. An improved core assay (planted core assay) developed at ICRISAT Center showed higher activity than that recorded by regular core assay. In greenhouse assay methods plants with shoots sustained higher activity than the ones whose tops were cut before the assay. Nitrogenase activity varied throughout the day, as well as over the season. Activity was maximum at the late flowering or early grain-filling stage and it was related to the ontogenetic development of the plant. Activity was favored in moist, warm (ca 35 deg C) soil and decreased with high levels of fertilizer N. Genotypic variation in sorghum and pearl millet lines for stimulating rhizospheric nitrogenase activity was observed. Out of 284 pearl millet lines tested, 18 lines stimulated high nitrogenase activity (460 nmol C2H4 h** (-1) 15 diam core** (-1)) in the rhizosphere. Two lines, Gam 73 and J 1407, were consistently active over several seasons. Similarly, 28 of 334 sorghum lines tested had high nitrogenase activity (460 nmol C2H4 h** (-1) 15 cm diam core** (-1)). At ICRISAT Center, pot-culture experiments with sorghum and pearl millet grown in a low-fertility Alfisol, or in unsterilized washed vermiculite, showed substantial positive balances for N. Long-term N balance studies in the field with sorghum and pearl millet cultivars are continuing. Using 15N2 it has been shown that 20-days-old sorghum seedlings fix N2 in the rhizosphere, and part of it is taken up by the plant within 3 days after exposure. The 15N isotope dilution technique has been evaluated for studying genotypic variation in sorghum and pearl millet cultivars for N2 fixation. Many different kinds of bacteria closely associated with the roots of sorghum and pearl millet plants show nitrogenase activity. Responses to inoculation in terms of increased dry-matter production and N uptake have been observed in pot experiments with sorghum and pearl millet grown in Alfisols and vermiculite. In five out of nine field trials with pearl millet cultivars, inoculations with N2-fixing bacteria significantly increased the grain and plant dry-matter yields in all the cultivars