Nitrogenase Activity Associated with Roots and Stems of Field-Grown Corn (Zea mays L.) Plants

Abstract

Collectively, the results provided strong evidence for the occurrence of nitrogenase activity associated with corn plants growing in a temperate climate and dependent upon indigenous N2-fixing bacteria.In corn roots, maximal nitrogenase activity has been reported to be associated with a variety of plant factors: stage of development, plant genotype, low N levels, high temperatures, etc. (1,5,6,13,15).In all of these studies, nitrogenase activity was measured, using excised roots, after prolonged preincubations (12-18 h) at reduced 02 tensions. The limitations of the use of preincubated roots have been discussed in recent reviews (13,19). Bacterial proliferation has been demonstrated to occur during long preincubation times at low 02 and interpreted as an overestimation of the actual nitrogenase activities occurring in situ (3,18). Most important, however, has been the confirmation of nitrogen fixation in grassbacteria associations by 15N2 incorporation into plant parts (7) and the recent report of immediate acetylene reduction by excised grass roots, assayed under air (21).

Manganese-resistant mutants ofAzospirillum brasilense: their response of associative nitrogen fixation, dry-matter production and nitrogen content of cheena (Panicum miliaceumL.) to different amounts of applied nitrogen in acid soil

“…1983). Azospirillum brasilense has been reported to be an associative N 2 fixer with roots of maize, rice, chick pea, minor millets and lentil (Barber, Russel & Evans, 1979;Rai, Prasad & Shukla, 1984) and the N 2 -fixing ability of different strains of A. brasilense was associated with soil types, environmental factors, nitrogen status of the soil, crop and crop varieties (Rai, 1985a;De-Polli, Boyer & Neyra, 1982). Grain yield of rice genotypes increased when inoculated with manganese-tolerant strains of A. brasilense at low levels of nitrogen in acid soil (Rai, 1986).…”

Endogenous Ethylene Production Is a Potential Problem in the Measurement of Nitrogenase Activity Associated with Excised Corn and Sorghum Roots

“…Measurements of immediate nitrogenase activity by freshly collected grass roots are desirable because they may more closely reflect the presence of and the rate of activity associated with the roots before sampling. Low rates of acetylene reduction by corn and wheat roots have been reported using measurements made within 1 to 2 h of their excision from plants (4,5,12). We are concerned that ethylene attributed to acetylene reduction by nitrogenase may actually be endogenous ethylene produced by root-associated bacteria or the excised roots themselves.…”

“…The presence of endogenous ethylene production during acetylene reduction assays was demonstrated by using either CO to inhibit nitrogenase activity or chloramphenicol to inhibit nitrogenase synthesis in freshly excised roots. problem in measuring acetylene reduction with soil cores having low nitrogenase activity (3,17). This has not been considered as a problem with excised roots because endogenous ethylene production has not been detected with samples incubated in air without acetylene (4,5,7,12). Ethylene production in soils is not detected during aerobic incubations because the rate of ethylene oxidation is greater than ethylene production (1).…”

Aluminum-tolerant strains of Azospirillum brasilense and their associative nitrogen fixation with finger millet (Eleusine coracana) genotypes in an acid soil.

“…Azospirillum colonization and proliferation on and in the roots are affected by plant genotypes and their interactions with Azospirillum strains (19). The Azospirillum-root association and functions are affected by soil type, environmental conditions and the fertility status of the soil (4,5,20). Biologists are increasingly concerned about the emergence of pollution resistant strains by either physiological adaptation or mutations.…”