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IYXX. Oxygen limitation of N, fixation in stem-girdled and nitrate-treated soybean. -Physiol. Plant. 73: 11S121.The effects of increasing rhizosphere PO, on nitrogenase activity and nodule resistance to O2 diffusion were investigated in soybean plants [Glycine max (L.) Merr. cv. Harosoy 631 in which nitrogenase (EC 1.7.99.2) activities were inhibited by (a) removal of the phloem tissue at the base of the stem (stem girdling), (b) exposure of roots to 10 mM NOj over 5 days (NOj-treated). or (c) partial inactivation of nitrogenase activity by an exposure of nodulated roots to 100 kPa O2 (O?-inhibited). In control plants and in plants which had been treated with 1 0 kPa 02, increasing rhizosphere 0, concentrations in 10 kPa increments from 20 to 70 kPa did not alter the steady-state nitrogenase activity. In contrast. in plants in which nitrogenase activities were depressed by stem girdling or by exposure to NOj. increasing rhizosphere p 0 2 resulted in a recovery of 57 or 67%. respectively. of the initial, depressed rates of nitrogenase activity. This suggests that the nitrogenase activity of stemgirdled and NO;-treated soybeans was O,-limited.O2 at 20 kPa and an apparent insensitivity of diffusion resistance to increases in external PO?.

The closed acetylene reduction assay has been used as a measure of nitrogenase activity and an indicator of N, fixation in Rhizobium/legume symbioses for 25 years. However, starting 10 years ago this assay has come under harsh criticism as being inaccurate. Currently, confusion exists regarding the conditions under which the acetylene reduction assay can be used accurately, or whether it can be used at all as a measure of nitrogenase activity. This article reviews the circumstance that has lead to this confusion. The author argues that under the proper assay conditions and with the appropriate checks, the closed acetylene reduction assay is still a valuable tool in assessing relative differences in nitrogenase activity in Rhizobium/legume symbioses.

In the mutualistic symbioses between legumes and rhizobia, actinorhizal plants and Frankia, Parasponia sp. and rhizobia, and cycads and cyanobacteria, the N 2 -fixing microsymbionts exist in specialized structures (nodules or cyanobacterial zones) within the roots of their host plants. Despite the phylogenetic diversity among both the hosts and the microsymbionts of these symbioses, certain developmental and physiological imperatives must be met for successful mutualisms. In this review, phylogenetic and ecological aspects of the four symbioses are first addressed, and then the symbioses are contrasted and compared in regard to infection and symbio-organ development, supply of carbon to the microsymbionts, regulation of O 2 flux to the microsymbionts, and transfer of fixed-N to the hosts. Although similarities exist in the genetics, development, and functioning of the symbioses, it is evident that there is great diversity in many aspects of these root-based N 2 -fixing symbioses. Each symbiosis can be admired for the elegant means by which the host plant and microsymbiont integrate to form the mutualistic relationships so important to the functioning of the biosphere.

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