Miyake and Wada (8) and Sweeney et al. (10) reported that the nitrogen enrichment between nitrate and phytoplankton was null.The purpose of the experiment presented in this study was to investigate the source of the discrepant results described above by studying the effect of time during the growing season and external N03 concentration on isotopic fractionation associated with N03 uptake.The results given in this paper suggest that the apparent discrepancy of the previous reports are not surprising. It will be shown in this paper that the observed isotope effect for NO3 uptake varies systematically in response to environmental conditions which have an impact on the relative rates of the relevant reactions. (15N/14N) of the increment of product which appears in an infinitely short time at time t, and R, the isotope ratio of the substrate at the same time. MATERIALS ANDThe isotope enrichment factor is:(p/ff= (ap/8 -1) X 1,000When the quantity of substrate is infinite compared to the quantity of product (6), the isotopic ratio of the product Rp is constant.

This paper expands upon previous reports of '5N elevation in nodules (compared to other tissues) of N2-fixing plants. N2-Fixing nodules of Glycine max (soybeans), Vigna unguiculata (cowpea), Phaseolus vulgaris (common bean), Phaseolus coccineus (scarlet runner bean), Prosopis glandulosa (mesquite), and Olneya tesota (desert ironwood) were The N of whole soybean plants which are grown with atmospheric N2 as the sole source of N has an isotopic composition within approx. 2 6'5N units2 of atmospheric N (2, 8). In contrast, we have consistently observed that soybean nodules are significantly enriched in 1 N (7,14). This observation has been recently confirmed by Turner and Bergersen (15). The difference in 6'5N between soybean nodules and whole plants ranged from +2.8 to +12.8, with an average (for 59 observations) of +8.3. The 15N abundance of other plant parts (roots, stems, foliage, pods, seeds) was much more uniform (7,14). The largest difference between any of the other plant parts was only about 2 815N units, a modest difference in comparison with the usually quite large difference between nodules and other plant parts. The homogeneity of isotopic composition of nonnodular tissue persisted throughout the growing season, even during times of massive mobilization and transport of N from vegetative to reproductive tissues, a time at which isotopic fractionation might be expected to result in the alteration of isotopic abundances. By measuring the '5N abun- strongly correlated with N2-fixing efficiency (9).Kohl et al. (9) previously reported that the 15N abundance of nodules from nonleguminous N2-fixing plants was not elevated. Certainly this difference in nodular 15N abundance between soybeans, with its rhizobial symbiont, and nonlegumes, which are infected with actinomycetes, is a reflection of differences in the metabolism of fixed N. We report here results which contribute to answering the question: How widespread is the elevation of 15N abundance in nodules and what metabolic characteristics distinguish those plants which do and which do not have nodules with elevated 15N abundance? On the basis of previously reported results Kohl et al. (9) rejected the hypothesis that elevated 15N in nodules is the result of the denitrifying capability of the symbiont. Instead they proposed that the 5N abundance in nodules is a result of the difference between isotopic fractionation associated with the synthesis of nodule tissue and that which accompanies the concurrent synthesis of the main form of N transported from the nodule to the rest of the plant. If this hypothesis is correct, then one might expect the "5N abundance of nodules (compared to the rest of the plant) to vary among plant species, depending on the form of the major transport compound. METHODSWe analyzed the concentration and 15N abundance of the total N of various tissues of a number of N2-fixing plants (both legumes and nonlegumes) by methods previously described (12,13

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The fine structure of three unclassified strains of Humicola and of H. grisea has been investigated. The hyphae of all the strains show septa with Woronin bodies of the ascomycetous type. The cytoplasm contains many nuclei per cell, mitochondria, ribosomes, and endoplasmic vesicles, all typical of fungal cells. Electron-microscopic studies of thin sections of mature aleuriospores reveal a thick multilayered cell wall and an accumulation, inside the spore, of β-hydroxybutyrate granules. Aleuriospores exhibit different types of cell surface; the outer wall layer of two strains is smooth, while the outer layer of the other strains is rough because of the presence of melanizing bodies on the cell wall matrix. The fine structure of phialospores and microconidia is also described. Differences in the fine structure among the strains studied are reported.