Observations on the ultrastructure ofFrankia sp. in root nodules ofDatisca cannabina L.

Summary The fine structures of the microsymbiont inside the root nodules ofDatisca cannabina have been studied by light, by transmission- and by scanning-electron microscopy. The endophyte is prokaryotic and actinomycetal in nature. The hyphae are septate and branched, diameter 0.3–0.5 m. The tips of hyphae are swollen to form electron-dense, clubshaped to filamentous vesicles, ranging in diameter: 0.4–1.4 m. The endophyte penetrates through walls of the cortial cells. The infected zone is kidney shaped and confined to one side of the acentric stele. The orientation of infection is reversed from other actinorhizae exceptCoriaria. The hyphae are near the host cell wall and vesicles are directed towards the central vacuole. Vesicles are aseptate and no collapsing of the vesicle cell wall (void area) has been observed. Vesicle clusters structures are globular with an opening at one side of the cluster. The host cell is multinucleate or contains a lobed nucleus. Groups of mitochondria are located in between the hyphae, suggesting a strong association between the host and the endophyte for energy supply and amino acid production. The consequences of the inability to separate the mitochondria from the vesicle clusters in nodule homogenates in physiological studies have been discussed.Isolated vesicles clusters showed dehydrogenase activity, indicated by the presence of formazan crystals, after incubation with NADH and NBT. Strongest reducing activity was found within the vesicles. The possible role of filamentous vesicles in nitrogen fixation has been discussed.     

Nitrogen fixation by subclover associations fertilized with sulfur

Summary The effect of S fertilization on symbiotic N₂ fixation was measured with the¹⁵N technique and the N difference method in a lysimeter study using Josephine loam (Typic Haploxurults). Nitrogen fixation by subclover (Trifolium subterraneum L.) was strongly enhanced by added S. The association of soft chess (Bromus mollis L.) or filaree (Erodium botrys (Cav.) Bertol.) with subclover increased the percentage of N in subclover that was fixed, with the results that N₂ fixation was increased beyond that due to the mere increase in subclover biomass. Nitrogen fixation estimates by¹⁵N dilution and N difference methods were highly correlated (r²=0.97), and S fertilizer did not result in any significant differences in N₂-fixation estimation by the two methods. Both methods were useful in distinguishing between soil N uptake and N₂ fixation where S applications produced highly significant increases in both uptake and fixation. Application of sulfur fertilizers to much annual rangeland has the potential to increase pasture productivity through enhanced N₂ fixation. Contribution of the University of California Hopland Field Station and Department of Agronomy and Range Science, Univ. of California, Davis, CA 95616.     

The effects of pH and aluminium toxicity on the growth and symbiotic development of cowpeas (Vigna unguiculata (L.) Walp)

Summary The interaction of pH (4 or 6), aluminium (0 or 16 ppm at pH 4) and N source (symbiotic or combined) on the growth and nutrient status of cowpea (Vigna unguiculata) was studied in a glasshouse experiment.Low pH significantly decreased the growth of the plants dependent on symbiotic nitrogen fixation but at pH 4 the addition of 16 ppm Al further depressed growth in both nitrogen regimes. Al-ions appear to exert their effect primarily on the root system, as shown by the reduction in total length and fresh weight. The symbiotic development of the plants was affected by low pH but more markedly by the Al treatment.Shoot nitrogen concentrations were reduced from ca. 2.6% at pH 6 to 1.8% and 0.9% at pH 4 without and with aluminium respectively. Calcium concentration was decreased by low pH and further by Al in both nitrogen regimes.In all Al-treated plants, the aluminium was mainly accumulated in the roots and was associated with an increase in their phosphorus concentration.     

Quantitative assessment of symbiotic nitrogen fixation in diverse mutant lines of field bean (Vicia faba minor)

Summary Homozygous mutant lines of field bean selected for (a) improved yielding potential and (b) altered plant architecture and/or physiological response were tested for symbiotic nitrogen fixing ability under field conditions in comparison with their parent cultivar.¹⁵N-tracer techniques were applied to determine %N derived from atmosphere. Data were collected on assimilate and nitrogen accumulation and distribution among various plant parts during two stages of reproductive growth.Symbiotic nitrogen fixation was closely correlated with total plant top biomass and nitrogen yield. A similar close association was found between crop yield and nitrogen harvest index. Both harvest indices tended to be negatively correlated with stage of maturity and with the amount of N derived from air per unit of area. The generally high %N derived from symbiotic N₂ fixation and its comparatively small variability implies that this parameter may be difficult to improve inVicia faba under field conditions.It is concluded, that the main genetic potential for improving the amount of biological nitrogen fixation in this crop depends upon factors that promote high photosynthetic productivity and efficient N-use under appropriate agronomic conditions and with effective rhizobial associations. The establishment of rational ideotypes with a possitive impact on yield appears to be of practical significance for increasing the amount of symbiotically fixed nitrogen.     

Seasonal patterns of ammonium and nitrate uptake in nine temperate forest ecosystems

Summary Seasonal patterns of net N mineralization and nitrification in the 0–10 cm mineral soil of 9 temperate forest sites were analyzed using approximately monthlyin situ soil incubations. Measured nitrification rates in incubated soils were found to be good estimates of nitrification in surrounding forest soils. Monthly net N mineralization rates and pools of ammonium-N in soil fluctuated during the growing season at all sites. Nitrate-N pools in soil were generally smaller than ammonium-N pools and monthly nitrification rates were less variable than net N mineralization rates. Nitrate supplied most of the N taken up annually by vegetation at 8 of the 9 sites. Furthermore, despite the large fluctuations in ammonium-N pools and monthly net N mineralization, nitrate was taken up at relatively uniform rates during the growing season at most sites.     

Optimization of L-phenylalanine production of Corynebacterium glutamicum under product feedback inhibition by elevated oxygen transfer rate.

Production feedback inhibition both on cell growth and on product formation of phenylalanine fermentation might be alleviated by elevated oxygen supply. Batch fermentations by a high phenylalanine producing strain Corynebacterium glutamicum CCRC 18335 at various initial phenylalanine concentrations (P(0)) ranging from 0 to 20 g/L and different oxygen transfer rate coefficients (K(L)a) ranging from 23 to 76 h(-1) were studied. The fermentation parameters with respect to P(0) were strongly dependent on K(L)a. Cell yield favored higher K(L)a and lower P(0). Product yield with respect to varying phenylalanine concentration was evaluated by the relative oxygen availability (ROA). The optimal ROA for phenylalanine formation was strongly dependent on the product concentration. While P(0) was low, the product inhibition was less significant and the maximum product yield occurred while ROA was at 0.5-0.6. While P(0) was high, the product inhibition was significant and the maximum product yield occurred while ROA was at 0.8-0.9. These results suggest that the product feedback inhibition of phenylalanine fermentation processes can be alleviated by a gradual increase in oxygen supply rate while the increasing product concentration is taken into account. The strategy is demonstrated in a fed-batch culture with elevated oxygen supply. The final phenylalanine concentration was 23.2 g/L, which was 45% better than that of the fed-batch fermentation without elevated oxygen supply. Likewise, the maximum productivity was improved by 42% at 0.37 g/(L x h).     

Selection of thermotolerant bradyrhizobial strains for nodulation of soybean (<Emphasis Type="Italic">Glycine max</Emphasis> L.) in semi-arid regions of Iran

Nodulation of soybeans grown in semi-arid region of southern parts of Iran is poor due to high air and soil temperatures. Here we identified thermotolerant isolates of soybean bradyrhizobia and evaluated the nitrogen fixation efficiency of the isolates under heat stress conditions in greenhouse and field experiments. The ability of fifty-six bradyrhizobial isolates to grow on solid or in liquid yeast extract mannitol medium at 38 and 41°C was evaluated. We identified 19 isolates, which were able to grow at 38°C and 10 isolates able to grow at 41°C. Greenhouse experiments were carried out at 28 and 38°C to study the nitrogen-fixing capacity of the isolates under optimal and high temperature conditions. Ten isolates had a symbiotic index of effectiveness of 80% or greater compared with nitrogen-fertilized treatments in greenhouse experiments at 28°C. Some thermotolerant isolates demonstrated good nitrogen-fixing performance at 38°C. Eight isolates were selected for use in a field trial in the natural high temperature environment of the Dezful region in Iran. Our results demonstrate that geographical origin can have a great influence on the successful selection of thermotolerant bradyrhizobia. Our thermotolerant isolates were mainly obtained from high-temperature regions, and improved shoot dry matter, nitrogen-uptake and seed yield of the plants.