Nitrogen fixation inAzolla-Anabaenasymbiosis as affected by mineral nutrient status

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Abstract
The growth and nitrogen-fixing capacity of Azolla imbricata were studied with special reference to the effects of the mineral nutrient status in the medium. The threshold levels of P, K, Mg, and Ca in the medium for Azolla growth were ca. 0.03, 0.4, 0.4, and 0.5 mmol · liter-1, respectively. Full development of nitrogenase activity was not realized at concentrations below 0.03, 0.6, 0.5, and 0.5 mmol · liter-1, respectively. Combined nitrogen was unfavorable for growth and nitrogen fixation. Deficiency of any one of the micronutrient elements, Fe, Mn, Co, Zn, Cu, Mo, and B, had an unfavorable effect on growth and developing nitrogenase activity. The threshold levels of Fe, Mn, Mo, and B for growth were 50, 20, 0.3, and 30 μg · liter-1, and those for nitrogenase activity were 20, 10, 1, and 20 μ · liter-1, respectively. Fe and Mo were very sensitive for developing nitrogenase activity. On the other hand, B was shown to affect growth specifically. The nitrogen-fixing capacity measured from the increase in total nitrogen in Azolla plants cultured with complete nutrient solution lacking only combined nitrogen, was 6.50 mg N·g-1 (D. W.)·day-1. On the other hand, the acetylene-reducing capacity of the plants was 34.6 μmole C2H4 produced·g-1 (D.W.)·hr-1. These two values are not so different from one another. The very low nitrogen-fixing capacities reported by pioneer workers were found to be raised considerably when the data were recalculated on the basis of the daily increase ratio in growth. Illumination (up to 6,000 lux) of the plants during the period of acetylene reduction, greatly enhanced nitrogen fixation. This effect is discussed in relation to the saturation point in photosynthesis.