- Soil science and plant nutrition (ISSN:00380768)
- vol.38, no.1, pp.65-73, 1992-03
The growth of Azolla in the field is often limited by the phosphorous supply. To develop a screening technique for the ability of Azolla to grow and fix N_2 more efficiently under P-deficient conditions, water culture experiments were conducted under concentration-controlled conditions. Azolla was grown in a 60-liter bucket with a P concentration ranging from 0.02 to 0.06 ppm. At 0.03 ppm, Azolla became P-deficient. Under P-deficient condition, Azolla pinnata strains #5 and #7001 produced a larger amount of biomass and showed a lower tissue P concentration than A. microphylla #4018 and A. mexicana #2026. The P concentration in the culture medium decreased from 0.03 to 0.02 ppm during the growth of Azolla. A device for the supply of acid or alkaline and P solutions was designed to maintain a constant pH and P concentration in the water culture medium. Even when the P concentration was kept at 0.03-0.035 ppm, Azolla suffered from P deficiency. Under this condition, the A. microphylla, A. mexicana, A. filiculoides, and A. nilotica strains produced a smaller amount of biomass and showed a higher tissue P content than the A. pinnata strains. Growth responses of Azolla species under P concentration-limited conditions were, thus, similar to those under quantity-limited conditions. Quantity-limited water culture is more convenient for screening a large number of Azolla strains for their ability to grow under P-deficient conditions.