Differential responses of azolla to phosphorus deficiency

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Abstract
Azolla growth in the fields is often limited by phosphorus supply. A screening technique for Azolla to grow and fix N2 more efficiently under P-deficient conditions was developed in water culture experiments using 3 species of Azolla under quantity-controlled conditions. Azolla was grown with a P supply ranging from 0.17 to 13.9 μg.cm-2.day-1. At a P supply lower than 1.39 μg.cm-2 day-1, Azolla became deficient in P with tissue P contents lower than 0.2-0.25%. Growth, N content, and N2 fixation decreased with the decrease in the P supply. Under P-deficient conditions, Azolla pinnata var. pinnata #7001 and A. pinnata var. imbricata #5 showed lower P contents, grew better, and fixed a larger amount of N2 than A. mexicana #2026 and A. microphylla #4018. The ratio of N/P in the tissues was also higher in A. pinnata. At a P supply higher than 1.39 μg.cm-2, A. microphylla #4018 exhibited a better growth and N2 fixation than the other strains. The growth responses of different Azolla strains to P deficiency at a P supply of 0.174 μg.cm-2.day-1 were similar regardless of the inoculation with P-starved or enriched Azolla. From the third week, when P deficiency symptoms appeared, A. pinnata strains produced a larger amount of biomass and fixed a larger quantity of N2 than the Euazolla section species (one A. mexicana and five A. microphylla strains). Biomass was harvested weekly and either half or the fixed quantity of biomass was inoculated. By the repetition of this procedure, biomass production approached a steady state. Both methods gave a similar ranking for the strains in terms of growth under P-deficient conditions.