Azolla, a free-floating, N2-fixing aquatic fern, is an established N biofertilizer for flooded rice. Phosphorus (P) is the most critical and limiting input for Azollarice cultivation (Majumdar et al 1993). Azolla absorbs P from the floodwater and makes it available to the plant. Different species of Azolla have different P requirements. An efficient P-scavenging strain is needed to ensure an adequate P supply.
We tested the P-scavenging ability of A. microphylla, the most efficient strain under north Indian climatic conditions. A. microphylla was obtained from germplasm collected at IARI. It was grown and maintained in N-free Espinas and Watanabe (E&W) medium in 8 × 10 × 2-in trays at 30 ± 2 ºC in a polyhouse (Watanabe et al 1977). To avoid drying, trays were topped with fresh medium every other day. The effect of P concentration on A. microphylla was studied using P-starved inoculum (Azolla fronds maintained in E&W medium minus P for 14 d). We also grew 0.5 g of P-starved fronds in 200-mL E&W media supplemented with different amounts of potassium dihydrogen phosphate to give 1-20 ppm P in 500 mL glass beakers.