INFLUENCE OF DIFFERENT PHOSPHORUS MANAGEMENT STRATEGIES ON THE SPORULATION AND GROWTH OF AZOLLA

Abstract

Phosphorus at the recommended dose of 10 kg P2O5 ha−1 divided between three equal applications, one on each of 0, 7 and 14 d after Azolla inoculation (DAI), increased biomass production but sporulation was invariably inhibited. Of the different options tested, lowering the phosphorus application rate to 4–8 kg P2O5 ha−1 significantly improved sporulation in A. microphylla (strain 202) but there was a substantial reduction in biomass production. On the other hand, changing the schedule of phosphorus application from 0, 7 and 14 DAI to 0, 3 and 6 DAI did not hamper biomass production and improved sporulation frequency and sporocarp number in A. microphylla (strain 202), A. caroliniana and A. pinnata. It was comparable to the no-P treatment for the number of sporocarps, with slightly lower sporulation frequency. In these species, the sporulation frequency and sporocarp number of Azolla enriched with 30 or 60 kg P2O5 ha−1 and then grown without any further added phosphorus were higher than those of unenriched Azolla grown with 10 kg P2O5 ha−1 and mostly comparable to those of Azolla grown without phosphorus. Foliar spray of 2.5 μg ml−1 gibberellic acid (GA) solution (7 DAI) along with the application of 10 kg P2O5 ha−1 (split between 0, 7 and 14 DAI) to unenriched Azolla increased the sporulation frequency and number of sporocarps in A. microphylla (strains 202 and 203), A. caroliniana and A. pinnata, not only over that of phosphorus application alone but also over the untreated control. Combining the use of P-enriched Azolla (A. microphylla strains 202 and 203) with application of GA was more effective for increasing sporulation than was the use of P-enriched Azolla without GA, or application of phosphorus plus GA to unenriched Azolla.