Study of the Physiology of Azolla and Its Use as a Green Manure for Rice

The evaluation of Azolla species as a nitrogen source for rice crops at a particular location is complex. Prevailing climate, which affects nitrogen fixation, must be known so that the opportunity to enhance the rice yield is maximized. Selecting the species adapted to a specific agricultural environment is necessary before implementing other efforts in the management of azolla. Five species were evaluated as being relatively tolerant to intense light and high temperature. Nitrogen fixation capability and other physiological characteristics of these species were evaluated in a series of experiments. Nitrogenase activity and growth of A. filiculoides, A. caroliniana, A. microphylla, A. pinnata and A. mexicana in response to light were studied under greenhouse conditions. Nitrogenase activity increased with increasing light and with the increase in total biomass of the azolla mat. A maximum biomass accumulation of + 2,500 g m-2 was obtained at either high or low initial inoculum. Nitrogenase activity increased with increasing frond area index. When azolla is intercropped with rice, light available to azolla depends on the population density of the rice plants. In the field experiment, different population densities of rice were obtained when rice seedlings were transplanted at different spacings. The relative growth rate (RGR), fresh weight and nitrogen accumulation were highest when azolla was intercropped with rice at a spacing of 20 cm x 20 cm than those either at a spacing of 25 cm x 25 cm or at a spacing of 20 cm x 15 cm. A. pinnata accumulated significantly more fresh weight and nitrogen than A. caroliniana. However, the increase in rice yield due to either A. pinnata or A. caroliniana fertilization was not significantly different. In the present study, average increases in grain yield over control plots were 27.8 and 38%, respectively, for plots fertilized with azolla only and plots fertilized with azolla plus 25 kg N ha-1 of urea. The benefit of azolla to the rice crop was equivalent to 25 kg N ha-1 nitrogen fertilizer. The physiological efficiency for azolla-treated plots with the addition nitrogen fertilizer was higher than for either azolla only or nitrogen fertilizer only. Rice yield components are usually influenced by the population density of the rice plants. Nutrient, light and space available to the individual tiller at a dense rice population might limit the development of yield components. In this experiment, the development of yield component was great at the spacing of 20 cm x 20 cm, therefore rice yield was the highest at the spacing treatment. A. filiculoides, A. pinnata and A. mexicana responded similarly to temperature and light treatments. However, A. pinnata had a relatively higher CER over time than did filiculoides or A. mexicana. A. filiculoides was consistently superior to the other two species in dry matter accumulation. Having a vertically growing fronds, A. filiculoides was able to grow when space was limited and growth in the other two species was retarded due to mutual shading. The carbon dioxide exchange rate per unit weight (CERw) and per unit frond area (CERA) was positively correlated with coefficients of correlation ranging from 0.65 to 0.98. The CERW and the CERA were positively correlated with coefficients of correlation ranging from 0.65 to 0.98. The CERW and the CERA were positively related to both dry matter content and relative growth rate at 5, 10, 25 and 30 DAI. The photosynthetic rates of A. mexicana, A. filiculoides and A. pinnata was optimum at 25°C. The CER increased with increasing light intensity and decreased as temperatures increased beyond 25°C. Significant interaction effects between photosynthetic photon flux density (PPFD), thermoperiods and species were not consistently observed either on the relative growth rate (RGR) or on the carbon dioxide assimilation rate per unit fresh weight (CERW). For the RGR and the CERW of azolla, the interactions were only significant, respectively, at 5 and 15 days after inoculation (DAI). However, for a successful performance of azolla, as indicated by high biomass production and high nitrogen accumulation, an integral effect of all environmental factors on the growth and nitrogen fixation has to be taken into account