OPTIMUM CONDITIONS FOR GROWING AZOLLA
Carrapiço and Pereira (2009) identified the IRRI2 medium, which was developed by the International Rice Research Institution (IRRI), as the best nutrient medium for promoting Azolla’s growth. Table 1 lists the nutrient medium concentrations in the IRRI2 medium as compiled by Francisco Carrapiço of the University of Lisbon
Water acidity (pH)
Azolla can survive within a pH range of 3.5 to 10. It cannot grow in acidic soils with a pH below 3.5 (Singh, 1977). Optimum pH for the IRRI2 medium is between 5.5 to 6.5
Azolla is also able to tolerant a wide range of temperatures and some species can survive in temperatures as low as -5oC. The growth of Azolla is typically reduced above 35oC and no species can survive if temperatures remain above 45oC for prolonged periods of time.
The optimum temperatures for most species are between 18oC and 28oC, although this can be as high as 30oC for species such as A. pinnata, A. mexicana, and A. caroliniana.
Light affects the photosynthesis and regulate nitrogenase activity in Azolla and Anabaena.
Azolla species generally grow best in less than full sunlight except in high latitudes during spring.
Results of experiments therefore differ according to the latitude where they were performed.
Under high sunlight intensities Azolla fronds turn brick red. Low light intensities, for example under a dense growth of rice, cause Azolla to suffer or die.
The optimum photoperiod for Azolla growth is 20 hours and the recommended illumination is 380E/m2/s with a 20 hour photoperiod (Kannaiyan & Kumar, 2005).
Azolla requires all the essential elements that are required by other plants, plus molybdenum (an essential constituent of nitrogenase) and cobalt, which are required for Anabaena’s nitrogen fixation.
Normally, nutrients must be available in the water, though in very shallow water Azolla may extract some nutrients from the soil. Adequate nutrient levels should be maintained throughout the period of growth.
Although Azolla does not require nitrogen in its growing medium, the level of nitrogen in the water affects its growth and nitrogen-fixation rates because nitrogen is assimilated by both Anabaena and Azolla from the atmosphere and from the water respectively. Relative growth rates of A. caroliniana and A. pinnata are higher at a nitrate level of 5 mM than in a nitrogen-free medium, but higher levels of nitrate reduced growth (Singh et al., 1992).
Phosphorus is the most important and often the limiting nutrient for Azolla growth. Its deficiency is indicated by smaller, less vigorous plants and may cause the plants to become pink to deep red and fragile and to develop very long roots.
Iron is a common limiting element in Azolla’s growth because it is an essential constituent of nitrogenase. Iron-deficient plants become yellow due to the depletion of chlorophyll. Roots become thin and whitish.
Calcium-deficient Azolla show an intense reddening of the dorsal lobes and fronds become fragmented. Roots become short, thin, and light in color.
In potassium-deficient plants, Azolla’s fronds become yellowish brown and its roots become dark brown and their growth is stunted.
Optimum threshold nutrients levels
Yatazawa et al. (1980) determined the threshold nutrients levels for Azolla growth and nitrogenase activity as follows:
Threshold macronutrients levels for Azolla growth.
• P: 0.03 mmol 1-1
• K: 0.4 mmol 1-1
• Mg: 0.4 mmol 1-1
• Ca: 0.5 mmol 1-1
Threshold macronutrients levels for full nitrogenase activity:
• P: 0.03 mmol 1-1
• K: 0.6 mmol 1-1
• Mg: 0.5 mmol 1-1
• Ca: 0.5 mmol 1-1
Threshold levels of the micronutrients for Azolla growth:
• Fe: 50 g1-1
• Mn: 20g1-1
• Mo: 0.3g1-1
• B: 30g1-1
Azolla is susceptible to attack by pests such as lepidopterous or dipterous insects or fungal diseases, particularly during hot, humid periods, and snails are a common pest for Azolla plants grown in rice fields
Different pesticides have varying effects on Azolla.
Molinate reduced the growth and nitrogenase activity of A. pinnata but increased its chlorophyll content, whereas carbofuran significantly increases its chlorophyll content and nitrogenase activity, but does not affect its growth (Ismail et al., 1995).