The growth of four species of Azolla as affected by temperature

The growth of 22 strains of Azolla pinnata R. Br., 3 strains of A. filiculoides Lam. and one strain each of A. mexicana Presl and A. caroliniana Willd. was tested separately in liquid culture media kept in controlled, artificial light (30 klux) growth cabinets. Three temperature levels were used: 33°C (37/29°C day/night), 29°C (33/25°C) and 22°C (26/18°C)/ Photoperiod was 12 h a day.

For most A. pinnata strains (except three) and an A. mexicana strain the maximum weekly relative growth rate was higher at 33°C than at 22°C, but not for A. filiculoides and A. caroliniana. The highest value of maximum relative growth rate corresponded to 1.9 doubling days and in most strains this occurred in the first week. As the plants grew, the growth rate slowed down more severely at higher temperatures. The maximum biomass was higher at 22°C than at 33°C in all strains. At 22°C, it took 30-50 days to attain maximum biomass and the highest value was 14 g N m ^-2 or 320 g dry m ^-2 by A. caroliniana, followed by 12 g N m ^-2 or 290 g dry wt. m ^-2 by one strain of A. filiculoides. At 29°C, the maximum biomass was attained in 20-35 days. The highest value was 6.3 g N m ^-2 or 154 g dry wt. m ^-2 by A. caroliniana. At 33°C, most A. pinnata strains gave a maximum biomass of less than 4 g N m ^-2 after 13-23 days, while some strains grew up to 30 days, resulting in a higher maximum biomass. The highest maximum biomass at 33°C was 5.5 g N m ^-2 or 140 g m ^-2 dry wt. by A. pinnata from Cheng Mai while the maximum biomass of A. filiculoides and A. caroliniana was much less. Azolla filiculoides requires lower temperature than other species for its growth. Azolla pinnata has the best tolerance to high temperatures among the four species. Azolla mexicana could not be discriminated from A. pinnata in its response to temperature. Azolla caroliniana may keep an intermediate position between A. filiculoides and A. pinnata in temperature response.

The formation of ammonia in the medium was examined and it occurred mostly under stationary growth conditions, but, at 33°C, some strains of A. pinnata and A. mexicana released or formed ammonia at 0.3-0.8 μg N ml ^-1 per week during their initial exponential growth stage.