Differential phosphorus requirements ofAzollaspecies and strains in phosphorus-limited continuous culture

Open Access
Publisher Website
Google Scholar
Abstract
Phosphorus represents a major limiting factor in the field for the growth of the Azolla-Anabaena symbiotic nitrogen-fixing system. The yield of fresh matter increased up to 5 ppm P in batch culture, following a quadratic type of relationship. However, the tissue phosphorus content increased with each incremental increase in phosphorus concentration. The threshold concentration of the phosphorus in Azolla appeared to be 0.2 to 0.3%. Below that, plant growth was proportional to the phosphorus content. A continuous-flow device for determining the minimum phosphorus level was developed. Azolla pinnata from Bangkok grew normally at 0.06 ppm P. At 0.03 ppm P, the biomass, contents of phosphorus, nitrogen, and chlorophyll, and the acetylene-reduction activity were decreased. Further phosphorus increases of more than 0.06 up to 0.12 ppm in the medium had no impact on the growth of Azolla. Eleven Azolla strains, including A. pinnata, A. mexicana, A. caroliniana, and A. filiculoides, were compared at the limiting concentration of phosphorus (0.03 ppm) in continuous-flow culture. Substantial differences among the species and strains were noted. For healthy growth, different species required widely differing phosphorus concentrations in the water medium. A. pinnata from Bangkok produced maximum yield and maximum frond area, and exhibited the better nitrogen-fixing capacity in the phosphorus limiting condition than did the other species compared. Other A. pinnata strains grew better in the phosphorus limited condition than the New World Azolla species.