Evaluation of Salt Tolerance in Azolla
The aquatic fern Azolla and the symbiotically associated cyanobacteria (blue-green alga) Anabaena azollae is a highly productive system for nitrogen fixation. The tolerance of several species and ecotypes of Azolla to increasing concentrations of salinity as NaCl was compared in environmental chamber studies using nitrogen-free medium. Growth of Azolla mexicana collected near Corrales, New Mexico, was not significantly inhibited in medium containing 50 milliequivalent (meq)/liter NaCl compared to the control without added NaCl. At 100 meq/liter NaCl yield of Azolla decreased to 42 percent of the control. Little growth occurred at 150 meq/liter NaCl. NaCl concentrations of 50 and 100 meq/liter correspond to very high salinity hazard irrigation waters that are unsuitable for ordinary irrigation use. Based on this investigation some saline waters appear to have application for the hydroponic production of Azolla as a nitrogen-rich organic material with potential for use as a feed or compost material.
Other species of Azolla and also stocks of A. mexicana collected from slightly saline waters in New Mexico were tested for NaCl tolerance. Several of these stocks were less salt tolerant than the Corrales A. mexicana and none demonstrated greater salt tolerance. Azolla was collected from the Bernardo, New Mexico area in water with a conductivity of 1.5 mmhos/cm; however, Azolla was not observed in more saline waters in either the Rio Grande or Pecos River valleys.
Azolla mexicana was found to be more tolerant of NaCl salinity than salinity imposed by addition of other salts including MgSO4, MgCl2, K2S04, KCl, Na2So4 or CaCl2. When A. mexicana was provided with fixed nitrogen in the medium, it was able to grow in 100 meq/liter NaCl without growth inhibition and at 150 meq/liter with 68 percent of the control yield. The symbiotic Anabaena and nitrogen fixation thus appear to be more sensitive to NaCl inhibition than the host fern when supplied with fixed nitrogen.