Effects of Humic Acid Application on Growth and Biomass Production of Azolla Pinnata in Symbiosis with Anabaena Azollae Under Semi-Arid Conditions

Azolla pinnata is a fast-growing aquatic fern known for its unique ability to form a symbiotic relationship with the nitrogen-fixing cyanobacterium Anabaena azollae. This association allows it to capture atmospheric nitrogen, which promotes the development of biomass rich in protein and nutrients. This symbiotic relationship represents an interesting biological model for semi-arid regions. Our study aims to evaluate the effect of humic acid, alone or combined with manure, on the growth of A. pinnata associated with A. azollae under a semi-arid climate. The materials used in the experiment were plastic containers filled with soil, water (distilled or well water), and fresh A. pinnata associated with A. azollae, subjected to three concentrations of humic acid (2, 4, and 6 g.L-1), with or without manure, over a 7-day period in a semi-arid climate. The results showed that the T0/C2 treatment (+ 4 g.L-1 humic acid without manure) induced strong growth (+48.6% biomass), accompanied by an increase in protein concentrations (21.54 mg. g?¹ FM) and carbohydrate concentrations (96.5 mg. g?¹ FM), reflecting improved photosynthesis and symbiosis under semi-arid conditions. In contrast, the T+/C3 treatment (manure + high humic acid) reduced growth, likely due to an excess of organic matter disrupting osmotic balance. The positive correlations observed were for protein (r = 0.904), carbohydrates (r = 0.779), chlorophyll content (r = 0.724) and negative correlation are observed for a stress marker proline (r = –0.649). In conclusion, humic acid, when used in moderate doses, stimulates the growth of A. pinnata in association with A. azollae under unfavorable conditions.