Effect of salinity stress on growth, chlorophyll, antioxidant enzymes and nutrient content in Azolla spp.

Azolla is an aquatic fern that has a symbiotic association with nitrogen-fixing cyanobacteria. It is mainly used as a biofertilizer in rice; however, its potential under salt-affected rice cultivated area was compromised. Therefore, the present study was undertaken to understand the effect of salinity stress on morpho-physiological, biochemical characteristics, photosynthetic efficacy, nutrient and High Affinity Potassium Transporter (HKT) genes in Azolla. The results indicated that out of 102, 8 Azolla (A. microphylla, BLCC 5, BLCC 18, BLCC 28, Pa Car WTY, R 18, R 54 and R 59) were found tolerant to 80 mM NaCl. The best species for salt tolerant (80 mM NaCl) was A. microphylla, whereas the least-tolerant was A. rubra. Fresh biomass production, frond length and width in A. microphylla were significantly (p < 0.05) higher in A. microphylla than A. rubra in both 40 and 80 mM NaCl. Moreover, chlorophyll a/b ratio, carotenoids and chlorophyll fluorescence (CHF)-derived F_O, F_m, F_v/F_m and root architecture (root length, average root diameter, root volume, projectile and surface area) were higher in A. microphylla than A. rubra under 40 and 80 mM NaCl. Contents of Na⁺ and Ca²⁺ increased in both A. microphylla and A. rubra, which can interfere with the uptake of essential macronutrients; however, these were accumulated comparatively less in A. microphylla than A. rubra, whereas a reverse trend was observed in cellular accumulation of K⁺ content. A. microphylla had higher superoxide dismutase (SOD), ascorbate peroxidase (APX), and proline activities in 40 and 80 mM NaCl than A. rubra. For the first time, twenty six HKT primers were designed as a molecular marker to identify salt-tolerant Azolla. Out of these, three HKT primers (Req 6, Aeq14, and Aeq16) were amplified in A. microphylla under NaCl stress, while their amplifications were not observed in A. rubra (salt susceptible). In A. microphylla, the expression of the Req 6 (HKT) gene were more under NaCl stress. Moreover, further research is needed to discover and validate the biochemical and molecular processes that confer salinity tolerance in Azolla plants.