Physiological interactions inAzolla-Anabaenasystem adapting to the salt stress

Abstract

The effect of the different levels of salinity on some important physiological parameters among six species of Azolla has been investigated. Salinity reduced the growth (dry weight) and nitrogenase activity in AzollaAnabaena association while heterocyst frequency was sharply increased. Minimum sodium accumulation and electrolyte leakage (EL) was observed up to 20 mM NaCl whereas nitrate was accumulated significantly. Beyond 20 mM NaCl, fronds having higher amount of Na ' and EL inhibited the accumulation and reduction of nitrate suggests that 20 mM NaCl is the critical concentration. Highest superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities and proline content might be the probable reason for the low level of EL, malondialdehyde (MDA) content and Na ' accumulation in Azolla species. Lower Na ' accumulation, solute leakage and lipid peroxidation along with the higher nitrogen demand and the proline accumulation with a high ability of oxygen radical scavenging in A. microphylla might be the reasons to grow at higher NaCl concentrations.Keywords: Azolla; salinity; antioxidants; electrolyte leakage; nitrate; proline IntroductionAzolla is a heterosporus, free floating, fast growing aquatic fern and is widespread in fresh water habitat of India, Sri Lanka, Japan, China and Philippines. The cyanobiont Anabaena azollae inhabits the leaf cavity of Azolla-Anabaena system and has the capacity to fulfill the total nitrogen demand of the association (Peters and Meeks 1989;Plazinski 1990). In comparison to the free-living cyanobacteria, Azolla association has several folds higher nitrogen fixation capacity and that is why the Azolla-Anabaena system is extensively used as a biofertilizer in rice and many other crops (Pabby et al. 2003;Nayak et al. 2004). However, salinity is a serious handicap for the cultivation and production of agricultural crops. High salinity not only affects the morphological, anatomical, developmental, physiological and biochemical processes of plants which can be observed in the form of plant productivity and/or as the death of the plants but also causes a water deficit condition, membrane disorganization and altered nutrition acquisition. This leads to the formation of reactive oxygen species (ROS) that disrupts normal metabolism through oxidative damage to the lipids (Wise and Naylor 1987) and to protein and nucleic acids (Imlay and Linn 1988). However, there is the ability in all the living systems, from bacteria to higher plants to adapt themselves to the osmotic stress with minor modifications. Plants always make various strategies to avoid the deleterious effect of NaCl (Munns et al. 2002;Parida and Das 2005). During the salt stress or adaptation towards salt, the usually associated mechanisms are major alteration in Na ' transport, i.e. influx and efflux. Salt-tolerant plants, besides involving in regulation of ion and water movements in the cells, also have an antioxidant system for effective removal of highly reactive oxygen species (ROS). It has been also reported that supe...

Sodium transport and mechanism(s) of sodium tolerance in Frankia strains

“…It is well known that salinity imposes ionic stress in which Na + inclusion is one of the major contributors. Each frankial strain showed a different level of salt tolerance as in other organisms . Considerable growth was observed only up to 500 and 250 mM NaCl respectively in HsIi10 and rest of the strains which proved that 500 and 250 mM NaCl are the critical concentrations for their respective strains.…”

Attenuation of metal toxicity by frankial siderophores

“…At higher concentrations, poor growth of the frankial strains might be the result of the minimum siderophore production, which would not be able to maintain the minimum intracellular metal ions (Mg 2þ , Cu 2þ , and Zn 2þ ) in Frankia strains. So, there would be heavy accumulation of the metals which ultimately damaged the metabolic activities as well as the growth through the reduced nutrient uptake and osmotic injury or specific ion toxicity as reported for the other organisms (Baccio, Navari-izzo, and Izzo 2004;Singh, Upadhyay, and Mishra 2008). There are efflux transporters, ATP binding cassette (ABC) transporters, metal efflux pumps, and unspecific membrane transporters present in some bacteria which either efflux toxic or over concentrated metals, or maintain low intracellular metal ions (Nies 1995;Borges-Walmsley, McKeegean, and Walmsley 2003;Nies 2003;.…”