Salinity tolerance mechanism in the aquatic nitrogen fixing pteridophyte Azolla : a review
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Salinity tolerance mechanism in the aquatic nitrogen fixing pteridophyte Azolla: a review Ravindra Kumar Yadav 1 & Apurva Chatrath 2 & Keshawanand Tripathi 2 & Manju Gerard 3 & Altaf Ahmad 4 & Vandana Mishra 1 & Gerard Abraham 2 Received: 6 July 2020 / Accepted: 14 October 2020 # Springer Nature B.V. 2020
Abstract Salinity is one of the major constraints for agricultural productivity and the problem of salinity is likely to increase further. The aquatic fern Azolla is employed as an important bio-inoculant for the cultivation of rice paddy. In addition to this it is also used as feed supplement for cattle and poultry. However, increasing soil salinity is a serious threat for its wider exploitation. Stressors such as salinity leads to drastic changes in the metabolic activities especially the photosynthesis and nitrogen fixation. The potential of plants to tolerate salinity is determined by efficient mechanisms operating at cellular and sub-cellular levels. Several adaptation strategies are required to cope with salinity stress. Complicated interactions between the host plant Azolla and its symbiont should be considered if this organism is employed as bio-inoculant. Therefore, in the present review we discuss on the effect of salinity on Azolla and its cyanobiont with an emphasis on the physiological, biochemical and molecular mechanisms of salinity tolerance. Keywords Azolla-Anabaena system . Bioinoculant . Cattle feed supplement . Nitrogen fixation . Photosynthesis . Salinity tolerance
1 Introduction The nitrogen fixing aquatic fern Azolla is found floating on the surface of water bodies and has a fast doubling time of 5– 10 days (Brouwer et al. 2015). It is distributed in both temperate and tropical regions and it grows luxuriantly in ponds, ditches, canals and rice paddy fields. Azolla plant is an extremely small sporophyte having a rhizome which is horizontal (0.5 to 7 cm in diameter) and densely arranged branches and overlapping leaves. The single leaf has a thin ventral and
* Gerard Abraham [email protected] 1
Bioresources and Environmental Biotechnology Laboratory, Department of Environmental Studies, University of Delhi, Delhi 110007, India
2
Centre for Conservation and Utilization of BGA, ICAR-Indian Agricultural Research Institute, New Delhi 110012, India
3
Division of Research Systems Management, ICAR-National Academy of Agricultural Research Management, Rajendra Nagar, Hyderabad, Telangana 500030, India
4
Department of Botany, Aligarh Muslim University, Aligarh, UP 202001, India
thick dorsal lobe and cavities present in the dorsal lobe house the symbiotic cyanobacteria (Peters and Mayne 1974). Azolla is an effective green manure and biofertilizer for flooded crops particularly rice (Bhuvaneshwari and Singh 2015). Gaind and Singh (2015) however, observed that in a rotating rice-wheat cropping system, beneficial effects of Azolla was also observed in wheat. Inoculation of Azolla improved the physico-chemical properties of the soil due to release of nutrients and organic matter (Bhuvaneshwa
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