Abstract
To search for an alternative to alfalfa under conditions of salinity and drought, a comparative study was carried out to explore the effect of salinity on the symbiosis of alfalfa and local esparcet species (Onobrychis transcaucasica and Onobrychis chorassanica) inoculated with their nodule bacteria. The salinity of up to 30 mM NaCl insignificantly affected the biomass growth of shoots and roots of alfalfa plants, but the increase in the salinity from 30 to as high as 140 mM NaCl led to the biomass decrease. The salinity produced a double effect on the nodulation process in inoculated alfalfa plants as follows: 1) at 30 - 100 mM NaCl the stimulation of nodulation and increased leghemoglobin activity were observed; 2) at salinity concentrations higher than 100 mM NaCl the suppression of both nodule formation and leghemoglobin activity was observed. Alfalfa plants under inoculation with the Sinorhizobium meliloti 10 strain obtained a considerable resistance to salinity (50 - 80 mM NaCl). The efficient symbiosis of O. transcaucasica plants with Rhizobium sp. OT111 and O. chorassanica plants with Rhizobium sp. OC109 enhanced the adaptation of plants to salinity up to 150 mM NaCl. The gradual growth suppression of both Onobrychis plants species started from 200 mM NaCl, and salinity concentration 300 mM NaCl was critical (sublethal) for plants independently of inoculation by nodule bacteria. In field conditions, O. chorassanica was more resistant to salinity than O. transcaucasica, but minimal irrigation for both species of Onobrychis showed a higher effect on their growth and development than the moderate salinity at the concentration 75 mM NaCl. The lower limit (drought threshold) of drought-resistance of Onobrychis plants was 6% - 8% of soil humidity. In shoot and roots of alfalfa, both Onobrychis plant species subject to salt stress, aldehyde oxidase and xanthine dehydrogenase enzymes and different number of their isoforms as well as their electrophoretic mobilities/activities were found.
Highlights
Legume grasses are used in agriculture under crop rotation for restoration of biological productivity of soils and their enrichment by biological nitrogen through the symbiosis of grasses together with nitrogen-fixing nodule bacteria
Alfalfa plants in symbiosis with S. meliloti No.10 nodule bacteria acquired a considerable resistance to salinity, i.e. at 50 - 80 mМ NaCl salinity the lowering of plant biomass came only to 11% 19% in comparison with control
If in irrigated agricultural lands alfalfa produces a good yield of green biomass under favorable conditions and its nitrogen-fixing symbiosis together with nodule bacteria is a very efficient measure to increase biological productivity and soil fertility, in saltaffected and drought-subjected soils it has low yield of green biomass and correspondingly its contribution into soil fertility restoration is small
Summary
Legume grasses are used in agriculture under crop rotation for restoration of biological productivity of soils and their enrichment by biological nitrogen through the symbiosis of grasses together with nitrogen-fixing nodule bacteria. Alfalfa is used widely in many countries for this purpose and it is sowed in the overall are about of 32 million ha. The cultivation of it is conducted preferably on well-irrigated non-saline soils. Since the zone of fertile lands is shrinking the sowing area for alfalfa shrinks because of a lower productivity of this agricultural crop in soils subject to such stresses as salinity and drought. Onobrychis, is a perennial leguminous grass that is widely distributed in many natural and climatic areas, some species are used in agriculture of many countries [1,2,3]. Onobrychis chorassanica grows in adyrs (non-irrigated steppe massifs) in Central Asia, but in sub-mountain and plain regions of Central Asia Onobrychis transcaucasica species was introduced before; it can be characterized as a leguminous grass that favorably develops under valley conditions without any need in special irrigation [4]
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