Effect of salinity on growth, nodulation and n2 fixation of Prosopis alba

FERRARI A; DIAZ G; WALL LG

SM de Tucumán

Congreso; VII Congreso Argentino de Microbiología General “SAMIGE del Bicentenario”; 2011

SAMIGE

Algarrobo blanco (Prosopis alba Griseb) is a legume tree native from arid zones of centre and north Argentina. It has been reported as tolerant to salinity. The ability of plant to adapt to saline soils can be strongly dependant to the establishment of effective symbiosis. In a greenhouse experiment, seedlings were inoculated with two rhizobia isolates. One was obtained from nodules of P. alba growing in Santiago del Estero province, and was used as a reference nodulating strain (R strain). The other was isolated from nodules of P. flexuosa growing on a substrate of a saline soil from Salina del Bebedero, San Luis province (S strain). After two months growing in a mineral solution without added salt, the seedlings were watered adding different concentrations of NaCl to the mineral solution: 0, 50, 100 and 200 mM. Inoculated plants were watered without added N while non-inoculated controls were watered with 5 ppm N. At the end of the experiment, N2 fixation activity of intact plants was measured by the Acetylene Reduction Activity (ARA) method, then plants were harvested and the different parts (shoot, root, nodules) were weighed to estimate plant biomass. Non inoculated plants showed an almost null growth rate for every salt level, suggesting a deficiency in nitrogen more than a salt stress. Inoculated plants showed a sustained growth rate along 40 days, even for the highest salt level. However, the shoot growth rate and the biomass decreased regularly when the salt concentration was increased. Plants inoculated with R strain grew more in height and biomass than the inoculated with S strain, specially without salt addition, suggesting that the symbiotic pair Rstrain x P. alba was more efficient, and specific for plant growth, than the pair with S strain, which was isolated from another Prosopis species. The Nitrogenase activity per plant (ARA) decreased regularly as the salinity level increased for both strains. However, when plants were inoculated with the S strain, isolated from saline soils, the Nitrogenase specific activity (ARA/nodule biomass) was rather constant and unchanged until a salinity level of 100 mM. This activity slightly decreased at the highest salt level (200 mM). A different response was shown by plants inoculated with the R strain, since the specific activity of those nodules decreased with salinity. In conclusion, both strains were able to fix nitrogen actively still at the highest salinity level, allowing a sustained plant growth. The salt stress reduced the nodulation or the activity of the nodules, but this effect was weaker in the strain isolated from a saline soil, suggesting a bigger potential of this rhizobia strain for projects of reforestation of saline lands with native trees.