Biochemical, morphologic and ecophysiological changes in Lotus genus plants under saline stress
Ingeniera Agrónoma Julieta Pesqueira
      
      ABSTRACT
         Lotus tenuis is an herbaceous legume naturalized in saline - alkaline lowlands in the Salado River Basin, highly valuable because of its contribution to the forage offer in the region systems. Nowadays, it’s necessary to increase pastures productivity, specially those which are already adapted to marginal areas with stressful soil conditions. 
         With this aim we have searched, characterized and selected for productive germplasm of Lotus tenuis already adapted to saline conditions. Specifically, we evaluated the response to salt stress, imposed either by shock or by acclimatizing, in two vegetative stages by monitoring morphophysiological and biochemical changes. We also identified and selected for individuals among a population with high variability in the salt stress tolerance. In order to increase the genetic resources, we included different species from the Lotus genus (Lotus corniculatus, Lotus creticus and four model species: Lotus japonicus MG20, Lotus japonicus Gifu, Lotus filicaulis, Lotus burtii), natural L. tenuis populations, as well as three comercial varieties of the forage legume with the aim of using them in future breeding program.
         Salt stress did not result in plant death in any of the different species and vegetative stages evaluated, but decreased both fresh and dry weight, especially in young plants. The growth parameters that account for the reduced weight were the number of stems and branching points per plant, and main stem diameter. Total leaf area under salt stress conditions only diminished in two out of the 19 L. tenuis populations under study and in the model species L. japonicus MG20. The L. tenuis populations with the largest growth under control conditions were the ones that experienced the highest inhibition of growth when exposed to salt stress. 
         In plantlets, as well as in adult plants, saline treatments (up to 150 mM NaCl) caused increased concentrations of Na+ and Cl- in their aerial parts. We also observed that basal leaves accumulated more Na+ and Cl- than the apical ones, and that the resultant K+/Na+ ratio in the apical tissues were positively correlated with dry matter accumulation under salt conditions. In general, L. tenuis populations accumulated more Cl- than model species and less than L. corniculatus, while L. creticus was one of the species with more Na+ accumulated in the leaves. Salt stress also induced proline accumulation in the shoots and, together with ion accumulation, was correlated with lower osmotic potentials in all species. Polyamine concentration was also altered when plants were exposed to salt. However, none of these quantitative organic changes have reflected a clear salt tolerance response to salinity.  
         Nevertheless, the analysis of the different parameters (water potential, photosynthetic pigment levels, carbohydrate and protein levels, Rubisco carboxylase activity) revealed intra and interspecific variability in the response to increased salinity tolerance which allowed for the selection of individuals of the L. tenuis species with good performance under both control and high salt conditions.