Fig. 1

Boron stress has an inhibitory effect on M. truncatula growth and nitrogenase activity. a Pictures of 4-week-old M. truncatula plants growing under a B gradient are shown. Upper panels (upper row of images) show plants fertilized with N (non-nodulated plants) growing under B deficiency (no B was supplemented into the media), control (media at a final B concentration of 0.1 mM B [OH₃]) or B toxic conditions (1 mM B [OH₃]). Middle row of images shows plants growing under symbiotic conditions (M. truncatula plants inoculated with S. meliloti 2011). Lower row of images shows nodules from plants grown under different B conditions, as indicated in the bottom labels. Bar = 3 cm (plant pictures) or 1 mm (nodule pictures). b Plant biomass as dry weight of roots and shoots of non-nodulated or nodulated plants under a B gradient. c Nodule fresh weight under a B gradient. Data are the Mean ± SD of two independent experiments with, at least, four biological replicates (n = 4). d Nitrogenase activity was determined in 4-week-old plants growing under a B gradient. Plants growing under control, B deficiency, or B toxic conditions were used to evaluate nitrogenase activity changes under different B conditions. Nitrogenase activity was analyzed by the acetylene reduction method (nmol ethylene generated per hour per plant). Data were analyzed using One-way analysis of variance (ANOVA) followed by t-student post-hoc test. Asterisks indicate significant differences when comparing B stress samples with B control treatment (t-Student, (“*” = p < 0.05, “**” = p < 0.01,, “***” = p < 0.001)