Integrating morphophysiological traits with salt-responsive gene expression uncovers cultivar-specific tolerance mechanisms in faba beans facing NaCl stress

Soil salinization constrains faba bean (Vicia faba L.) production across North Africa and the Near East. Three Egyptian cultivars, Nubaria 1, Giza 716, and Sakha 5, were subjected to 0, 150, and 200 mM NaCl under controlled greenhouse conditions. Twenty morphological, physiological, and biochemical traits were quantified, along with quantitative RT-PCR profiling of 11 salt-responsive genes. Two-way ANOVA detected significant effects of cultivar, NaCl treatment, and their interaction for a subset of traits, including root length, leaf K⁺ concentration, net photosynthesis, stomatal conductance, intercellular CO₂, and three epicuticular wax fractions, indicating that these traits responded differently to salinity across genotypes. For a separate subset branch number, root dry weight, root-to-shoot ratio, and leaf nitrogen, only the cultivar term was significant, with no detectable effect of treatment or interaction. Nubaria 1 showed the smallest reductions in shoot dry weight (14% at 200 mM NaCl), maintained leaf K⁺ concentration across all salinity levels (13.3-13.6 g kg⁻¹), and retained the highest pod number at 200 mM NaCl (1.78 pods per plant). It also showed the lowest fold induction across all 11 genes at both stress levels. Sakha 5 showed the largest increase in root length under stress, from 3,577 cm in the control to 8,664 cm at 200 mM NaCl (+ 142%), the highest fold-induction across all gene categories, and produced zero pods at 200 mM NaCl in all replicates. Giza 716 had the highest net photosynthetic rate under control conditions (10.33 µmol CO₂ m⁻² s⁻¹) and was intermediate for most traits across treatment levels. In all three cultivars, net photosynthesis declined, and intercellular CO₂ rose simultaneously with increasing NaCl, from 327 to 364 µmol mol⁻¹ under control to 378-413 µmol mol⁻¹ at 200 mM NaCl. Total epicuticular wax reached its maximum at 150 mM NaCl in all cultivars and declined at 200 mM NaCl in Sakha 5 and Giza 716, despite continued CER1 induction in Sakha 5. Principal component analysis explained 70.7% of total variance across two components (PC1: 39.5%; PC2: 31.2%), with cultivar samples remaining separated along PC1 across all treatment levels. Leaf K⁺ concentration and net photosynthesis under stress showed the most consistent differences among cultivars and are identified as practical traits for early-stage phenotyping in salt-tolerance screening programs.