The aim of this study was to determine the effect of salt stress on morphological and physiological changes of Brassica oleracea acephala group seedlings. Seedlings of kale cultivar Red Russian (RR) and collard Croatian population Konavle 2 (K2) were grown in a floating hydroponic system in Tifton, Georgia, USA. Seedlings were treated with seven different nutrient solutions (NS). The control NS (EC 2 dS m^-1) was concentrated to achieve EC 4, 6 or 8 dS m^-1. Three additional salt treatments included addition of NaCl solution to the control NS to get: EC 4 NaCl (2 NS + 2 NaCl), EC 6 NaCl (2 NS + 4 NaCl) and EC 8 NaCl (2 NS + 6 NaCl) dSm^-1. Leaf gas exchange parameters decreased with increased EC. Seedlings treated with EC 6 NaCl and 8 NaCl dS m^-1 had the lowest leaf relative water content (less than 59%). Seedlings treated with 2 dS m^-1 had the greatest (187 cm²) leaf area (LA). Cultivar RR had greater LA (131 cm²) than population K2 (84 cm²). Increased percentage of shoot (14.1%) and root (10.4%) dry weight (DW) was recorded in seedlings treated with EC 8 dS m^-1, c. Population K2 had higher shoot (10.9%) and root (10.4%) DW percentage compared with cv. RR. In conclusion, the nutrient solution of EC 4 NaCl had negative effect on morphological characteristics, compared to the same solution without NaCl. Increased concentrations of NS significantly affected the leaf thickness (SLA) of B. oleracea acephala group seedlings. This can be used as production tool for seedlings hardening.