Metal-dependent inhibition of germination and seedling growth in two onion cultivars under in vitro conditions

Heavy metal contamination can reduce seed germination and early seedling growth. However, plant responses may change depending on the metal and cultivar. This study examined the effects of copper (Cu), cadmium (Cd), lead (Pb), nickel (Ni), and cobalt (Co) on two onion ( Allium cepa L.) cultivars, Gence and Karbeyazı, under in vitro conditions. Seeds were cultured on Murashige and Skoog medium containing 0, 100, 300, or 500 µM of each metal. Germination rate, mean germination time, germination index, shoot and root length, and fresh and dry biomass were recorded. Cultivar, dose, and their interaction significantly affected most traits. In general, higher metal doses reduced germination performance, root growth, and biomass accumulation. Cu and Cd decreased germination rate from 77.50 to 54.00% and from 80.00 to 52.67%, respectively. Root length was markedly inhibited at the highest metal doses, with Cu, Pb, and Co showing pronounced reductions based on relative inhibition values. Ni showed a different pattern. It increased final germination at low to medium doses but reduced germination vigor. Overall, Gence maintained higher final germination, whereas Karbeyazı better preserved root-related traits. These results show that early onion responses to heavy metals are both metal- and cultivar-dependent.