Among cereals, rice is known as the major components of our food consumption worldwide; however, recently accumulation of heavy metals in soil and water has emerged as big constraints for rice yields. This study was undertaken with analyzing the impacts of various heavy metals (Hg, Co, Ni, Cd, Zn and Cu) stress treatments on seed germination and early seedling growth in coastal red rice (Oryza sativa L. cv. Poongar) crop. Responses in terms of germination percentage mean were recorded as partial germination and full germination after 5-days and 10-days of stress treatments respectively. Moreover, stress responses of heavy metals during early seedling growth were measured in terms of root and shoot lengths of the seedlings after 10-days of treatments. Mature seeds were initially treated with HgCl2 (0.01%, 0.1% and 1.0%) and equivalent concentrations of CoCl2, NiCl2 and CdCl2 (1.0mg/L, 5.0mg/L, 10mg/L, 25mg/L and 50mg/L) followed by ZnSO4 and CuSO4 solutions (0.5mg/L, 1.0mg/L, 2.5mg/L and 5.0mg/L). Results reveal that HgCl2 even at very low concentration (0.01%), was found to be the lethal for seed germination (5±0.0%), however, these poorly germinated seeds were further failed to grow into seedlings. Like HgCl2, CoCl2 was also found to exhibit strong toxicity but at high concentration (50mg/L) where seed germination was completely lacking. Furthermore, unlike CoCl2, NiCl2 solution was found to be less toxic where germination frequency was recorded as (12±0.0%) and suppressed root formation completely at high concentration (50mg/L). Moreover, among chloride solutions of cobalt, nickel and cadmium, CdCl2 was proved as little weak inhibitor because complete seedling development with root-shoot length was observed at high concentration (50mg/L) and root-shoot length ratio (0.28±0.19cm/0.96±0.23cm) was recorded as compared to control seedlings (3.97±0.71cm/4.52±0.45cm). Furthermore, in case of sulphate solutions of zinc and copper, ZnSO4 stress proves to be strongly lethal even at very low concentration (5.0mg/L) and seed germination was completely lacking in comparison to CuSO4 treatment (15±0.0%). However, CuSO4-treated germinated seeds were grown into incomplete seedlings without roots (0.0cm/0.05±0.02cm) after 10-days of treatments. Hence this study shows that HgCl2 proves to be the most toxic heavy metal for seed germination and early seedling growth followed by ZnSO4, CuSO4, CoCl2 and NiCl2 while CdCl2 was emerged as the least inhibitory heavy metals among all tested metals in rice crop.
Key words: Abiotic stress, Heavy metal, Rice, Seed germination, Seedling