Effects of exogenous thiocyanate on mineral nutrients, antioxidative responses and free amino acids in rice seedlings

The effects of exogenous thiocyanate (SCN-) on amino acids composition, content of mineral nutrients and antioxidative systems in plants were investigated. Young rice seedlings (Oryza sativa L. cv. XZX 45) were grown in nutrient solutions amended with potassium thiocyanate (KSCN). Activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) in plant materials were analyzed in vivo. Mineral nutrients and free amino acids in rice seedlings were also measured to determine metabolic responses to SCN- exposure. A significant reduction in transpiration and relative growth was recorded with all treatments (p < 0.05), while changes of total chlorophyll content in leaves was negligible (p > 0.05). SCN-induced toxicity appeared to be more sensitive to activities of POD in shoots and APX activities in roots than the others. The content of nutrient elements in rice seedlings exposed to exogenous SCN- was variable, while the effects were more evident at the highest SCN-treatment (p < 0.05). Although the change of total free amino acids in shoots of SCN-exposed seedlings was negligible (p > 0.05), responses of different amino acids to SCN- application were quite different. Among fifteen free amino acids detected, serine (Ser), proline (Pro), and methionine (Met) increased, while asparagine (Asp) decreased with an increase of the doses of SCN- supplied. Phyto-transport of SCN- was apparent and the removal rates were positively correlated to the doses, suggesting that phyto-assimilation of SCN- is an enzymatic process through a potentially un-identified degradation pathway.