Alleviation of manganese toxicity and manganese-induced iron deficiency in barley by additional potassium supply in nutrient solution

Barley plants were grown hydroponically at two levels of K (3.0 and 30 mM) and Fe (1.0 and 10 microM) in the presence of excess Mn (25 microM) for 14 d in a phytotron. Plants grown under adequate K level (3.0 mM) were characterized by brown spots on old leaves, desiccation of old leaves, interveinal chlorosis on young leaves, browning of roots, and release of phytosiderophores (PS) from roots. These symptoms were more pronounced in the plants grown under suboptimal Fe level (1.0 microM) than in the plants grown under adequate Fe level (10 microM). Plants grown in 10 microM Fe with additional K (30 mM) produced a larger amount of dry matter and released less PS than the plants grown under adequate K level (3.0 mM), and did not show leaf injury symptoms and root browning. On the other hand, the additional K supply in the presence of 1.0 microM Fe decreased the severity of brown spots, prevented leaf desiccation, and increased the leaf chlorophyll content, which was not sufficient for the regreening of chlorotic leaves. These results suggested that the additional K alleviated the symptoms of Mn toxicity depending on the Fe concentration in the nutrient solution. The concentration (per g dry matter) and accumulation (per plant) of Mn in shoots and roots of plants grown in 10 uM Fe and 30 mM K were much lower than those of the plants grown in 10 microM Fe and 3.0 mM K, indicating that additional K repressed the absorption of Mn. The concentration and accumulation of Fe in the shoots and roots of the plants grown in 10 microM Fe and 30 mM K were higher than those of the plants grown in 10 microM Fe and 3.0 mM K, indicating that the additional K increased the absorption of Fe under excess Mn level in the nutrient solution. The release of PS, chlorophyll content, and shoot Fe concentration were closely correlated.