Roles for potassium in the iron‐stress response mechanisms of strategy I and strategy II plants

An adequate supply of K is critical to proper function of the Fe‐stress response mechanisms of both monocotyledonous and dicotyledonous plants. This review summarizes findings of a series of hydroponic experiments conducted with Fe‐deficiency stressed muskmelon (Cucumis melo L.), soybean (Glycine max Merr.), tomato (Lycopersicon esculentum Mill.), and oat (Avenae byzantina C. Koch.) grown at varying levels of K and relates them to other studies to clarify the K‐Fe interaction. A differential disruption of the activity of Fe‐deficiency stress mechanisms by K deficiency was observed among muskmelon, soybean and tomato (a Strategy I species). In Fe‐stressed soybean and tomato, H+ ion release and reduction of Fe3+ to Fe2+ declined in K‐deficient versus K‐sufficient plants. Whereas K‐deficiency stress in muskmelon did not interrupt H+ ion release, it did diminish reduction of Fe3+ to Fe2+. In monocotyledonous Coker 227 oat (a Strategy II species), significantly more phytosiderophore was released by roots of plants grown with than without adequate solution K. The diminished capacity to operate specific Fe‐stress responses in the absence of K resulted in more intense Fe deficiency chlorosis and lower leaf Fe concentrations for all four species studied. This review will suggest several specific physiological mechanisms in both Strategy I and II plants by which K may operate to make Fe more available to plants. The differential responses observed among species could provide the key to further elucidate its role.