The critical range of Zn2+ activity in nutrient solution required for optimum growth of barley (Hordeum vulgare L. cv Herta) was studied using the synthetic chelating agent N-(2-hydroxyethyl)ethylenedinitrilotriacetic acid to buffer micronutrient metal ions. The activity of Zn2+ was varied over a wide range from approximately 0.1 x 10(-11) to 22 x 10(-11) m Zn2+. The dry weight of barley shoots reached a maximum at Zn2+ activities above approximately 3 x 10(-11) m and was clearly depressed when Zn2+ activities were below about 1 x 10(-11) m. The relationship in shoots between dry weight and Zn concentrations supports the view that there is a critical Zn concentration of about 25 mug g-1 dry weight in whole shoots of barley seedlings. When Zn2+ activities in solution were near or below approximately 3 x 10(-11) m, barley shoots accumulated higher concentrations of P, Mn, Ca, Mg, and Na, whereas Cu concentrations were reduced. P and Mn began to accumulate in the shoots before differences in dry weights were apparent and provided the earliest index of Zn deficiency. In Zn-deficient roots, concentrations of Ca and Mg increased by 25 to 30%, and those of Fe and Mn more than doubled. Zn appears to play a special role in regulating uptake of several mineral nutrients in barley.
