Absorption of copper, zinc, and manganese by sugarcane leaf tissue

The absorption of Cu²⁺, Zn²⁺, and Mn²⁺ by leaf tissue of 4-month old sugarcane plants (Saccharum officinarum L., var. H53-263) has been investigated. After the “apparent free space” fraction was desorbed, the absorption of Cu²⁺, Mn²⁺, and Zn²⁺ yielded a curve typical of many ion uptake processes when measured as a function of the external concentration. However, only 1 absorption mechanism was evident for each cation. The pH optimum for Cu²⁺ and Zn²⁺ uptake was 5.0 to 6.0, whereas that for Mn²⁺ absorption was 4.5 to 6.0. Absorption was competitively inhibited by H⁺, and this inhibition was reversible when 0.5 mm Ca²⁺ was present. Cu²⁺ and Zn²⁺ were absorbed through the same carrier sites, as concluded from their mutually competitive activities. Mn²⁺ was absorbed through a second, independent mechanism. Uptake of each cation was strongly inhibited by uncouplers of oxidative phosphorylation, by Amytal and Nembutal², by 5 × 10⁻²m succinate, and by ADP and P_i. Absorption of Cu²⁺, Zn²⁺, and Mn²⁺ was concluded to be coupled to oxidative phosphorylation, and specifically to energy-conservation Site I.