Regulation of Zinc and Manganese Transport into Developing Wheat Grains Having Different Zinc and Manganese Concentrations

Grains of wheat were produced with differing zinc (Zn) or manganese (Mn) contents by culturing detached ears from anthesis onwards in solutions of four different concentrations of Zn or Mn (0.1, 1.0, 10, and 50 mu M). After 20 days, ears were labeled with Zn-65 or Mn-54 at (i) the pretreatment concentrations of Zn or Mn, or (ii) at 10 mu M Zn or Mn regardless of the pretreatment. Accumulation of Zn or Mn in the grain was greater as the pretreatment concentration of Zn or Mn increased from 1.0 to 10 mu M, however, accumulation was less in ears cultured at 50 mu M Zn or Mn. Accumulation of Mn in grain of different Mn status labeled at 10 mu M Mn was similar in the 0.1, 1.0, and 10 mu M Mn pretreatments, but accumulation in the grain pretreated at 50 mu M Mn was reduced. In contrast, accumulation of Zn in grains of different Zn status when labeled at 10 mu M Zn was highest in ears pretreated at 10 mu M Zn, but substantially lower in ears of a lower Zn status (those pretreated at 0.1 or 1.0 mu M Zn) as well as in those preheated at 50 mu M Zn. These results suggest that Zn-deficient grain was not a strong sink for Zn, while at high concentrations of solution Zn, a protective barrier exists preventing excessive accumulation of Zn in the grain. Proportionally more Zn was distributed to the inner pericarp and generally less to the endosperm, outer pericarp, and embryo as the Zn status of the grain increased. This work demonstrates that loading of Zn and Mn into, and distribution within, wheat grain is regulated by the nutritional status of the grain.