Dynamics of zinc and manganese movement in developing wheat grains

Zinc and Mn accumulate in the pericarp of the developing wheat grain during the middle stages of the grain-development period. The dynamics of Zn and Mn loading into, and retranslocation within, wheat grain during different development stages, as well as the role of the crease phloem and the pericarp vascular system in distribution of Zn and Mn in various grain tissues, are not known. This study showed that most Zn-65 and Mn-54 were distributed within the grain via the crease phloem, while Zn-65 could be transported to the embryo via a phloem-only strand originating in the grain stalk. Neither Zn nor Mn was transported circumferentially from the crease to the dorsal pericarp tissues. After 24-h pulse-labelling of detached ears on day 15 post-anthesis, accumulation of Zn-65 and Mn-54 in the grain increased rapidly for 5 days but declined thereafter. The relatively largest amounts of Zn-65 and Mn-54 were located in the crease/inner pericarp tissues (>60%), followed by the endosperm. The outer pericarp contained only small amounts of Zn-65, but relatively large amounts of Mn-54. Little Zn-65 and Mn-54 was found in the embryo. Pulse-labelling ears with Zn-65 and Mn-54 at various stages of grain development showed that they are mostly transported to the crease/inner pericarp tissues, but increasingly more to the endosperm and the embryo as the grain matures. Retranslocation of Mn from the crease/inner pericarp and outer pericarp tissues was coincident with an increased accumulation of Mn-54 in the embryo. It is concluded that the rates of transport of Zn and Mn into, and retranslocation within, wheat grains change during grain development.