Differential transport of Zn, Me and sucrose along the longitudinal axis of developing wheat grains

Abstract

The hypothesis that Zn and Mn are transported within the grain in a similar manner to sucrose was investigated in the developing wheat grain. Detached ears were cultured in solution containing 65Zn, 54Mn and [14C]‐sucrose for 10 to 120 min at 18–22 days post‐anthesis. At different times the grain was cut transversely into 1‐mm sections and the radioactivity in each section determined The embryo region was damaged in some grains to investigate the effect of reduced accumulation rate on the transport of 65Za, 54Mn and [14C]‐sucrose to the embryo. The distribution of 65Zn. 54Mn and [14C]‐sucrose between the endosperm cavity sap. endosperm, embryo and pericarp in grains labelled for 2.5 and 6 h at 18–22 days post‐anthesis was also determined. [14C]‐su‐crose was initially high in the first, embryo‐containing section of the grain but decreased progressively to the distal end of the grain. The amount of 65Zn along the longitudinal axis of the grain was distributed evenly in each 1‐mm section, whilst 54Mn accumulated exponentially in the first proximal 1‐mm section of the grain and was distributed evenly in the remaining sections. Damaging the embryo had no effect on 65Zn and 54Mn transport to the section containing the embryo. The pericarp contained almost all of the grain 65Za and 54Mn, with small amounts found in the embryo, endosperm and endosperm cavity sap. Increasing amounts of [14C]‐sucrose were found in the endosperm as time progressed. The rate of accumulation of 65Zn, 54Mn and [14C]‐sucrose was much higher in the embiyo than the endosperm: the difference between the embryo and endosperm was especially large for 65Zn and 54Mn. It is suggested that 65Zn and 54Mn are not transported within the grain in the same way as [14C]‐sucrose. [14C]‐sucrose moves laterally out of the vascular system of the crease into the endosperm cavity and is subsequently taken up and stored in the endosperm. In contrast, 65Zn and 54Mn appear to be retained within the vascular system of the crease and may be transported more slowly to grain parts such as the embryo and pericarp tissue.