DIFFERENTIAL TRANSPORT OF ZN, MN AND SUCROSE ALONG THE LONGITUDINAL AXIS OF DEVELOPING WHEAT GRAINS

The hypothesis that Zn and Mn are transported within the grain in a si milar manner to sucrose was investigated in the developing wheat grain . Detached ears were cultured in solution containing Zn-65, Mn-54 and [C-14]-sucrose for 10 to 120 min at 18-22 days post-anthesis. At diffe rent times the grain was cut transversely into 1-mm sections and the r adioactivity in each section determined. The embryo region was damaged in some grains to Investigate the effect of reduced accumulation rare on the transport of Zn-65, Mn-54 and [C-14]-sucrose to the embryo. Th e distribution of Zn-65, Mn-54 and [C-14] sucrose between the endosper m cavity sap, endosperm, embryo and pericarp in grains labelled far 2. 5 and 6 h at 18-22 days post-anthesis was also determined. [C-14]-sucr ose 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 Zn-65 along the longitudinal axis of the grain was distribut ed evenly in each 1-mm section, whilst Mn-54 accumulated exponentially in the first proximal 1-mm section of the grain and was distributed e venly in the remaining sections. Damaging the embryo had no effect on Zn-65 and Mn-54 transport to the section containing the embryo. The pe ricarp contained almost all of the grain Zn-65 and Mn-54, with small a mounts found in the embryo, endosperm and endosperm cavity sap. Increa sing amounts of [C-14]-sucrose were found in the endosperm as time pro gressed, The rate of accumulation of Zn-65, Mn-54 and [C-14]-sucrose w as much higher in the embryo than the endosperm: the difference betwee n the embryo and endosperm was especially large for Zn-65 and Mn-54. I t is suggested that Zn-65 and Mn-54 are not transported within the gra in in the same way as [C-14]-sucrose, [C-14]-sucrose moves laterally o ut of the vascular system of the crease into the endosperm cavity and is subsequently taken up and stood in the endosperm. In contrast, Zn-6 5 and Mn-54 appear to be retained within the vascular system of the cr ease and may be transported more slowly to grain parts such as the emb ryo and pericarp tissue.