TRANSPORT OF ZINC AND MANGANESE TO DEVELOPING WHEAT GRAINS

An understanding of the transport pathway used by Zn and Mn to enter d eveloping grains may allow measures to increase the Zn and Mn content of wheat grain grown on Zn/Mn deficient soils. For this reason, transp ort of Zn and Mn into developing grains of wheat (Triticum aestivum L. cv. Aroona) was investigated. Detached ears (18-22 days post anthesis ) were cultured for 48 h in a solution containing 185 kBq of Zn-65 and 185 kBq of Mn-54. Transport of Zn-65 to the grain was unaffected by r emoval of glumes but was slightly reduced after the lemma was removed. Heat girdling the peduncle slightly reduced the amount of Zn-65 trans ported to the grain, whilst heat girdling the rachilla reduced transpo rt of Zn-65 to the gl ain to a greater degree, suggesting phloem trans port to the rachilla. The transport inhibitor CCCP (carbonyl cyanide m -chlorophenyl hydrazone) blocked Zn-65 transport to grain but not to l emma and glumes. Removing glumes and lemma and heat girdling the pedun cle did nor affect transport of Mn-54, but transport was slightly affe cted by heat girdling the rachilla, indicating xylem transport. CCCP b locked transport of Mn-54 into the grain but not to lemma and glumes. It was concluded that xylem-to-phloem transfer of Zn occurs in the rac his and to a lesser extent in peduncle and lemma. The results suggest that the lemma may be an important site for phloem loading when the co ncentration of Zn within the xylem is high. The data also suggest that Mn was predominantly translocated to the spikelets in the xylem, but that transport to the grain was dependent upon membrane transport befo re entering the grain. Phloem loading of Mn into the grain vascular sy stem may have occurred at the site of xylem discontinuity in the flora l axis.