When the process of xylem-to-phloem exchange does occur, it should be noted that the transported micronutrient must be currently moving within the xylem stream. Thus, continued root influx of micronutrients, and transfer to the xylem pathway during the period of seed fill, wouid be necessary to ensure seed import of the nutrients. Limitations to root ion influx during reproductive growth, such as caused by the Fe plaque that forms on rice roots in flooded soils (Chen et al., 1980), could minimize the potential for xylem-to-phloem exchange.