Xylem and phloem transport of Cd, Zn and Fe into the grains of rice plants (Oryza sativa L.) grown in continuously flooded Cd-contaminated soil
Xylem and phloem transport of cadmium (Cd), a toxic element, into rice grains was compared with transport of the nutritional elements zinc (Zn) and iron (Fe) in rice plants (Oryza sativa L.) grown under continuously flooded soil conditions. To explore their transport, the concentrations of Cd, Zn and Fe in xylem and phloem saps and in different tissues were determined at four (10th-leaf, 14th-leaf, early grain-filling, mature) stages using semi-dwarf rice plants (cv. Kantou) grown on soil contaminated with Cd fourfold higher than average Cd contamination in Japan. Phloem saps were collected from mature leaf sheathes at the first two stages and from the uppermost internodes at early grain filling. Xylem saps were collected from cut stems. The Cd concentrations in the xylem and phloem saps collected at the 14th-leaf and early grain-filling stages were lower than those at the 10th-leaf stage, and the Cd concentration in dehusked grain was low (0.05 mg kg−1 dry weight [DW]). In contrast, Zn and Fe concentrations in the xylem and phloem saps were maintained throughout the growing season, and their grain concentrations were high (57 mg Zn and 29 mg Fe kg−1 DW). Schemes of metal transport to the grains during early grain-filling to mature stages for different metals are proposed. Cadmium may be transported from the flag leaf blade to the grains via the phloem and additionally after xylem-to-phloem transfer at the stem and spikelet nodes if supplied from the roots. Zinc in the grains and partly in the husks may be actively supplied via the phloem after mobilization from the blades of the flag and upper leaves and also by xylem-to-phloem transfer in the nodes. Iron stored in the leaves may be transported to the grains via the phloem.