Phloem and xylem features in prolific and nonprolific maize hybrids

To compare the relative amount of conductive tissue in two maize hybrids (Zen mays L.) with contrasting prolificacy, the number of vascular bundles and the phloem and xylem area per bundle were quantified in cross-sections of internodes, peduncles and pedicels. Samples were collected 30 days after pollination in five plants of each hybrid. The number of vascular bundles and phloem and xylem areas in the internodes of the prolific hybrid were 12%, 21% and 31% higher than the nonprolific type. In peduncles of both hybrids, the values were very similar, except for the peduncle of the subapical ear of the nonprolific hybrid, which presented a 29% reduction in the number of bundles, a 58% decrease in phloem, and a 61% decrease in xylem areas compared to the prolific type. The phloem area was significantly higher in the peduncles of the apical ear than in the internodes, suggesting a trade off. The reduction in xylem area and the increment in phloem area may improve the movement of assimilates towards the ear. In pedicels, phloem area was 8 to 10 times larger than xylem area, suggesting an adaptation in phloem tissue of the kernels, which favors the accumulation of biomass at high rates during the kernel filling stage.