Patterns of Translocation, Respiratory Loss, and Redistribution of 14 c in Maize Labeled after Flowering 1

Maize (Zea mays L.) growing in the field was labeled with ¹⁴CO₂ at four leaf positions 15 to 18 days after flowering. On four occasions after labeling, entire plants were harvested and ¹⁴C in several plant fractions was analyzed by liquid scintillation spectroscopy to reveal patterns of translocation within the plants, losses of ¹⁴C by respiration, and patterns of mobilization and redistribution of ¹⁴C among and within organs. The first and third leaf blades above the ear (+1 and +3, respectively), and the first blade below the ear (∗1), exported ¹⁴C mainly kernels of the developing ear. Among these three leaf blades, the extent of export of ¹⁴C was more rapid from upper positions than from lower ones. Movement of ¹⁴C from the fifth leaf blade (∗5) below the ear was much slower than that from the upper three blades, but kernels of the developing ear ultimately became the principle sink for ¹⁴C exported by this blade. Compared with the upper three leaf blade positions, however, appreciable label from ∗5 accumulated in the lower stem and roots. Labeling plants bearing partly-fertilized ears caused a large accumulation of ¹⁴C in the stem, roots and to a lesser extent, husks, compared with completely-fertillzed controls. About 20% of the ¹⁴C assimilated by the +1, +3, and ∗1 blades was lost by respiration in the 5 weeks following labeling. During the same interval, nearly 70% of the ¹⁴C assimilated by the ∗5 blade was lost by respiration. Approximately half of the carbon assimilated by +3, +1, and ∗1 blades was translocated directly to kernels within 24 hr after assimilation. A similar quantity of label assimilated by +3, +1, and ∗1 blades was deposited in cobs and husks within 24 hr after assimilation. Estimates of redistribution of label within the plants suggest that 60 to 80% of the ¹⁴C accumulated in kernels between days 1 and 35 following labeling was derived from ¹⁴C originating in upper leaves and deposited in the cob and husk within 24 hr.