SOLUTE CONCENTRATIONS IN THE PHLOEM AND APEX OF THE ROOT OF ZEA MAYS

Abstract

Plasmolytic studies utilizing a graded series of mannitol solutions (0.1–1.4 M in 0.1 M increments) were conducted on adventitious roots of Zea mays to determine solute concentrations of cell types at various locations in the root. Results indicated that mature sieve‐tube members had the highest solute concentration as determined by their C 50 (the estimated mannitol concentration plasmolyzing an average of 50% of a given cell type) of any cell type in the root. In tissue 12 cm from the tip, C 50 values calculated for proto‐ and metaphloem sieve‐tube members were 1.15 and 1.19 M, respectively, while in tissue 0.5 cm from the root tip, values for the same cell types were 0.68 and 0.46 M, respectively. The C 50 values for sieve elements in tissue 5 cm from the tip were intermediate (1.08 and 1.11 M). Although the companion cells generally plasmolyzed at nearly the same concentrations of mannitol as the sieve elements, their C 50 values were slightly lower than adjacent mature sieve elements. The lowest C 50 (0.35 M) for any cell type examined was associated with meristematic cells in tissue 0.1 cm from the root tip. Taken collectively, the results indicate that positive concentration gradients exist between mature sieve tubes and meristematic cells of the root apex of maize.