The postulate that single roots of Zea mays transport their absorbed phosphorus nonuniformly to the leaves was tested. Plants were grown under growth chamber conditions for three to four weeks in nutrient solution. At this stage of growth a series of plants was placed into a system in which two roots on each plant were allowed to absorb either 33P or 32P from uptake solutions for time intervals of up to 24 hours. Plants subsequently were harvested such that each leaf was partitioned into samples containing tissue from one side or the other of the midrib. All samples were assayed for 33P and 32P and the results were expressed as the amount of total P transported into different plant parts from a single root. Nonuniform P accumulation in the leaves occurred and different patterns of accumulation, dependent on the type of root chosen for uptake were observed. Nearly uniform P accumulation occurred between one side and the other of a given leaf when transport was from radicle roots. In marked contrast, transport from adventitious roots resulted in an alternating pattern of accumulation between one side and the other of each successive leaf up the stem. The seminal root system supplied more P to the older leaves than did the adventitious root system. The nature of these nonuniform P transport patterns is attributed to the vascular organization between roots and leaves.

The implications of nonuniform P distributions in the root zone of corn (Zea mays L.) on growth and P uptake were explored utilizing a split root system. Maximal dry matter accumulation occurred only when the entire root system was exposed to an external P supply. Reduction in shoot growth of nearly 20% was associated with those P distributions in which part of the root system was devoid of an external P supply. There was a concomitant decrease in root growth in “no P” zones such that shoot/root ratios remained constant regardless of whether all or part of the root system was exposed to P. The growth rate of roots dependent on an internal supply of P via translocation (roots in “no P” zones), as contrasted to that of roots with an external supply, is considered to have been limited by the rate at which inorganic P was supplied to the growing points.These effects were not due to the lack of total P uptake for it was shown that a portion of the root system could take up high amounts of P if the external supply was sufficient.For P distributions with one root zone devoid of an external P supply, significant differences in P concentrations, leaf widths, and dry weight occurred between opposite sides of some leaves. It is postulated that these nonuniform effects on leaves were the result of nonuniform P transport from the roots to leaves.