Studies of Root Function in Zea mays

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Abstract
The cut ends of excised Z. mays roots were sealed to a pressure transducer and their root pressures recorded. These rose approximately hyperbolically to a maximum value of 4.21 .+-. 0.34 bar after 30-40 min. Xylem exudate could not be collected at this pressure since the flow rate was zero. Samples of exudate were collected at lower applied pressures (.DELTA.P), and .DELTA..pi., the osmotic pressure difference between them and the solution bathing the root, was measured by freezing point depression. A plot of .DELTA.P/.DELTA..pi. against Jv/.DELTA..pi. where Jv is the volume flux was a straight line whose intercept, equal to .sigma., the reflection coefficient, was 0.853 .+-. 0.016. The maximum xylem concentrations of various chemical species were found by a similar extrapolative method and compared with those in the cell sap. Ca2+, Mg2+, NO-3, SO2-4 and most amino acids move from the cells to the xylem down an electrochemical potential gradient; relative to these ions H+, NH+4, glutamine and asparagine are actively transported into the xylem; and H2PO4- and K+ are actively retained in the symplasm.