Root xylem embolisms and refilling. Relation to water potentials of soil, roots, and leaves, and osmotic potentials of root xylem sap

Embolism and refilling of vessels was monitored directly by cryomicroscopy of field-grown corn (Zea mays L.) roots. To test the reliability of an earl ier study showing embolism refilling in roots at negative leaf water potent ials, embolisms were counted, and root water potentials (Psi(root)) and osm otic potentials of exuded xylem sap from the same roots were measured by is opiestic psychrometry. All vessels were full at dawn (Psi(root) -0.1 MPa). Embolisms were first seen in late metaxylem vessels at 8 AM. Embolized late metaxylem vessels peaked at 50% at 10 AM (Psi(root) -0.1 MPa), fell to 44% by 12 PM (Psi(root) -0.23 MPa), then dropped steadily to zero by early eve ning (Psi(root) -0.28 MPa). Transpiration was highest (8.5 mu g cm(-2) s(-1 )) between 12 and 2 PM when the percentage of vessels embolized was falling . Embolized vessels were refilled by liquid moving through their lateral wa lls. Xylem sap was very low in solutes. The mechanism of vessel refilling, when Psi(root) is negative, requires further investigation. Daily embolism and refilling in roots of well-watered plants is a normal occurrence and ma y be a component of an important hydraulic signaling mechanism between root s and shoots.