Root xylem embolisms: implications for water flow to the shoot in single-rooted maize plants

The recent measurements of high percentage embolized vessels in a sample of maize axile roots during the day raised the question whether such interruptions to water flow could occur in plants that had only a single water-supplying root. Zea mays L. plants were grown in the field on a root system developed entirely from the single primary root. The formation of vessel embolisms in this root was monitored by cryo-scanning electron microscopy. Vessels were sap-filled at dawn, but 60% of late metaxylem (LMX) vessels were embolized by 0830 h. This high percentage persisted until early afternoon, then declined to reach 30% by 1800 h. Leaf balance pressures rose from about 0.3 MPa at dawn to a peak mean of 1.2 MPa at 1300 h, then declined to 0.5 MPa by 1800 h. Peak embolism in LMX vessels was reached in the early morning at balance pressures of 0.7-0.8 MPa. Some refilled in the afternoon while balance pressures were 0.5-0.7 MPa. The six to eight LMX vessels at the root base would deliver most of the transpiration stream to the shoot. Our finding that up to 60% were embolized during the day, while the plants continue to transpire, suggests that the mean flow velocity in the vessels is higher than the value of 270 m h(-1) estimated recently by others for completely full vessels in the mesocotyl of single-rooted corn plants.