XVI International Botanical Congess

The proportion of embolized vessels in the veins of maize leaves was measured during 24 h by direct counting in snap-frozen samples in the cryo-scanning electron microscope. All vessels were sap filled at night. Vessels of intermediate and small veins, and the tracheids of lateral veins, were sap filled throughout the 24 h. The large metaxylem vessels of lateral veins were embolized during the day. The percentage of these vessels embolized was maximum (>70%) at 1400 h, and declined during the afternoon to 20% at 1800 h. Leaf water potential reached a minimum (-1.2 MPa) at this time. The protoxylem cavity of the lateral veins was much less embolized than the large vessels, although it was of comparable diameter. The observations are interpreted in terms of the refilling hypothesis which is part of the compensating pressure theory of water transport.