Summary• Xylem sap under high tension is in a metastable state and tends to cavitate, frequently leading to an interruption of the continuous water columns. Mechanisms of cavitation repair are controversially discussed.• Magnetic resonance (MR) imaging provides a noninvasive, high spatial and temporal resolution approach to monitor xylem cavitation, refilling, and functionality.• Spin density maps of drought-stressed maize taproots were recorded to localize cavitation events and to visualize the refilling processes; c. 2 h after release of the nutrient solution from the homemade MR imaging cuvette that received the root, late metaxylem vessels started to cavitate randomly as identified by a loss of signal intensity. After c. 6 h plants were rewatered, leading to a repair of water columns in five out of eight roots. Sap ascent during refilling, monitored with multislice MR imaging sequences, varied between 0.5 mm min −1 and 3.3 mm min −1. Flow imaging of apparently refilled vessels was performed to test for functional repair. Occasionally, a collapse of xylem vessels under tension was observed; this collapse was reversible upon rewatering.• Refilling was an all-or-none process only observed under low-light conditions. Absence of flow in some of the apparently refilled vessels indicates that functionality was not restored in these particular vessels, despite a recovery of the spin density signal.

Conversion-electron Mossbauer spectroscopy of ultrathin pure ' Fe(110) films on W(110) coated by Ag provides detailed information on the mode of growth, the film structure, and the local structure of magnetic order, in particular its thermal decrease, because difterent structural components are marked by strongly differing magnetic hyperfine fields.A related CEMS analysis of Fe(110)/Ag (100) multilayers' recently showed relaxation broadening of the lines, indicating a fine-grained structure in the monolayer regime, in accordance with the surface energies [y", =2.0 J m, yA = l. 1 J m ' (Ref. 11)], which indicate a nonwetting condition. ' ' Conversely, for Fe(110) on the

The effect of N nutrition on root xylem (water) flow in Phaseolus vulgaris was studied by using the 1 H NMR flow imaging technique. Plants pre-cultivated on CaSO 4 -solution were transferred to NMR-compatible split-root cuvettes filled with halfstrength nitrate medium (2.5 mM). After 6 days fullstrength (5 mM) nitrate medium was supplied to both compartments (NN treatment), or only to one compartment, whereas the other one was perfused with full-strength (5 mM) ammonium medium (NA treatment). Imaging was performed 7-8 days later. Root growth, xylem flow velocity and xylem pressure (measured with the xylem pressure probe) were largely unaffected by N nutrition. Under NA conditions, volume flow was significantly lower in ammonium-compared to nitrate-fed roots due to a reduced number of functional xylem elements. This was apparently not related to anatomical differences nor to an enhanced number of cavitated vessels in ammonium-fed roots. Vessels in these roots may become dysfunctional by clogging to prevent cavitation.

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