Different aspects of cavitation resistance in Ceratonia siliqua, a drought-avoiding mediterranean tree.

Potted plants of Ceratonia siliqua L., growing in a greenhouse, were used to detect xylem cavitation (in terms of ultrasound acoustic emissions AE) in internodes and node-to-petiole (N-P) junctions, after different periods of drought (9, 16 and 23 d). Diurnal AE were only 100 in internodes of watered (W) plants but 320, 1250 and 2460 in 9-, 16- and 23-d stressed ones. In N-P junctions, AE were only 15 to 20% with respect to internodes. Stem perfusion with dye allowed measurement of the percentage of xylem conduit transverse area blocked by cavitation. This was 2% in internodes of W-plants and 5-2, 13-8 and 40-4% in those of 9-, 16- and 23-d stressed ones. In N-P junctions, 18-5% of the xylem conduit transverse area was blocked in the 23-d stressed plants only. The major resistance to cavitation exhibited by the N-P junctions is interpreted in terms of their greater number of narrow xylem conduits. The percentage of blocked xylem conduits within a range of diameters showed that the narrower a xylem conduit, the less likely it was that cavitation would occur. After rewatering, the release of the xylem blockage caused by cavitation occurred within 2 h. Our data suggest that C. siliqua can be considered to be a cavitation avoider, especially in its stem-to-leaf transition zones .