Abstract:

Water stress-induced xylem embolism has important physiological consequences for plant growth and survival in most species. Effects of soil water conditions on leaf xylem cavitation and embolism occurrence were investigated throughout the major growth stages in field-grown winter wheat (Triticum aestivum L. cv. Shixin 733) plants. The xylem cavitation and embolism were measured by monitoring ultrasonic acoustic emission (AE) from leaf xylem. It was found that daily AE generally started around 6:00–8:00 a.m. and ceased between 21:00 p.m. and 3:00 a.m. of the following day. Embolism occurred to a much greater extent in leaves of water stress (WS) plants than in well-watered (WW) control. The total number of AE occurred in a period of 24 hours (24 h AEs) from WS plants was higher than that from WW control between jointing and mid grain filling stage. The difference between treatments peaked between anthesis and initial grain filling, with 24 h AEs being 4.6 times greater in WS than in WW plants. The number of 24 h AEs from WS plants, however, dropped greatly to values similar to or even lower than that of WW plants under more severe soil water stress after mid grain filling, which phenomenon is termed “AE depletion”. Vulnerability curves, which were fitted to Logistic function by plotting cumulative AEs versus corresponding leaf water potential (Ψl), showed that leaf xylem vessels were generally most vulnerable at Ψl of about －1.00 MPa. Soil water stress led to much more vulnerable leaf xylem, which greatly reduced leaf embolism resistance and consequently impaired xylem safety. The higher degree of cavitation and embolism occurrence in WS plants resulted basically from a more vulnerable water conducting system, and much less from the greater xylem water tensions, as compared with WW control.

Key words: Xylem cavitation and emboilsm, Leaf water potential, Vulnerability curves, Acoustic emission, Winter wheat