Relationship of Xylem Embolism to Xylem Pressure Potential, Stomatal Closure, and Shoot Morphology in the Palm Rhapis excelsa.

Xylem failure via gas embolism (cavitation) induced by water stress was investigated in the palm Rhapis excelsa (Thumb.) Henry. Xylem embolism in excised stems and petioles was detected using measurements of xylem flow resistance: a decrease in resistance after the removal of flow-impeding embolisms by a pressure treatment indicated their previous presence in the axis. Results supported the validity of the method because increased resistance in an axis corresponded with: (a) induction of embolism by dehydration, (b) increased numbers of cavitations as detected by acoustic means, (c) presence of bubbles in xylem vessels. The method was used to determine how Rhapis accommodates embolism; results suggested four ways. (a) Embolism was relatively rare because pressure potentials reach the embolism-inducing value of about -2.90 megapascals only during prolonged drought. (b) When embolism did occur in nature, it was confined to the relatively expendable leaf xylem; the stem xylem, which is critical for shoot survival, remained fully functional. (c) Even during prolonged drought, the extent of embolism is limited by complete stomatal closure, which occurred at the xylem pressure potential of -3.20 +/- 0.18 megapascals. (d) Embolism is potentially reversible during prolonged rains, since embolisms dissolved within 5 h at a pressure potential of 0.00 megapascals (atmospheric), and xylem sap can approach this pressure during rain.

[1]  J. Milburn,et al.  STUDIES OF CAVITATION IN ISOLATED VASCULAR BUNDLES AND WHOLE LEAVES OF PLANTAGO MAJOR L. , 1974 .

[2]  M. Tyree,et al.  Ultrasonic Acoustic Emissions from the Sapwood of Thuja occidentalis Measured inside a Pressure Bomb. , 1984, Plant physiology.

[3]  J. Grace,et al.  Concurrent measurements of stem density, leaf and stem water potential, stomatal conductance and cavitation on a spaling of Thuja occidentalis L. , 1984 .

[4]  W. R. N. Edwards,et al.  Relations between water content, potential and permeability in stems of conifers , 1982 .

[5]  M. Zimmermann,et al.  Transport and Storage of Water , 1982 .

[6]  M. Zimmermann.  Hydraulic architecture of some diffuse-porous trees , 1978 .

[7]  M. Zimmermann,et al.  Anatomy of the Palm Rhapis excelsa, IX. Xylem Structure of the Leaf Insertion , 1983, Journal of the Arnold Arboretum..

[8]  M. Tyree,et al.  Ultrasonic acoustic emissions from the sapwood of cedar and hemlock : an examination of three hypotheses regarding cavitations. , 1984, Plant physiology.

[9]  D. A. Baker.  Water Flow in Plants , 1980 .

[10]  D. Macdougal.  Reversible variations in volume, pressure, and movements of sap in trees , 1925 .

[11]  M. Tyree,et al.  Water stress induced cavitation and embolism in some woody plants , 1986 .

[12]  M. Tyree,et al.  Cavitation Events in Thuja occidentalis L.? : Utrasonic Acoustic Emissions from the Sapwood Can Be Measured. , 1983, Plant physiology.

[13]  G. M. Smith,et al.  The hydrostatic-pneumatic system of certain trees : movements of liquids and gases , 1929 .