Xylem Embolism in the Palm Rhapis Excelsa

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Abstract
Xylem failure via gas embolism (cavitation) was investigated in Rhapis excelsa (Palmae). Embolism was detected using measurements of xylem flow resistance in excised stems and petioles: a decrease in resistance after the removal of flow-impeding embolisms by a pressure treatment indicated their previous presence in the axis. Results suggested that Rhapis avoids serious damage from embolism in at least four ways. 1) Xylem pressure potentials reached embolism-inducing levels (c. -2.90 MPa) only during prolonged drought. 2) When embolism did occur, it was confined to leaf xylem; stem xylem, most critical to shoot survival, remained fully functional. This is due in part to hydraulic architecture: 70 to 85% of shoot xylem resistance is in the leaf, and thus xylem pressures are much lower in leaves than stems. 3) Even during prolonged drought, the amount of embolism is probably limited by complete stomatal closure, which occurred at xylem pressure potentials of -3.20 ± 0.18 MPa. 4) Embolism is potentially reversible during prolonged rains, since embolism dissolved within 5 h at zero pressure (atmospheric), and xylem pressure potential can reach zero during extended rain.

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