Water‐stress‐induced xylem embolism in three species of conifers

Abstract

Abstract. The mechanism of water‐stress‐induced xylem embolism was studied in three species of conifers: Abies balsamea (L.) Mill., Picca rubens Sarg, and Juniperus virginiana L. Each species showed a characteristic relationship between xylem tension and the loss of hydraulic conductivity by air embolism. Abics balsamea and Picca rubens began to embolize at tensions between 2 and 3 MPa and were completely non‐conducting between 3 and 4 MPa. Juniperus virginiana was least vulnerable, beginning to embolize at 4 and still retaining approximately 10% conductivity at 10 MPa. As with a previous study of the vessel‐bearing Accr saccharum Marsh., a brief perfusion of branch segments with an oxalic acid and calcium solution (10 and 0.1 mol m−3. respectively) increased the vulnerability of the xylem to embolism; this was especially pronounced in Abies balsamea. In order to test whether embolism was caused by aspiration of air into functional tracheids from neighbouring embolized, ones (the ‘air‐seeding’hypothesis), hydrated branch segments were injected with air at various pressures and measured for embolism. Results supported the air‐seeding hypothesis because the relationship between injection pressure and embolism for both native and oxalic‐calcium‐treated segments was essentially the same as for embolism induced by xylem tension. Structural and experimental evidence suggested the air seeding occurred through inter‐tracheid pit membranes when the thickened torus region of the membrane became displaced from its normal sealing position over the pit aperture. Thus, the embolism‐inducing tension may be a function of pit membrane flexibility. This tension is of ecological significance because it reflects to some extent the range of xylem tensions to which a species is adapted.