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

The mechanism of water-stress-induced xyletn embolism was studied in three species of conifers: Abies balsamca (L.) Mill., Picea rtibens Sarg. and ./uniperus virginiana L. Each species showed a characteristic relationship between xylem tension and the loss of hydraulic conductivity by air etnbolistn. Abies balsamea and Picea rubens began to etnbolize at tensions between 2 and 3 MPa and were completely non-conducting between 3 and 4 MPa. Juniperus virginiana was least vulnerable, beginning to etnbolize at 4 and still tetaining apptoxitnately 10% conductivity at 10 MPa. As wilh a previous study of the vessel-bearitig Aeer saccharum Marsh., a brief perfusion of btanch segnienls with an oxalic acid and calciutn solution (10 and 0.1 mol m ', respectively) iticteased the vulnerability of the xylem to embolism; ihis was especially pronounced in Abies balsamea. In order lo test whether etnbolisin was caused by aspiration of air into functional tracheids frotn neighbouring etnbolized ones (the 'air-seeding' hypothesis), hydrated branch segtnents were injected with air al various presstuTS and tneasured for etnbolistn. Restills supported lhe air-seeding hypothesis because the telationship between injection ptessure and etnbolistn for both native and oxalic-calciumtrcated segtnents was essentially the same as for embolism itiduced by xylem tension. Structural and experimental evidence suggested the air seeding occurted through inter-ttacheid pit tnembranes when the thickened torus region of the metnbrane became displaced frotn its nortiial sealing position over the pit aperture. Thus, the etnbolistn-inducing tension tnay be a ftinction of pit tnembtane flexibility. This tension is of ecological significance because il reflects to sotne extent the range of xyletn tensions to which a species is adapted. Key-words: .Abies balsamea (L.) Mill,: Pieea rubens Sarg,: Juniperus virginiana L.: water slress; xylem embolism: hydraulic conductivity: conifers: wood anatomy.