Use of Positive Pressures to Establish Vulnerability Curves

Abstract

Loss of hydraulic conductivity occurs in stems when the water in xylem conduits is subjected to sufficiently negative pressure. According to the air-seeding hypothesis, this loss of conductivity occurs when air bubbles are sucked into water-filled conduits through micropores adjacent to air spaces in the stem. Results in this study showed that loss of hydraulic conductivity occurred in stem segments pressurized in a pressure chamber while the xylem water was under positive pressure. Vulnerability curves can be defined as a plot of percentage loss of hydraulic conductivity versus the pressure difference between xylem water and the outside air inducing the loss of conductivity. Vulnerability curves were similar whether loss of conductivity was induced by lowering the xylem water pressure or by raising the external air pressure. These results are consistent with the air-seeding hypothesis of how embolisms are nucleated, but not with the nucleation of embolisms at hydrophobic cracks because the latter requires negative xylem water pressure. The results also call into question some basic underlying assumptions used in the determination of components of tissue water potential using "pressure-volume" analysis.

Is sclerophylly of Mediterranean evergreens an adaptation to drought?

“…(xylem and mechanical tissue) and consequent water On the basis of the above considerations, we can release to symplast, thus supporting a possible expect that the larger the apoplastic volume in a functional role of sclerophylly in adaptation to plant organ (mainly represented by xylem and drought, {b) mechanical cells play a role in leaf remechanical tissue) and the more prone to cavitation watering by a capillarity effect (Yang & Tyree, it is (Cochard et al, 1992), the larger might be the 1992), thus providing an additional driving force for contribution of apoplastic water to that globally leaf water uptake and favouring leaf recovery from expressed from the sample during pressurization.…”

“…water potential, enabling plants to keep stomata open Recently, positive air pressures have been used at low W^ values (Lo Gullo & Salleo, 1988; Salleo for inducing water cavitation in xylem conduits & Lo Gullo, 1990;Pigott & Pigott, 1993); {d) (Cochard et al, 1992;Salleo et al, 1992; mechanical cells make leaves difiicult for herbivores Sperry & Saliendra, 1994). This procedure has to ingest and digest (Turner, 1994;Turner et ah, convincingly confirmed that xylem cavitation is 1993).…”

“…Hard-leaved species are widespread throughout Cochard, Cruiziat & Tyree (1992) were well aware of the world in both arid and humid environments such a problem, especially in the study of leafy (Turner, 1994). Four hypotheses have been adshoots or species with wide vessels or with large vanced for interpreting the possible functional role apoplastic volumes.…”

Hydraulic architecture of trees: main concepts and results

“…Repeating such small increase of pressure with time will lead to a regular dehydration of the sample and to more and more negative values of its water potential (an intuitive image of this process is the progressive squeezing of a sponge full of water). At each chosen step, the level of embolism is estimated by the measured conductivity K h expressed as a percentage of the maximum K h obtained after removal of embolism [17,107].…”

Vulnerability to air embolism of three European oak species (Quercus petraea (Matt) Liebl, Q pubescens Willd, Q robur L)

“…Natural development of embolism in Q petraea Figure 5 (Cochard et al, unpublished data), on 2-4 year-old conifer branches (Abies alba) (Cochard, 1992), and in the current year twigs of 2 diffuse-porous species (Salix alba and Populus deltoides; Cochard et al, 1992 …”