Xylem wall collapse and cavitation in Pinus spp. COCHARD HERVÉ, MAYR STEFAN, COUTAND CATHERINE, JERONIMIDIS GEORGE Xylem wall collapse and cavitation in Pinus spp. COCHARD HERVÉ, MAYR STEFAN, COUTAND CATHERINE, JERONIMIDIS GEORGE International Symposium on Wood Sciences OCTOBER 24-29, 2004, MONTPELLIER, FRANCE

Sap ascent in trees : a vulnerable pipeline ?   Sap is transported in xylem conduits under negative pressures Forces on water : Sap Cavitation Forces on wall : Wall Collapse  Two theoretical physical limitations for such a transport How vulnerable actually are xylem conduits in trees? … in Pinus ?

1- Xylem vulnerability to cavitation Walnut leaf blade Walnut leaf petiole (Optical microscopy) (cryo-SEM) Direct observations

Initial Conductance Saturated Conductance PLC = 1- Embolised conduit Hydraulic technique (Sperry et al 1988) % embolism = % loss of hydraulic conductance XYL’EM

Loss of xylem conductance in Pinus during dehydration  cav =-2.5 MPa Pinus sylvestris

Ultrasonic Acoustic technique (Tyree et al 1984) Cavitation Event I15I DSM 4615 Physical Acoustic Corp. ( khZ) Ultrasonic Acoustic Events =

+ Water Utrasonic Acoustic Events in Pinus during dehydration  cav = MPa Pinus sylvestris -2.5 MPa

Liquid water P = 0 MPa Possible underestimation with the hydraulic technique? Vapor water P=-0.1 MPa

Microscope 0 r Light Upstream reservoir Downstream reservoir Microscope Xylem negative pressure P= -0.5    R 2 Xylem conductance K= (dr/dt) / 0.5  2 [R 2 – (R-r) 2 ] Use of the centrifugal force to generate cavitation (Cochard 2002)

Xylem conductance under negative pressure in Pinus  cav = -2.5 MPa Pinus sylvestris

Dehydration versus centrifugation in Pinus -2.5 MPa Pinus sylvestris

Xylem Cavitation in Pinus : conclusions No detectable xylem embolism above -2.5 MPa 1. ? Acoustic events above -2.5 MPa are not cavitation events in functional tracheids 2.

2- Xylem vulnerability to wall collapse Pinus nigra, stem at -2.5MPa Cavitation but no collapse What about tracheids in needles ?

Wall collapse in pine needles during dehydration Pinus cembra 0 MPa Cryo-SEM

Wall collapse in pine needles during dehydration Pinus cembra -4 MPa No cavitation Wall deformation for tracheids in contact with living cells (thinner walls)

Wall collapse in pine needles during dehydration Pinus cembra -4.6 MPa No cavitation Wall deformation for most tracheids

Wall collapse in pine needles during dehydration Pinus cembra -5 MPa No cavitation Xylem entirely collapsed

Wall collapse in pine needles during dehydration Pinus cembra <-5 MPa Cavitation Wall relaxed

Observation of frozen needles with a fluorescent microscope 0 MPa

Xylem anatomy and collapse thresholds in Pinus spp cembramugo nigrasylvestris 40 µm -3.4 MPa-3.5 MPa -2.6 MPa-1.8 MPa

A biomechanical model of xylem wall collapse Actual geometry of a vascular bundle Finite elements (MARC) Preliminary results Well watered needle

A biomechanical model of xylem wall collapse Moderate water stressHigh water stress

Xylem wall collapse in Pinus : conclusions In stems: No evidence of xylem wall collapse (cavitation occurs first) In stems: No evidence of xylem wall collapse (cavitation occurs first) In needles: Evidences of xylem wall collapse during dehydration (P<-2MPa) In needles: Evidences of xylem wall collapse during dehydration (P<-2MPa) ( Evidences of rapid reversal of wall collapse upon rehydration) Tracheid wall in stems is stiffer because reinforced for tree mechanical support

Functional consequences of xylem collapse and cavitation Stomatal closure prevents xylem cavitation in stems … but not xylem wall collapse in needles (hydraulic signal?)

Pines (trees) are remarkably invulnerable to xylem cavitation and collapse Pines (trees) are remarkably invulnerable to xylem cavitation and collapse The actual physical limitation for water transport occurs only at very negative pressures (<-2MPa) The actual physical limitation for water transport occurs only at very negative pressures (<-2MPa) Under severe water stress, these limitations constrains tree water loss and stomatal function Under severe water stress, these limitations constrains tree water loss and stomatal function Sap ascent in trees : a vulnerable pipeline ?