Plant pneumatics: stem air flow is related to embolism – new perspectives on methods in plant hydraulics

Abstract

SummaryWood contains a large amount of air, even in functional xylem. Air embolisms in the xylem affect water transport and can determine plant growth and survival. Embolisms are usually estimated with laborious hydraulic methods, which can be prone to several artefacts.Here, we describe a new method for estimating embolisms that is based on air flow measurements of entire branches. To calculate the amount of air flowing out of the branch, a vacuum was applied to the cut bases of branches under different water potentials.We first investigated the source of air by determining whether it came from inside or outside the branch. Second, we compared embolism curves according to air flow or hydraulic measurements in 15 vessel-and tracheid-bearing species to test the hypothesis that the air flow is related to embolism.Air flow came almost exclusively from air inside the branch during the 2.5-min measurements and was strongly related to embolism. We propose a new embolism measurement method that is simple, effective, rapid and inexpensive, and that allows several measurements on the same branch, thus opening up new possibilities for studying plant hydraulics.

The Pneumatron: An automated pneumatic apparatus for estimating xylem vulnerability to embolism at high temporal resolution

“…Similar to the optical method (Brodribb et al, ), microtomography (Brodersen et al, ), and acoustic emissions approach (Milburn, ; Vergeynst, Dierick, Bogaerts, Cnudde, & Steppe, ), the pneumatic method estimates embolism instead of the percentage loss of conductivity. However, previous comparisons showed a good agreement between the estimated vulnerability curves when using the pneumatic method and the hydraulic apparatus (Pereira et al, ; Zhang et al, ) or Cavitron (Zhang et al, ). In the same way, the curves estimated with the Pneumatron showed a strong agreement with hydraulic measurements for S. terenbithifolius branches, although slight differences for E. camaldulensis branches were found (Figure ).…”

“…As described in Pereira et al (), the increase in moles of AD in the tubes (Δn, mol) was calculated according to the ideal gas law using the initial ( P i , in kilopascal) and final ( P f ) pressure measured: where n i (mol) is the initial number of moles of air, and n f (mol) is the final number after a predetermined time. R is the gas constant (8.314 kPa·L·mol −1 ·K −1 ), T is the room temperature (20°C = 293.15 K), and V r is the discharging tube volume (L).…”

“…The vulnerability curves may be estimated either by directly measuring the loss of conductivity due to embolism formation or by quantifying the number or volume of embolized vessels (Venturas et al, ). The loss of conductivity is measured directly by using a hydraulic apparatus (Sperry, Donnelly, & Tyree, ) or Cavitron centrifuge (Cochard, ), whereas embolism is quantified through 2‐D or 3‐D images (Brodersen, McElrone, Choat, Matthews, & Shackel, ; Brodribb et al, ), acoustic emissions (Milburn, ), or airflow using the pneumatic method (Pereira et al, ). Although measuring xylem conductivity in intact plants would be desirable, the use of plant segments coupled with a hydraulic apparatus is subject to interferences such as the background flow (Hacke et al, ; Pereira & Ribeiro, ), wounding response, introduction of air bubbles (Espino & Schenk, ), ionic effect (Jansen et al, ), or potential refilling of embolized conduits (Melcher et al, ).…”

Hydrological niche segregation defines forest structure and drought tolerance strategies in a seasonal Amazon forest

“…Both methods provide similar estimates of P 50 and P 88 values (i.e. water potentials at which the PLC is 50 and 88%; Pereira et al., ; Zhang et al., ).…”

“…We measured xylem vulnerability to embolism as the relationship between the percentage loss of xylem conductivity (PLC) and xylem water potential (Ψ x in MPa). PLC was estimated from percentage of air discharged (PAD) using the pneumatic method (Pereira et al., ). For the two understorey species, Rinorea pubiflora and Amphirrhox longifolia , we used the hydraulic bench method to calculate the PLC (Sperry, Donnelly, & Tyree, ).…”