Seeing is believing: what visualising bubbles in the xylem has revealed about plant hydraulic function

Abstract

Maintaining water transport in the xylem is critical for vascular plants to grow and survive. The drought-induced accumulation of embolism, when gas enters xylem conduits, causes declines in hydraulic conductance (K) and is ultimately lethal. Several methods can be used to estimate the degree of embolism in xylem, from measuring K in tissues to directly visualising embolism in conduits. One method allowing a direct quantification of embolised xylem area is the optical vulnerability (OV) technique. This method has been used across different organs and has a high spatial and temporal resolution. Here, we review studies using the OV technique, discuss the main advantages and disadvantages of this method, and summarise key advances arising from its use. Vulnerability curves generated by the OV method are regularly comparable to other methods, including the centrifuge and X-ray microtomography. A major advantage of the OV technique over other methods is that it can be simultaneously used to determine in situ embolism formation in leaves, stems and roots, in species spanning the phylogeny of land plants. The OV method has been used to experimentally investigate the spreading of embolism through xylem networks, associate embolism with downstream tissue death, and observe embolism formation in the field.

Extreme drought can deactivate ABA biosynthesis in embolism‐resistant species

“…Graphs were generated using the Sigmaplot software (v.10; Systat Software). Leaf vulnerability curves were plotted as mean Ψ l at 5% increments of total embolized xylem area as recommended by Cardoso et al (2022).…”

Abscisic acid acts essentially on stomata, not on the xylem, to improve drought resistance in tomato

“…To analyze the relationship between plant water potential and embolism formation, a linear regression was fitted between time and Ψ w , so the Ψ w of each image could be determined. Mean and standard errors for vulnerability curves were determined for every 1% of the embolized xylem area and plotted as per (Cardoso et al, 2022). The Ψ w at 12%, 50%, and 88% cumulative embolism (P 12 , P 50 and P 88 ) were obtained for each curve.…”

“…Mean and standard errors for vulnerability curves were determined for every 1% of the embolized xylem area and plotted as per (Cardoso et al, 2022). The Ψ w at 12%, 50%, and 88% cumulative embolism (P 12 , P 50 and P 88 ) were obtained for each curve.…”

“…Consequently, plants that have evolved xylem highly resistant to embolism are likely more tolerant to low water potentials and drought (McAdam and Cardoso, 2019). Xylem resistance to embolism has been shown to vary enormously across (Choat et al, 2012) and within species (Cardoso et al, 2020(Cardoso et al, , 2022Johnson et al, 2022;Rodriguez-Dominguez et al, 2018). Species with narrower xylem vessels are often more resistant to embolism than species with wider conduits (Isasa et al, 2023).…”

Enhanced drought resistance in tomato via reduced auxin sensitivity: delayed dehydration and improved leaf resistance to embolism

“…Great effort has been devoted to understanding the structural drivers for improved embolism resistance (lower P 50 ), and pit membranes seem to be central in this matter (Kaack et al 2021). Lower P 50 has been mechanistically linked to thicker pit membranes and smaller pit membrane areas (Kaack et al 2019; Cardoso et al 2022; Levionnois et al 2022). Studies on pit membrane structure, however, can only be performed using transmission electron microscopy, which is labor‐intensive and time‐consuming (Cardoso 2022) as well as prone to artifacts (Zhang et al 2020).…”