The wounds of drought: imaging of embolized xylem conduits in plants

Over the recent decades, the frequency and severity of extreme drought and heat waves have apparently increased in several areas of the globe. Because water availability is a major limiting factor for plant growth and survival, it should not surprise that extreme droughts have already impacted natural vegetation and crops. Massive dieback and mortality of some tree species are being reported with increasing frequency, and in some cases are leading to large-scale forest decline processes, with important consequences on ecosystems and related services.
Physiological mechanisms leading to tree death under drought are only partially understood. Plants release huge amounts of water vapor to the atmosphere in order to assure adequate CO2 uptake for photosynthetic processes. To maintain the hydration status and avoid desiccation, water lost to the atmosphere must be replaced with water absorbed at the root level and transported to leaves via xylem conduits, a complicated network of tubes with diameters ranging from 0.01 to 0.5 mm, made up by the remnant walls of dead cells and connecting roots to foliage. Forces necessary to uplift water are generated by water evaporation at the level of leaf cells, where the adhesive and cohesive properties of water, coupled to the ultrastructure of cell walls, generate curved menisci. In this region a significant negative pressure (= tension) develops and is transmitted down to the root system via continuous water columns in the xylem. Lumen size and wall properties of xylem conduits favor the maintenance of water in the liquid phase, but water under tension is metastable and can undergo a sudden phase change to water vapor in a process known as cavitation. Under drought conditions, limited soil water availability and enhanced evaporative demand can force plants to develop xylem tension exceeding critical thresholds, and resulting cavitation in xylem conduits leads to formation of embolism that occupies the xylem conduit lumen and prevents water transport through the embolized elements, thus reducing plant water transport efficiency. Eventually, embolism can propagate to other conduits through small apertures known as inter-vessel pits, leading to complete blockage of the long-distance water transport system and to plant ‘hydraulic failure’ and death.
Quantifying the vulnerability of plants to drought requires measurements of embolism levels suffered at different water status. The most commonly used techniques rely on measurements of the hydraulic conductance of stems, roots and leaves exposed to air-dehydration to induce progressively lower xylem pressure. Embolism rates in the xylem are quantified by measuring water flow rates through excised stem or root segments while perfusing samples under relatively low pressure (< 10 kPa) to avoid embolism displacement, and after higher pressure (0.1-0.2 MPa) flushes to remove gas entrapped in conduits. This procedure allows to estimate the embolism-induced percentage loss of hydraulic conductance (PLC) of the sample.

Figure 1. Hydraulic measurements of embolism-induced loss of stem conductance (a) were paralleled by in vivo imaging of the functional status of xylem at the SYRMEP beamline (b) to visualize gas-filled and water-filled conduits (c)

Several controversies around the validity of classical hydraulic measurements, and related data interpretation, have been raised in recent years.In particular, it is possible that hydraulic measurements lead tothe occurrence of artefactual increases of PLC when stems are excised from plants, even if under water, while the xylem is under substantial tension. This calls for validation experiments based on in vivo imaging of the functional status of xylem conduits. By using high resolution X-ray phase contrast micro-tomography (microCT) setup available at the SYRMEP beamline, we were able to observe the progressive increase in the number of gas filled xylem conduits in plants of Laurus nobilis L. (Laurel) exposed to drought stress (Fig. 1), and compare image-based estimates of PLC to classical hydraulic measurements (Fig. 2). 


Our data suggest that Laurel stems undergo significant blockage of the water transport conduits at xylem tension between -2.0 and -3.5 MPa, and that hydraulic measurements are consistent with microCT-derived embolism rates.

There is an urgent need to expand microCT analysis of xylem conduits to different plant organs and different species exposed to drought, in order to improve our understanding of the functional limits leading to plant failure under water shortage, and identify key structural and physiological traits underlying the resistance and/or resilience of forest trees and crops to global-change-type droughts.










 

 

Figure 2. Comparison of percent loss of stem hydraulic conductance measured with traditional hydraulic techniques of calculated on the basis of microCT imaging of dehydrated stems of Laurus nobilis L.

 

This research was conducted by the following research team:

Andrea Nardini1, Tadeja Savi1, Sebastiano Salleo1, Adriano Losso2, Stefan Mayr2, Giai Petit3, Serena Pacilè4, Giuliana Tromba4, Patrizia Trifilò5 and Maria A. Lo Gullo5

 

Dipartimento di Scienze della Vita, University of Trieste, Trieste, Italy
2 Institut für Botanik, University of Innsbruck, Innsbruck, Austria
3 Dipartimento Territorio e Sistemi Agro-forestali, University of Padova, Legnaro (PD), Italy
4 Elettra-Sincrotrone Trieste, Area Science Park, Trieste, Italy
5 Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche e Ambientali, University of Messina, Messina, Italy



Contact person:

Andrea Nardini, email: nardini@units.it

 

Reference

Andrea Nardini, Tadeja Savi, Adriano Losso, Giai Petit, Serena Pacilè, Giuliana Tromba, Stefan Mayr, Patrizia Trifilò, Maria A. Lo Gullo, Sebastiano Salleo, “X‐ray microtomography observations of xylem embolism in stems of Laurus nobilis are consistent with hydraulic measurements of percentage loss of conductance”, The New Phytologist 213, 1068-1075 (2017); DOI: 10.1111/nph.14245
 
Last Updated on Monday, 05 June 2017 14:56