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Methods to induce xylem cavitation
Several methods have been developed in order to explore the vulnerability to cavitation on the whole tree or on a segment of the tree. To construct a vulnerability curve, the dehydration level has to be controlled. The xylem pressure potential (P, MPa) is the key variable that is followed in relation with the embolism rate (Sperry et al., 1988). Cavitation could be induced by various methods such as bench dehydration, air pressurization and centrifugation (Cochard et al., 2013).
Bench dehydration
Bench-dry method (Sperry and Tyree, 1988) is considered a reference method because it is a natural way for inducing cavitation. Plant segment is left to dry out and afterward xylem pressure is measured using a pressure chamber on non-transpiring covered leaves or withstem psychrometers. The relevant pressure is the most negative pressure the plants have experienced during the drought treatment, usually during midday. This method requires a long time to induce cavitation in xylem. More, it is necessary to use a relatively large sample size (typically a leafy branch >1 m long) because very fast dehydration could induce a high heterogeneity of water stress in the branch and it should be avoided. The organ is cut from an intact plant and left to freely dehydrate in the air.
The study of Tyree et al. (1992) verified that VCs obtained from intact plants and cut branches are similar. However, the latter procedure is preferable as branch water status is better controlled. To construct a vulnerability curve for evaluating the xylem vulnerability to cavitation, samples have to be exposed to several xylem pressures and their embolism rate measured.
New samples which are exposed to increasing water stress are required. In general, it requires sizeable samples to construct a curve and as a consequence, the vulnerability curve obtained A customized pressure sleeve is applied to the center of branch segment where cavitation is induced by air pressurization. The distal end of branch segment is connected with a vertical solution-filled tube, solution flows from this tube through the segment and exits at the proximal end due to difference in water pressure. The solution is collected and weight to calculate flow rate. Modified from Ennajeh et al. (2011b).
Vessel length and reliability of the techniques for measuring xylem vulnerability to cavitation
Among the techniques to induce xylem cavitation, the dehydration or bench-dry technique is considered a reference way to induce cavitation. Indeed, dehydration of large branch segments is similar to what happened in nature. However, due to it requires a long period of time to let the segment to dry off, the use of faster techniques like Cavitron and air-injection are increased. Despite several advantages of Cavitron and air-injection technique, there is an ongoing debate on the reliability of these techniques especially when measuring xylem vulnerability to cavitation on long-vessel species (Choat et al., 2010; Cochard et al., 2010; Cochard et al., 2013; Delzon and Cochard, 2014; Ennajeh et al., 2011b; Jacobsen and Pratt, 2012; Sperry et al., 2012; Tobin et al., 2013; Torres-Ruiz et al., 2014).
Several studies have pointed out the need in considering vessel length of the species when measuring vulnerability to cavitation (Choat et al., 2010; Cochard et al., 2010; Delzon and Cochard, 2014; Ennajeh et al., 2011a; Torres-Ruiz et al., 2014). These studies have demonstrated that the xylem conduits were found far more vulnerable to cavitation when using Cavitron and air-injection methods on segments that have vessel length exceeding the segment length.
The first evidence of this concern comes from a study testing the effects of stem length on the vulnerability to cavitation using Cavitron (Cochard et al., 2010): as the samples become shorter, they could become more vulnerable to cavitation when spun in the Cavitron (Figure 18A). The xylem vulnerability to cavitation measured on 4 species with different vessel lengths and different techniques is presented. Oak (Quercus robur L.) is a ring-porous species with very long vessels. Birch (Betula pendula Roth) is a diffuse-porous species with very short vessels. Peach (Prunus persica (L.) Batsch) has vessels of intermediate length and Scots pine (Pinus sylvestris L.) is a coniferous species with tracheids. Clear changes in shapes of VC from sigmoidal shape (call “s”-shape) to exponential-shape (called r-shape) are observed The sigmoidal curve, with s-shape, (solid line) is considered a normal vulnerability curve with a safety range of xylem pressure (gray zone) whereas the exponential curve, with r-shape, (dash line) would be an anomalous curve without safety pressure zone. This latter type of vulnerability curve is suspect to be a result of cut open vessel artifact using the centrifugal or air-injection techniques. From Cochard et al. (2013).
Variation of xylem vulnerability to cavitation
The P50 value could varies in different levels such as across the species, within the species or even between different organs on the same plant. These variations are considered as the result of the genetic control and/or the adjustment of the individual to environmental conditions, that is to say the phenotypic plasticity. In this following section, the variation of xylem vulnerability to cavitation is discussed in two aspects: the interspecific variation and the intraspecific variation. In addition, on the intraspecific level, the genetic variability and phenotypic plasticity are discussed.
Interspecific variation
Numerous studies have shown that the P50 is differed across the species (Choat et al., 2012; Maherali et al., 2004; Pockman and Sperry, 2000; Tyree et al., 2003), with a large range of variation. The value could be as high as −0.04 MPa which was found on a liana or woody vine species, Celastrus orbiculatus (Tibbetts and Ewers, 2000) or very low as −14.1 MPa for conifers (Willson et al., 2008). Several studies were carried out to investigate the xylem vulnerability to cavitation of herbaceous species such as in common bean (Phaseolus vulgaris), fireweed (Chamerion angustifolium), maize (Zea mays) and rice (Oryza sativa) with differences vulnerability to cavitation between these species (Cochard, 2002a; Holste et al., 2006; Maherali et al., 2009; Mencuccini and Comstock, 1999; Stiller et al., 2005). Focusing on broadleaved tree species, the vulnerability to cavitation found on angiosperms might rank from −0.09 MPa in Albizia julibrissin (Li et al., 2008) up to −8.12 MPa in Ceratonia siliqua according to Cochard et al., unpublished date in Choat et al. (2012). In gymnosperms, it could vary from −1.74 MPa in Podocarpus latifolius (Vander Willigen et al., 2000) to −14.1 MPa in Actinostrobus acuminatus and Juniperus pinchotii (Choat et al., 2012; Willson et al., 2008).
Within tree variation in xylem vulnerability to cavitation
Vulnerability to cavitation was also different between organs of the same plant (Hacke and Sauter, 1996; Rood et al., 2000; Sangsing et al., 2004; Tyree et al., 1993). The study of Tyree et al. (1993) showed that petiole xylems of Juglans regia were more vulnerable to cavitation than stem xylems. This finding suggests segmentation in vulnerability to cavitation which allows plant discarding expendable organs in order to preserve more important organs from dehydration (Zimmermann, 1983). In contrast, some other studies have found that leaf xylem such as in the midribs and petioles are less vulnerable to cavitation when compared to branch xylem (Hacke and Sauter, 1996; Sangsing et al., 2004). Hacke and Sauter (1996) found that petioles of Populus balsamifera, a deciduous tree, were less vulnerable to cavitation compared to branches and its roots were the most vulnerable organ. A study on Hevea brasiliensis (Sangsing et al., 2004) has also found that xylem in midrib are far less vulnerable to cavitation compared to petioles and branches with P50 ranking from −2.72 to −1.22 MPa.
Furthermore, the study of Cochard et al. (1997) demonstrated that even with the same type of organ but at the different positions on an individual tree, vulnerability to cavitation could differ. Leaf rachises from the lower canopy layer were found less vulnerable compared to the ones from upper layer; the P50 values of leaf rachises of Fraxinus exculsior from upper and lower layers were −2.5 and −3.5 MPa, respectively. This contrasted vulnerability to cavitation is related to xylem efficiency; the leaf rachises in upper layer had lower leaf specific conductance and smaller vessel sizes. The vulnerability to cavitation is not only varied because of a genetic control but it is also under the influences of environmental condition and the interaction of both. These influences need to be clearly understood in order to use the vulnerability to cavitation as a criterion for drought tolerance selection in agronomic species. The relationship of this trait with drought tolerance within a species has to be firstly studied since the species might adopt different.
Walnut trees: the investigation of xylem vulnerability to cavitation
The investigation on xylem vulnerability to cavitation using Cavitron technique was conducted on two species of Juglans: Persian walnuts (Juglans regia L.) and hybrid walnuts (J. regia x J. nigra). Branch samples were harvested from 15 to 20 years old trees grown in orchard at INRA site Crouël, Clermont-Ferrand in south-central France (45°46′27″N, 3°8′36″E; altitude 338 m).
The Persian walnut trees were comprised of 48 individuals from six cultivars: Chandlers (Ch, 4 individuals), Fernettes (Ft, 3), Fernors (Fo, 4), Franquettes (Fq, 17), Laras (La, 15), and Serrs (Se, 4). They are important commercial cultivars for nut productions in France and the USA with different origins and parentages (McGranahan and Gale, 1994) (Table 4). On October 2011, 17 branches were randomly harvested from some of the 48 Persian walnut trees for a preliminary test with the Cavitron. The branches were mature, current year, at the least 0.45 m long and harvested from the southern side of the canopy (fully exposed to sunlight). After they were harvested, branches were immediately defoliated, wrapped with moist paper then placed in plastic bag to avoid dehydration and stored in cold storage (4°C) for maximum three days before the analysis was completed. These branches were used to test the suitable length of sample (0.28 and 0.38 m) and the direction of water flow through sample while spinning on the Cavitron (sense and antisense flows). The results from these measurements were used as a protocol for further investigation of xylem vulnerability to cavitation.
Three to six branches were collected from each individual Persian walnut tree on November 2011 for the investigation of genetic variability of vulnerability to cavitation. The sampling was done following similar harvesting and preparing protocol previously mentioned. Twenty branches were randomly chosen from these sampled populations for native embolism Nine out of ten clones used in this study are presented in the diagram except clone RRIT 408 which has not been analyzed for its genetic. Figure is provided by André Clément-Demange, UMR AGAP, CIRAD, Montpellier.
Table of contents :
I. Drought stress
1. Effects of drought stress
2. Defense mechanisms against water deficit
2.1 Drought avoidance mechanisms
2.2 Drought tolerance mechanism
II. Water properties and hydraulic architecture
1. Water properties
2. Water potentials
3. The ascent of water
3.1 Water absorption and soil-root boundary
3.2 Transpiration and stomatal control
3.3 Hydraulic properties in xylem: conductance or resistance
III. Xylem: cell types and structure
IV. Xylem cavitation and embolism
1. The mechanism of cavitation
2. Resistance to xylem implosion and cavitation resistance
3. How to evaluate the drought-induced xylem cavitation?
3.1 Cavitation and embolism detections
Acoustic detection
Anatomic detection
Hydraulic detection
3.2 Methods to induce xylem cavitation
Bench dehydration
Air pressurization
Centrifugation
3.3 Vessel length and reliability of the techniques for measuring xylem vulnerability to cavitation
4. Variation of xylem vulnerability to cavitation
4.1 Interspecific variation
4.2 Intraspecific variation
Genetic variability
Phenotypic plasticity
Within tree variation in xylem vulnerability to cavitation
V. Pros and cons of the investigated species
1. Walnut tree
2. Rubber tree
3. Apple tree
