Testing hypotheses that link wood anatomy to cavitation resistance and hydraulic conductivity in the genus Acer
Summary (2 min read)
Introduction
- Xylem cavitation represents an important constraint on plant survival, and xylem hydraulic conductance potentially limits plant productivity (Rood et al., 2000; McDowell et al., 2008; Brodribb & Cochard, 2009).
- The few integrative studies that have combined anatomical and experimental approaches mostly lack indepth anatomical observations using electron microscopy (Wheeler et al., 2005; Sperry et al., 2006; Cochard et al., 2008; but see Hacke & Jansen, 2009; Pittermann et al., 2010).
- One of the hypotheses the authors tested in the present paper was to evaluate whether pit structure (especially pit membrane thickness and porosity, and pit chamber depth) is more crucial than pit quantity per vessel (whether measured as Ap or Np) for explaining differences in cavitation resistance between Acer species.
- A major hypothesis emerging from previous work is that small conduits are more resistant to cavitation than large ones (Ellmore & Ewers, 1985; Hargrave et al., 1994; Cai & Tyree, 2010), resulting in a trade-off between safe xylem (small conduits) and efficient xylem (large conduits).
- The authors combined detailed anatomical observations at the tissue level (i.e. the three-dimensional hydraulic network) and intervessel pit level with empirical measures on xylem hydraulics for seven Acer species.
Materials and Methods
- Plant material Branches c. 1 m in length and 8–11 mm in diameter belonging to different parts of an individual’s crown were sampled around midday for a total of seven Acer taxa during the months of May–June 2009.
- The fraction of silicone-filled vessels at each length was counted and the data analyzed for vessel length distribution as explained in Christman et al. (2009).
- Measurements were performed on five dried wood samples (without pith and bark) per species using the water displacement method (Chave et al., 2006).
- Horizontal pit membrane diameter (Dm), pit aperture shape (APf, short ⁄ long axis of pit aperture), and aperture fraction (Fap = aperture area per pit membrane area) were measured on 50 pits per species from several vessels.
- Afterwards, specimens were stained with 118 mM uranyl acetate dissolved in ethanol for at least 30 min at 37 C, and rinsed three times with propanol 100%.
Results
- The species studied showed a wide range of cavitation resistance (Fig. 1).
- More negative MCP was also associated with an increase in the fraction of vessel wall area occupied by pits (Fp, P = 0.054).
- This trade-off is also reflected in the stem-specific conductivities shown in Fig. 1: more cavitation-resistant species tended to have a lower initial KSa than the more vulnerable species.
- There was a marginally significant trend for KXa to increase with greater Np (P = 0.06).
- No other measured pit parameter was found to be important (Dm, Dp, Ap, APf, Fig. 6).
Discussion
- Superficially, the wood anatomy of the seven Acer taxa studied lacked obvious variation using standard light microscope observations (Fig. 4), despite a 2 MPa range in MCP and a clear difference in stem-specific hydraulic conductivity (Fig. 1).
- Greater cavitation resistance was also associated with features that would minimize mechanical stresses on the aspirated pit membrane (Fig. 2; smaller Lp, larger APf, and smaller Fap).
- The seeming unimportance of vessel diameter for cavitation resistance in Acer contradicts observations from other woody angiosperm comparisons where species with wider vessels tend to be more vulnerable (Carlquist, 1977; Ellmore & Ewers, 1985; Hargrave et al., 1994; Wheeler et al., 2005; Hacke et al., 2006; Cai & Tyree, 2010).
- Shorter vessel length was also strongly related to lower KXa (Lv, Lv*, Fig. 2), more so than narrower Dv (Fig. 2).
- The correlations the authors observed for Le may result from the link between Le and Lv (Table 4).
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Frequently Asked Questions (16)
Q2. What have the authors stated for future works in "Testing hypotheses that link wood anatomy to cavitation resistance and hydraulic conductivity in the genus acer" ?
Further research is required to verify whether this asymmetrical sculpturing pattern in Acer can be related to the possible functions put forward in the literature for intact helical thickenings. Potential traits explaining this trade-off are identified in Fig. 7.
Q3. What is the role of vessel wall thickening in Acer?
Vessel wall thickening may also decrease the contact angle between water and conduit walls to nearly zero, causing enhanced wettability of the walls that may reduce embolism formation and facilitate refilling (Kohonen & Helland, 2009).
Q4. What is the pressure at which implosion of the vessel occurs?
The pressure at which implosion of the vessel occurs is a function of the ‘thicknessto-span’ ratio, that is, the double vessel wall thickness per lumen diameter (Tw Dv–1).
Q5. What was the main focus of the study?
Their major focus was to evaluate previously published anatomical hypotheses with respect to cavitation resistance and conducting efficiency, and to reveal traits implicated in any trade-off the authors might observe between safety and efficiency within the genus.
Q6. What is the importance of a slit-like aperture in conifers?
The hydraulic importance of proportionally smaller apertures (Domec et al., 2006; Pittermann et al., 2010) and reduced chamber depth (Hacke & Jansen, 2009) has also been identified in conifer species with torus-margo membranes.
Q7. How many Acer species were collected during the month of May?
Plant materialBranches c. 1 m in length and 8–11 mm in diameter belonging to different parts of an individual’s crown were sampled around midday for a total of seven Acer taxa during the months of May–June 2009.
Q8. How many dried wood samples were used?
Measurements were performed on five dried wood samples (without pith and bark) per species using the water displacement method (Chave et al., 2006).
Q9. What is the original interpretation of the relationship between vessel strength and resistance?
The original interpretation (Hacke et al., 2001) was that cavitation-resistant plants tend to have more negative sap pressures, and hence need stronger walls.
Q10. What is the reason for the thickenings?
A simple explanation is that the thickenings acts to strengthen the vessel wall in more arid-adapted taxa, which experience more negative xylem pressures.
Q11. What is the significance of the two mechanical parameters?
Because both Tw Dv–1 and WD increase with cavitation resistance in many broad-scale studies (Hacke et al., 2001; Jacobsen et al., 2005, 2007; Chave et al., 2009; Onoda et al., 2009), the authors investigated whether these two mechanical parameters are also significant at the genus level.
Q12. What is the effect of a slit-like aperture on the pit border?
Once aspirated, further stress would be minimized by narrower slit-like apertures (greater APf) that occupy a proportionally smaller area of the pit border (smaller Fap).
Q13. What is the relationship between vessel wall thickness and MCP?
The extent of vessel wall thickenings in Acer is correlated with MCPThis new finding (Figs 2, 5) supports the idea that the presence of vessel wall thickenings can be linked with drought adaptation (Carlquist, 1966, 1975), although vessel wall sculpturing is also common in the less xeric Mediterranean and the far more mesic cool temperate flora that experience frost (Carlquist, 1966; Baas et al., 1983; Baas & Schweingruber, 1987).
Q14. How many measurements were taken to define the thickness of the intervessel pit membranes?
Image analysis was undertaken using ImageJ software on at least 25 measurements to define the thickness of the intervessel pit membranes (Tm).
Q15. What was the relationship between vessel diameter and MCP?
Dv was not well related to MCP (Fig. 2; P = 0.075), but the untransformed mean vessel length Lv was: longer vessels tended to be more vulnerable than shorter ones by this measure (P = 0.006; Fig. 2).
Q16. What is the significance of the trade-off between Lv and KXa?
As discussed earlier, Lv emerges as a trait that is arguably central to the safety–efficiency trade-off in Acer, being functionally linked to both MCP and KXa.