Relationship between dehydration tolerance of California ferns and the mechanical strength of their stipes

Background/Question/Methods

In vascular plants, water moves upwards through xylem vessels/tracheids due to negative pressures created by the evaporative pull of water. Under severe dehydration, extreme negative pressures are known to cause xylem cavitation and embolism. In seed-bearing plants, the mechanical support of stem xylem has been shown to be a good predictor of cavitation resistance, presumably because sclerenchyma fibers buttress against micro-fracture or collapse of conduit walls. In spore-bearing plants, such as ferns, current anatomical theory indicates that mechanical support lies external to underlying xylem, in the outer hypodermal region, leaving the central xylem in ferns without any fiber support. In light of this anatomical difference, we hypothesized that there would be no correlation between the mechanical strength of fern stipes and their cavitation resistance. In this study we used dehydration tolerance of twelve California fern species as a surrogate for cavitation resistance. We used a Scholander-Hammel pressure chamber to examine minimum seasonal water potential (Ψ_min) and pressure-volume curves to estimate the osmotic potential of leaf tissue at the turgor loss point (Ψ_s,tlp). We used a four point bending test via an Instron Mechanical Testing Device to estimate stipe mechanical strength (Modulus of Elasticity, MOE). 

Results/Conclusions

We found large differences among our 12 fern species.  Minimum seasonal water potential varied between -1.1 and -8.3 MPa and mean Ψ_s,tlp varied between -1.1 and -3.4 MPa.  Modulus of Elasticity of fern stipes varied nearly ten-fold, between 0.0025 N/m² and 0.023 N/m². Consistent with our initial hypothesis, we found no correlation between the mechanical strength of stems (MOE) and their dehydration tolerance, either in terms of Ψ_s,tlp (r² = 0.005) or Ψ_min (r² = 0.011). A strong positive correlation between bulk modulus of elasticity, obtained from the pressure-volume curves, and MOE, obtained from the four-point bending test (r² = 0.669), gave us confidence in our assessment of mechanical properties.  Taken together, we speculate that the lack of a correlation between stipe mechanical strength and dehydration tolerance reflects the relictual separation of water transport conduits from mechanical support tissues in ferns.