Resistance to xylem cavitation in evergreen ferns correlates with seasonal dehydration levels, not mechanical strength

Background/Question/Methods

In our previous study, we examined the patterns of seasonal water use in eight fern species during historic drought in southern California. Surprisingly, minimum seasonal water potentials (Ψ_min) ranged from -1.2 to -8.4 MPa among the three evergreen species examined. Five additional species experienced seasonal dormancy by defoliation or tissue desiccation. In order to identify traits that might contribute to this broad range of response to drought conditions, we measured resistance to water stress-induced cavitation of stipe xylem (xylem embolism) and mechanical strength of the fern stipes. Interestingly, in the evergreen chaparral overstory of our ferns, it has been shown that cavitation resistance correlates with both Ψ_min and stem mechanical strength. Consequently, we hypothesized that cavitation resistance (water potential at 50% loss of conductivity, Ψ₅₀) would also correlate with Ψ_min in the eight fern species examined. In contrast to chaparral, we hypothesized that stipe mechanical strength would be uncoupled from cavitation resistance due to the absence of sclerenchyma fibers adjacent xylem conduits. Cavitation resistance was assessed using a centrifuge to generate negative tensions comparable to those experienced by the ferns in nature and an apparatus to measure hydraulic conductivity. Mechanical strength was assessed using an Instron Mechanical Testing Device.

Results/Conclusions

Contrary to our expectations, cavitation resistance did not correlate strongly with minimum seasonal water potential across all eight species of ferns (r² = 0.431). However, when only the three evergreen species were considered, the correlation improved (r² = 0.991), presumably because the tissues of evergreen fronds must seasonally persist and withstand extreme differences in seasonal water potential (e.g. -1.2 to -8.4 MPa). This result is consistent with the previous observation for the evergreen chaparral overstory that experiences Ψ_min between -2 to -11 MPa. Consistent with our initial hypothesis, mechanical strength (modulus of elasticity, MOE) of fern stipes was not a good predictor of cavitation resistance (Ψ₅₀, r² = 0.0003). This is in contrast to previous results for evergreen woody chaparral, in which fibers provided mechanical support to xylem conduits to resist cavitation. It appears that the differing geometry of fern stipes in contrast to chaparral, without supportive fibers to buttress xylem conduits, may uncouple mechanical support from cavitation resistance.