Protections against drought-induced xylem embolisms in shrubs: Resistance, repair, and redundancy

Background/Question/Methods

In engineering terms, the hydraulic system of a plant is a “negative pressure flow system” in which conducting cells in the xylem are linked from roots to leaves.  This type of hydraulic system is prone to failure due to the introduction of air bubbles, which create embolisms. Plants frequently develop xylem embolisms, especially under water limited conditions.  Any pressure flow system can be protected against hydraulic failure in three ways: resistance, reparability, and redundancy.  Resistance to hydraulic failure in plants has been a major focus of recent research and requires plants to possess xylem pit membranes that are penetrable to gas only under large pressure and reinforced hydraulic systems that can withstand large pressures.  Embolism repair depends on living cells in and adjacent to xylem.  Redundancy reflects the number of root to leaf conducing pathways and their interconnectedness (hydraulic integration).  The objectives of this study were (1) to determine if interactions and tradeoffs exist between the three protection strategies against xylem embolisms and (2) if protection types and interactions between them change with increasing habitat aridity.  The study included 21 shrub species from nine plant families growing in five field sites in California, Texas, Georgia, and North Carolina.

Results/Conclusions

Resistance to embolism formation was documented by xylem vulnerability curves, embolism repair by diurnal measurements of stem hydraulic conductance, and redundancy by dye tracer movement in the xylem and by the ratio of radial to axial hydraulic xylem conductivity.  Wood anatomical analyses were conducted to determine the structural bases for protection strategies.  Across all species there was no evidence for functional or structural trade-offs between the three types of protection, but within a given habitat species fell into groups of “resisters” or “repairers”.  “Repairers” had larger vessel diameters, lower vessel densities, and higher proportions of vessel-associated axial parenchyma than “resisters”.  High redundancy, i.e. a high degree of hydraulic integration, was mostly associated with the resistance strategy, possibly because the higher vessel densities necessitated by having narrower vessels crates dense packing of vessels with higher connectedness among them.  The findings point to the importance of interpreting functional and structural traits within the appropriate environmental context for each organism.