The mechanism of water‐stress‐induced embolism in two species of chaparral shrubs

The mechanism of water-stress-induced embolism of xylem was investigated in Malosma laurina and Heterome- les arbutifolia, two chaparral shrub species of southern California. We tested the hypothesis that the primary cause of xylem dysfunction in these species during dehy- dration was the pulling of air through the pores in the cell walls of vessels (pores in pit membranes) as a result of high tensions on xylem water. First, we constructed vul- nerability-to-embolism curves for (i) excised branches that were increasingly dehydrated in the laboratory and (ii) hydrated branches exposed to increasing levels of external air pressure. Branches of M. taurina that were dehydrated became 50% embolized at a xylem pressure potential of —1*6 MPa, which is equal in magnitude but opposite in sign to the +1-6 MPa of external air pressure that caused 50% embolism in hydrated stems. Dehydrated and pressurized branches of//, arbutifotia reached a 50% level of embolism at -6-0 and +6-4 MPa, respectively. Sec- ondly, polystyrene spheres ranging in diameter from 20 to 149 nm were perfused through hydrated stem segments to estimate the pore size in the vessel cell walls (pit mem- branes) of the two species. A 50% or greater reduction in hydraulic conductivity occurred in M. laurina at perfu- sions of 30,42, 64 and 82 nm spheres and in H. arbutifolia at perfusions of 20 and 30 nm spheres. Application of the capillary equation to these pore diameters predicted 50% embolism at xylem tensions of -2-2 MPa for M. taurina and -6-7 MPa for H. arbutifotia, which are within 0-7 MPa of the actual values. Our results suggest that the size of pores in pit membranes may be a factor in determining both xylem efficiency and vulnerability to embolism in some chaparral species. H. arbutifolia, with smaller pores and narrower vessels, withstands lower water potentials but has lower transport efficiency. M. taurina, with wider pores and wider vessels, has a greater transport efficiency but requires a deeper root system to help avoid catastro- phically low water potentials.

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