Drought experience and cavitation resistance in six shrubs from the Great Basin, Utah

Abstract

Shrubs of the Great Basin desert in Utah are subjected to a prolonged summer drought. One potential consequence of drought is a reduced water transport capability of the xylem. This is due to drought-induced cavitation. We used the centrifuge method to measure the vulnerability of root and stem xylem to cavitation in six native shrub species. The shrubs fall into three categories with regards to rooting depth, vegetative phenology and plant water status during drought. The “summer green” group ( Chrysothamnus viscidiflorus, Atriplex canescens, Atriplex confertifolia ) sustains summer drought with a relatively shallow root system (<2.5 m), but maintains leaf area. A “drought deciduous” group ( Grayia spinosa , Tetradymia glabrata ) has shallow roots but responds to drought by dropping leaves. A “phreatophytic” group (e.g. Chrysothamnus nauseosus ) avoids low water potentials by developing a deep root system that gives them access to deeper soil moisture. We hypothesized that cavitation resistance across these groups was adjusted to the amount of drought stress a species experienced. Drought experience was quantified by measuring plant water potentials over two growing seasons. As expected, we found a significant relationship between the cavitation resistance of roots and stems of a species and the minimum seasonal water potential. Shallow rooted, drought deciduous species showed the lowest water potentials prior to leaf shedding and they had the most resistant xylem. The phreatophytic shrub Chrysothamnus nauseosus always maintained favourable water potentials and had the most susceptible xylem, and summer green species were intermediate. This correlation between cavitation resistance and water potential suggests that a safe xylem is associated with some “cost” for the plant. We could identify two costs or trade-offs. The first trade-off was between safety and transport efficiency as reflected by vessel diameter. In 5 out of 7 species, there was an intra specific relationship between vessel diameter and cavitation resistance. Roots, having larger vessels than branches, were also more vulnerable to xylem dysfunction. There was also an inter specific correlation between vessel diameter and cavitation resistance, but only for combined root and stem data. A second trade-off was found between safety and construction cost. Resistant xylem was strongly correlated with high wood densities, suggesting that thicker cell walls and/or narrower conduits translate into lower air permeability. Air entry into the conduit is known to be the cause of cavitation.