Water transport to the leaves is influenced in part by leaf-specific hydraulic conductance (LSC). Studies have found that LSC may be sensitive to environmental factors such as the supply of water in the soil or the evaporative demand. Among individuals LSC can vary due to changes in biomass allocation such as differences in the ratio of leaf area/sapwood area (AL/AS). Increased sapwood specific hydraulic conductivity (KS) also affects LSC but leads to increased xylem vulnerability to embolism. These two aspects of LSC were examined separately in seedlings of Heteromeles arbutifolia, a native California chaparral species. H. arbutifolia is present in shade as well as full sun, where summer drought conditions resulting in water stress are intensified by the combination of incoming radiation and high air temperatures. In order to examine the effects of light and water on hydraulic architecture, H. arbutifolia seedlings were grown in a common outdoor garden under a factorial design with sun and shade conditions and high and low soil water availability. Physiological measurements made on the seedlings at the end of the growing season, including stomatal conductance and maximum transpiration rates, indicated significant differences in water use in the four treatments. Aspects of canopy architecture were also measured before seedlings were harvested to quantify biomass allocation patterns. Hydraulic conductance was determined using a bench-top conductance set-up in order to examine variation in LSC. In particular plasticity in different aspects of LSC was examined to determine whether AL/AS varied in response to higher transpiration rates while KS responded to variation in soil water supply.

Key words: chaparral, Heteromeles arbutifolia, hydraulic architecture, leaf-specific conductance, plasticity