Herbivory and plant water status of jojoba [Simmondsia chinensis (Link) Schn.] in the Sonoran Desert in Arizona

Abstract

Herbivory may alter the ability of a plant to compete with ungrazed neighbors for limited resources such as water in arid environments. Water status parameters of grazed and ungrazed jojoba [Simmondsia chinensis (Link) Schn.], a drought-tolerant, evergreen shrub of the Sonoran Desert, were compared over a 2-year period in Arizona. Ungrazed shrubs were larger than grazed shrubs but had similar canopy leaf area to larger grazed shrubs and about four times the leaf area of smaller grazed shrubs. Xylem water potential, transpiration rate, and stomatal resistance were similar for grazed and ungrazed shrubs both in early spring and later in early summer at the peak of the drought period. Xylem water potential of jojoba was generally as low or lower than wolfberry (Lycium exsertum A. Gray) and was much lower than paloverde [Cercidium microphyllum (Torrey) Rose & Johnston], which apparently had deep roots in contact with wetter soil than jojoba. The ability of jojoba to transpire and take up water while enduring low plant water potential probably allows it to reduce the soil matric potential around its roots below that available to possible competitors. As evaporative demand increased and water potential decreased into the drought period, jojoba transpiration was greatly reduced by stomatal closure in mid-afternoon which resulted in increased xylem water potential as water uptake allowed recovery of transpirational water loss. The ability of jojoba to grow during short seasonal periods of water availability in the Sonoran Desert and its ability to endure low plant water potential while continuing limited photosynthesis are suggested as important factors in jojoba tolerance to herbivory as well as drought.