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Gaseous Exchange of Simmondsia chinensis (Jojoba) Measured with a Double Isotope Porometer and Related to Water Stress, Salt Stress, and Nitrogen Deficiency¹

Gaseous exchange of Simmondsia chinensis [(Link) Schneider] (a desert shrub which may provide a substitute for sperm whale oil) was studied by exposing leaves simultaneously to radioisotopes of water (³HHO) and CO₂ (¹⁴CO₂) to determine conductances to water vapor and total conductances to CO₂ assimilation, respectively. Fixation rates of CO₂ and mesophyll conductances to CO₂ were calculated. Leaf conductance to water vapor and CO₂ fixation rates decreased markedly only at very low values of soil-water and leaf-xylem pressure potentials and recovered to maximum values after having been subjected to soil water potentials below --40 bars, indicating the extreme drought tolerance of S. chinensis. The ratios of
increased as water stress became more intense, suggesting that transpiration was reduced relatively more than photosynthesis, which could have considerable value under drought stress. Plants under intense water stress generally had little or no reduction in mesophyll conductance to CO₂.

There was no significant decrease in conductance to water vapor, CO₂ fixation rate, or mesophyll conductance to CO₂ with increasing salinity down to root medium osmotic potentials as low as —9 bars. This is consistent with the reported high salt tolerance of S. chinensis. Nitrogen-deficient plants had significant reductions in CO₂ fixation rate and mesophyll conductance to CO₂ but not in conductance to water vapor.

Key Words: CO₂ fixation rate • Conductance to water vapor • Mesophyll conductance to CO₂ • Transpiration ratio • Tritiated water • ¹⁴C • Soil water potential • Xylem pressure potential

² Post doctoral researcher, assistant professor of biology, professor of soil science, and professor of plant science, Univ. of California, Riverside, respectively. Part of the senior author's thesis for partial fulfillment of the Ph.D. degree at the Univ. of California, Riverside. Senior author's present address is Bureau of Land Management, USDI, 1695 Spruce St., Riverside, CA 92507.

Received for publication May 24, 1976.