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Responses of Stomata and Water, Osmotic, and Turgor Potentials of Jojoba to Water and Salt Stress¹

Jo]oba (Simmondsia chinensis (Link) Schneider) is desert shrub which can provide a much needed substitute for sperm whale oil. Because of the absence of soil-plant-water data for ]o]oba and the limited amount of such information for desert plants in general, selected plants were water- and salt-stressed in greenhouse soil and sand culture experiments. Responses of leaf water, osmotic, and turgor potentials to stress and associated stomatal behavior were studied.

Leaf water potentials of well watered jojoba plants (measured with thermocouple psychrometers) averaged —20 bars in the daytime and were generally below —10 bars at night, very low in comparison to most mesophytic plants. The recovery of plants which were water stressed to leaf water potentials below —50 bars (soil water potentials below —40 bars) indicates great drought tolerance. Turgot potential decreased linearly with decreasing leaf water potential, showing a lack of osmotic adjustment as leaf water potentials fall below about —30 bars. Turgot potential reached zero at a leaf water potential of about —35 bars, and dropped as low as —10 bars under the most severe water stress without wilting of mature leaves. Plants in the salinity experiment showed osmotic adjustment down to a root medium osmotic potential of —9 bars.

Values of leaf conductance decreased markedly only at very low xylem pressure potentials (pressure chamber measurements) and soil water potentials (—40 bars and —20 bars, respectively), corresponding to about the same values at which turgor potential reached zero. Stomatal closure was continuous with increasing water stress, rather than occurring abruptly at a threshold. There was no significant decrease in leaf conductance at root medium osmotic potentials as low as —9 bars in the salinity experiment.

Conductances of well watered plants to water vapor on lower leaf surfaces were typically similar or greater in value than those of upper leaf surfaces, whereas plants under water stress showed significantly higher conductances on their upper leaf surfaces rather than lower leaf surfaces.

Leaf xylem pressure potentials decreased with increasing transpiration rate under non-limiting soll water conditions and decreased below values predicted for well watered plants at the same transpiration rate as soil water potentials decreased. Thus it was possible to make an estimate of edaphic limitations of water absorption based on leaf xylem pressure potentials despite variation in the pressure potentials in response to microclimatic changes.

Key Words: Conductance to water vapor • Diffusion resistance for water vapor • Soil water potential • Xylem pressure potential • Transpiration rate • Soil-plant-atmosphere continuum

² Soil scientist (Bur. of Land Manage.), professor of soil science, associate professor of plant physiology, research agricultural engineer (U.S. Salinity Laboratory), and professor of plant science, 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: Bur. of Land Manage., U.S. Dep. Interior, 1695 Sprnce Street, Riverside, CA 99507.