RESPONSES OF STOMATA AND WATER, OSMOTIC, AND TURGOR POTENTIALS OF JOJOBA TO WATER AND SALT STRESS

Leaf water potentials of well-watered jojoba [Simmondsia chinensis (Link) Schneider] 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. Turgor potential decreased linearly with decreasing leaf water potential, showing a lack of osmotic adjustment as leaf water potentials fell below about -30 bars. Turgor 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; 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 soil 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 estimate the edaphic limitations of water absorption based on leaf xylem pressure potentials despite variation in the pressure potentials in response to microclimatic changes.