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

Abstract

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.