Summer water relations of the desert phreatophyte Prosopis glandulosa in the Sonoran Desert of southern California

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Abstract
Prosopis is a genus of phreatophytic trees inhabiting hot deserts and semiarid grasslands of the world. Although desert trees are exposed to unusual environmental temperature and water stress, few investigations have evaluated their water relations. This is particularly true for Prosopis species growing in areas where a large portion of their water use comes from ground water. Water relations components for Prosopis glandulosa were studied at Harper's Well, near the Salton Sea, California during the summer months of 1980. Maximum temperatures (49° C), irradiance (2,000 μE/m2/sec), and vapor pressure deficit (5.3 kPa) were reached in July. During this time Prosopis glandulosa predawn xylem pressure potentials were below-3.0 MPa. Prosopis glandulosa at Harper's Well is able to maintain open stomata during high temperatures, high vapor pressure deficit and at low estimated turgor pressure. Leaf resistance measurements indicate that stomata are open primarily in the morning, but may reopen in the afternoon in trees with greater water resources. Osmotic potentials of juvenile shoots were higher (-1.0 to-2.5 MPa) than mature shoots (-3.5 MPa). Estimated turgor potential remained low (0.1–0.2 MPa) during the morning and early afternoon. Estimated turgor pressure increased from August to September as temperatures and vapor pressure deficit decreased. Leaf conductance was strongly associated with leaf vapor pressure deficit and estimated turgor potential but poorly associated with xylem pressure potential. Prosopis stomata seem to be uncoupled from tissue water potential until-4.8 MPa is reached.