Diurnal and seasonal water relations of the desert phreatophyte prosopis-glandulosa (honey mesquite) in the sonoran desert of california
Diurnal and Seasonal water relations were monitored in a population of Prosopis glandulosa var. torreyana in the Sonoran Desert of southern California. Prosopis glandulosa at this research site acquired its water from a ground water source 4-6 m deep. Measurements of diurnal and seasonal cycles of aboveground environmental conditions, soil moisture, and soil water potential (to 6 m depth) were taken to ascertain environmental water availability and water stress. Leaf water potential, leaf conductance, leaf transpiration, relative saturation deficit of leaves, osmotic potential, and turgor potential were measured to evaluate plant adaptations to environmental water stress. Soil water potential was low (-4.0 to -5.0 MPa) in surface soil in relation to deep soil (-0.2) MPa). This difference was due to high surface soil salinity and low surface water content. the climatic conditions at the research site produced extreme water stress conditions in summer months when temperatures reached 50-C, vapor pressure deficit (VPD) reached 8 kPa, and surface soil water potential was below -4.5 MPa. Although considerable plant water stress developed in these trees (midday leaf water potential -4.8 MPa), osmotic adjustment occurred and turgor was maintained on a diurnal and seasonal cycle. Prosopis glandulosa has adapted to avoid water stress by utilizing deep ground water, but this phreatophyte has also evolved physiological adaptations, such as osmotic adjustment and seasonally changing stomatal sensitivity to VPD, which result in greater tolerance of water stress.