Browsing by Author "Sharifi, M. Rasoul"

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    Comparative water relations of phreatophytes in the sonoran desert of california
    Nilsen, Erik T.; Sharifi, M. Rasoul; Rundel, Philip W. (Ecological Society of America, 1984)
    The seasonal and diurnal water relations were compared among six desert phreatophytes, two evergreen shrubs, and one deciduous shrub. All species were located in one wash woodland in the Sonoran Desert of southern California. There are several mechanisms by which these Phaenicia have adapted to the desert environment. One group of winter-deciduous phreatophytes (Olneya tesota, Prosopis glandulosa, and Acacia greggii) experienced summer midday leaf water potentials below -4.0 MPa. These phreatophytes had a series of physiological mechanisms for tolerating summer water stress, including seasonal and diurnal osmotic adjustment and the maintenance of high leaf conductance at low leaf water potential. Osmotic adjustment of these three phreatophytes was similar to or greater than that of two evergreen species (Larrea tridentata and Simmondsia chinensis). Dalea spinosa, a stem-photosynthetic phreatophyte, avoided water stress by maintaining a very small leaf area. The summer-deciduous phreatophytes (Hyptis emoryi, and Chilopsis linearis) demonstrated mechanisms of drought avoidance such as change in leaf biomass and low summer leaf conductance. Little osmotic adjustment occurred in the summer-deciduous phreatophytes. The phreatophytic species studied in this investigation have evolved adaptations to water stress that are similar to those of deciduous and evergreen shrubs of the Sonoran Desert. Desert phreatophytes are a complex group of species with varied adaptive mechanisms to tolerate or avoid drought and should not be considered simply as a group of species that avoid desert water stress by utilizing deep ground water unavailable to other desert species of drought tolerance and avoidance.
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    Diurnal and seasonal water relations of the desert phreatophyte prosopis-glandulosa (honey mesquite) in the sonoran desert of california
    Nilsen, Erik T.; Sharifi, M. Rasoul; Rundel, Philip W.; Jarrell, Wesley M.; Virginia, Ross A. (Ecological Society of America, 1983)
    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.