Seasonal Water Potential Components of Sonoran Desert Plants

Abstract

Seasonal water potential components were measured for seven Sonoran Desert plant species representing a variety of growth forms. Three of the species (Amsinckia intermedia, Erodium cicutarium, and Baccharis sarothroides), which occupy the most mesic microhabitats, possessed the highest osmotic potentials and lowest pressure potentials, with very little seasonal adjustment in either parameter. In constrast, two species (Larrea tridentata and Atriplex polycarpa), which experienced substantial periods of water stress, had relatively low osmotic potentials and high pressure potentials with seasonal adjustments occurring in both. Two species (Encelia farinosa and Olneya tesota) exhibited intermediate responses for dawn plant water potential. Encelia also exhibited intermediate osmotic and pressure potential values. However, in Olneya these latter values were very similar to those for Larrea and Atriplex. The capacity for osmoregulation was noted in four of the seven species, with fluctuations in the osmotic potential at full turgor ranging from 0.3 to 0.8 MPa. Adjustments in tissue elasticity and pressure potential were also noted in four of the species. In general these adjustments allow the species to maintain higher turgor pressures at most relative water contents during periods of drought stress. The results indicate that a plant's capacity to avoid water stress is reflected in its water potential components.