Six-month-old honey mesquite (Prosopis glandulosa DC) and buttonwood (Conocarpus erectus L.) tree seedlings were grown under different soil water potentials (Ψsoil) in order to determine the minimum soil water potential at which both species can survive and grow fairly well, and to study the soil-plant water relationship at different irrigation regimes. The results showed that growth of Prosopis was not affected significantly until soil water potential was lower than –0.12MPa, while growth of Conocarpus seedlings was affected at water potential of about –0.08MPa, below which, plant height, leaf area and shoot and root dry weights were severely reduced by soil water deficit. Water stress decreased osmotic potential (Ψπ ) of leaves and roots of both species, however the decrease in Ψπ of Conocarpus leaves and roots were much more pronounced (10–15%, resp.) than that of Prosopis. Leaves tended to osmoregulate their cell sap through osmotic adjustment as their content of soluble sugars increased, which occurred in Prosopis at water stress higher than that needed for Conocarpus. The satisfying survival under low Ψsoil could be related to increased osmotic adjustment. Ψsoil values seem to be a more useful tool than the field capacity values to estimate plant water requirements and can be used over an extended period of time, for plant grown under different soil types and different environmental conditions. The study indicates that Conocarpus seedlings can withstand reasonable water stress and can survive at moderately low water potential, but can not be classified as a high drought tolerant or resistant species, while Prosopis can survive at an extremely low water potential and may be classified as a drought tolerant species.
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