Water is the key to the growth and survival of desert plants. It can be stored in the shoots, which is especially apparent for succulent plants such as Agave deserti (the common desert agave) and Ferocactus acanthodes (a barrel cactus). Internal redistribution of such stored water can influence water flow in the transpiration stream, the pathway from the soil to the root, stem, and leaves, and lead to non-steady state (transient) effects. Such transient effects. which can be considered by incorporating capacitors in electrical circuit analogs of the transpiration stream, have received little attention for plants, although the non-steady state is actually the rule, not the exception, for plants under natural conditions. Succulent plants such as A. deserti and F. acanthodes employ Crassulacean acid metabolism, where CO2 uptake occurs predominantly at night when temperatures are lower and hence water loss from the plant to the air is less. The CO2 taken up at night leads to the formation of malate, with a consequent large increase in osmotic pressure. This increase in osmotic pressure influences water movement by osmosis, a complication that has previously not been included in electrical circuit analogs for plant water relations. A new method will be employed that allows incorporation of this non-linear factor. New imaging methods developed in medical research will also be used to observe root location in the soil. Using an electric circuit analog, water movement and water potential will be simulated at various locations with computer programs developed to solve electrical circuits. Each component (such as a leaf) in the transpiration stream will be assigned a capacitance, which will be connected through a storage resistance to the main axial flow of the vascular tissue. Field measurements of desert succulents will also be made to interpret further the key adaptations of desert succulents to water stress and water use. Synthesizing the data into a dynamic model should enhance our quantitative understanding of the cellular properties of plants and the importance of capacitance and osmotic pressure in maintaining water potentials favorable for plant survival in the field, especially when they are exposed to natural or man- made changes in environmental conditions.
