Responses of a CAM Plant to Drought and Rainfall: Capacitance and Osmotic Pressure Influences on Water Movement

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Abstract
Daily and seasonal patterns in water flow and water potential were investigated for the Crassulacean acid metabolism succulent Agave deserti during an extended summer drought and for a period following rainfall. Field measurements of transpiration and of osmotic pressure changes over selected 24 h periods were used as input variables for a computer model of water flow that was based on an electrical circuit analog of the whole plant. Parameters such as root resistance and tissue capacitance were also varied to reflect the effects of changing plant or soil water status. The model predicted internal water flow and water potential during the drought cycle and was used to assess the role of tissue osmotic properties in water uptake from the soil and in internal water redistribution. For plants under wet soil conditions, 55% of the night-time transpiration was derived from water storage, this storage being recharged during the day. As drought progressed, transpiration and the nocturnal increase in osmotic pressure declined, although the osmotic pressure itself increased. The difference in osmotic pressure between the water storage tissue and the chlorenchyma caused a net flow of water into the chlorenchyma after 3 weeks of drought, thereby increasing chlorenchyma turgor pressure. Simulations also indicated that a large increase in root resistance must occur to prevent substantial water loss from the plant to the dry soil. After rainfall, recharge of plant water storage was complete within one week, although full recovery in the amplitude of daily osmotic pressure variations took longer.