External Pressures Based on Leaf Water Potentials Do Not Induce Xylem Sap to Flow at Rates of Whole Plant Transpiration from Roots of Flooded or Well-drained Tomato and Maize Plants. Impact of Shoot Hydraulic Resistances

We tested the hypothesis that leaf water potentials (ψL) are good guides to the positive pneumatic pressure needed to drive water flow through detopped root systems at the rate of whole plant transpiration. Unless sap flows at this rate, solute concentrations and rates of delivery are poor estimates of those carried by the transpiration stream of intact plants. In flooded and well-drained tomato (Lycopersicon esculentum L.) and maize (Zea mays L.) plants, pressures based on ψLproduced flow rates that were two–four times faster than whole plant transpiration. Covering leaves for several hours resulted in a less negative ψL . The smaller pneumatic pressures based on these measurements gave slower sap flow but rates still exceeded whole plant transpiration. The overrun is ascribed to the removal of hydraulic resistance when the shoots were excised for sap collection. In tomato, differences in ψLacross putative sites of resistance, or of increased sap flow when resistances were removed surgically, located major hydraulic resistances in leaf laminae and at stem–petiole junctions. Pneumatic pressures to generate realistic sap flow rates were therefore chosen by reference to experimentally derived pressure–flow curves and not to ψL . An alternative method of sap collection was tested where xylem sap was sampled from whole plants with roots pressurized until hydrostatic water potential of the xylem slightly exceeded atmospheric pressure. Calcium and osmolality concentrations in sap from a cut petiole close to the shoot base of tomato plants were similar to those obtained from detopped plants using pressures that produced transpiration-like flow rates. Sampling from different positions revealed that concentrations and delivery rates of calcium and osmolality decreased as xylem sap was transported upwards through the shoot.

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