Rectifier-like Activities of Roots of Two Desert Succulents

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Abstract
Axial and radial water flows for roots in response to applied hydrostatic pressure drops, water loss from roots after various periods of drying, and development of new roots after rewetting droughted plants were examined for two sympatric desert succulents. Agave deserti Engelm. and Ferocactus acanthodes (Lemaire) Britton and Rose. For a 40 kPa hydrostatic pressure drop applied to 20 mm long root pieces, radial water flows from the epidermis to the root xylem were 2- to 5-fold greater at the tip than at midlength and were much less than axial flows along the xylem. Upon drying detached roots in air at 20 °C and a water vapour saturation deficit of 1.2 kPa (50% relative humidity), radial water flow decreased more than 10-fold in 3–6 h, and could recover to the original level 6 h after rewetting. The rate of water loss from attached roots of plants dried in air at 20 °C and a 1.2 kPa saturation deficit decreased about 200-fold in 72 h, which would greatly limit water loss from the plant to a drying soil. At 96 h after rewetting roots of A. deserti that had been exposed to air at 20 °C and a 1.2 kPa saturation deficit for 120 h, rehydration of existing roots and development of new roots contributed about equally to water uptake by the whole plant. In summary, roots of these desert succulents can readily take up water from a wet soil but do not lose much water to a dry soil, thus effectively acting like rectifiers with respect to plant-soil water movement.