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Changes in Root Hydraulic Conductivity for Two Tropical Epiphytic Cacti as Soil Moisture Varies

Gretchen B. North and Park S. Nobel
American Journal of Botany
Vol. 81, No. 1 (Jan., 1994), pp. 46-53
Stable URL: http://www.jstor.org/stable/2445561
Page Count: 8
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Changes in Root Hydraulic Conductivity for Two Tropical Epiphytic Cacti as Soil Moisture Varies
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Abstract

The tropical epiphytic cacti Epiphyllum phyllanthus and Rhipsalis baccifera experience extreme variations in soil moisture due to limited soil volumes and episodic rainfalls. To examine possible root rectification, whereby water uptake from a wet soil occurs readily but water loss to a dry soil is minimal, responses of root hydraulic conductivity (Lp) to soil drying and rewetting were investigated along with the underlying anatomical changes. After 30 d of soil drying, Lp decreased 50%-70% for roots of both species, primarily because increased suberization of the periderm reduced radial conductivity. Sheaths composed of soil particles, root hairs, and mucilage covered young roots and helped reduce root desiccation. Axial (xylem) conductance increased during drying due to vessel differentiation and maturation, and drought-induced embolism was relatively low. Within 4 d of rewetting, Lp for roots of both species attained predrought values; radial conductivity increased for young roots due to the growth of new branch roots initiated during drying and for older roots due to the development of radial breaks in the periderm. The decreases in Lp during drought reduced plant water loss to a dry soil, and yet maximal water uptake and transpiration occurred within a few days of rewetting, helping these epiphytes to take advantage of episodic rainfalls in a moist tropical forest.

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