CHANGES IN ROOT HYDRAULIC CONDUCTIVITY FOR 2 TROPICAL EPIPHYTIC CACTIAS SOIL-MOISTURE VARIES

The tropical epiphytic cacti Epiphyllum phyllanthus and Rhipsalis bacc ifera experience extreme variations in soil moisture due to limited so il volumes and episodic rainfalls. To examine possible root rectificat ion, whereby water uptake from a wet soil occurs readily but water los s to a dry soil is minimal, responses of root hydraulic conductivity ( L(p)) to soil drying and rewetting were investigated along with the un derlying anatomical changes. After 30 d of soil drying, L(p) decreased 50%-70% for roots of both species, primarily because increased suberi zation of the periderm reduced radial conductivity. Sheaths composed o f soil particles, root hairs, and mucilage covered young roots and hel ped reduce root desiccation. Axial (xylem) conductance increased durin g drying due to vessel differentiation and maturation, and drought-ind uced embolism was relatively low. Within 4 d of rewetting, L(p) for ro ots of both species attained predrought values; radial conductivity in creased for young roots due to the growth of new branch roots initiate d during drying and for older roots due to the development of radial b reaks in the periderm. The decreases in L(p) during drought reduced pl ant water loss to a dry soil, and yet maximal water uptake and transpi ration occurred within a few days of rewetting, helping these epiphyte s to take advantage of episodic rainfalls in a moist tropical forest.