Phloem water transport maintains stem growth in a drought-stressed crop cactus (Hylocereus undatus)

Hylocereus undatus [(Haworth) Britton and Rose] is a vine cactus from central America that has been established as a new fruit crop (pitaya) in many tropical and subtropical countries. In order to develop improved irrigation practices, the relationships between water parameters and growth were studied in rooted stem cuttings growing in pots with sandy soil under well-watered and drought-stressed conditions, in a controlled environment. Soil water potential rapidly decreased from -0.02 to -1.5 MPa during the first 5 days of drought. However, growth of new stems emerging from the succulent mature stems only decreased significantly after 3 weeks of drought. After 3 weeks of drought, the water content of growing stems decreased by 2% (P < 0.05) and their water potentials by 0.05 MPa (P > 0.05), as compared with the irrigated controls. At the same time, water content in drought-treated mature stems decreased by 4% (P < 0.05) and water potentials by 0.25 MPa (P < 0.05). Several lines of evidence indicated that active phloem supply of assimilates and associated water reserves from mature stems was the mechanism that allowed developing stems of H. undatus to maintain growth under well-watered and drought conditions: 1) Girdling the phloem of growing stems rapidly inhibited stem elongation. 2) Secretion of sucrose-containing nectar by growing stems was maintained during drought. 3) The water potential gradient was in the wrong direction for xylem transport from mature to young growing stems and axial hydraulic conductivity in young stems was either zero or comparatively low.