WATER POTENTIALS FOR DEVELOPING CLADODES AND FRUITS OF A SUCCULENT PLANT, INCLUDING XYLEM-VERSUS-PHLOEM IMPLICATIONS FOR WATER-MOVEMENT

Developing cladodes had tower water potentials and developing fruits h ad higher water potentials than the underlying cladodes of the widely cultivated prickly pear cactus, Opuntia ficus-indica. The 0.06 MPa low er value in 4-week-old daughter cladodes indicated a typical water pot ential gradient from the underlying cladode along the xylem of -0.2 MP a m(-1); the 0.17 MPa higher value in 4-week-old fruits, which decreas ed to 0.07 MPa by 10 weeks, implicated the phloem as their supplier of water. The phloem sap of the underlying cladodes had an osmotic press ure of only 0.90 to 0.98 MPa, so the phloem could supply a relatively dilute solution to the photosynthetically dependent fruits (daughter c ladodes of O. ficus-indica are photosynthetically independent at 4 wee ks). Although the water potentials were similar for adjacent tissues, the osmotic pressures were lower for the water-storage compared with t he photosynthetic tissue; the osmotic pressures were higher for xylem sap from fruits, for which xylary flow apparently occurred toward the underlying cladodes, than for daughter cladodes. The relative capacita nce (change in relative water content divided by change in tissue wate r potential) was approximately 0.71 MPa(-1) for the water-storage tiss ue and the photosynthetic tissue of both daughter cladodes and fruits at 4 weeks of age. When these organs approached maturity at 10 weeks, the relative capacitance increased about 40% for their water-storage t issue, but decreased 30% for their photosynthetic tissue. As the plant water content decreases during drought, about twice as much water wil l thus be lost per unit volume of the water-storage tissue compared wi th the photosynthetic tissue of maturing fruits and cladodes.