Raviv, M., Agricultural Research Organization, Department of Ornamental Horticulture, Newe ya'Ar Research Center, PO Box 1021, Ramat Yishay 30095, Israel
Blom, T.J., Department of Plant Agriculture, HRIO University of Guelph, Vineland Station, 4890 Victoria Ave., Guelph, Ont. L0R 2E0, Canada

Low matric and to a lesser extent osmotic potential reduce significantly leaf area and rose yield. Net assimilation rate and transpiration are also negatively affected although less dramatically. Low water potential causes an increase in the water use efficiency of greenhouse roses when tested in closed, no-discharge systems. When a stable osmotic potential is maintained in open systems, using increased leaching fraction (LF), low osmotic potential results in lower water use efficiency. Osmotic potential in porous media serving for greenhouse cut-rose production is usually lower than the matric potential. However, low matric potential in porous media is usually accompanied by very low unsaturated hydraulic conductivity, causing localized zones of very low matric potential adjacent to the root-medium interface. This phenomenon, that cannot be measured using tensiometers, is the main limiting factor to water uptake by plant roots. Restricted water uptake results in low leaf water potential and cessation of leaf and shoot expansive growth. Combined effects of drought and salinity on photosynthesis have been studied for a number of agronomic crops but studies on roses have been limited. In most greenhouse crops a close relationship between total water potential in the root zone (Ψt soil) and in the shoot (Ψt shoot) is found and there are good indications about the plant's ability to make osmotic adjustments in order to lower Ψt shootand prevent excessive water losses from the leaves thus maintaining the plant's turgidity. Future studies conducted with roses can provide a better insight into the adaptive processes within the plants when exposed to salt or water stresses. © 2001 Elsevier Science B.V.