The effect of water availability and quality on photosynthesis and productivity of soilless-grown cut roses

Low matric and to a lesser extent osmotic potential reduce significantly le af area and rose yield. Net assimilation rate and transpiration are also ne gatively affected although less dramatically. Low water potential causes an increase in the water use efficiency of greenhouse roses when tested in cl osed, no-discharge systems. When a stable osmotic potential is maintained i n open systems, using increased leaching fraction (LF), low osmotic potenti al results in lower water use efficiency. Osmotic potential in porous media serving for greenhouse cut-rose production is usually lower than the matri c potential, However, low matric potential in porous media is usually accom panied by very low unsaturated hydraulic conductivity, causing localized zo nes of very low matric potential adjacent to the root-medium interface. Thi s phenomenon, that cannot be measured using tensiometers, is the main limit ing factor to water uptake by plant roots. Restricted water uptake results in low leaf water potential and cessation of leaf and shoot expansive growt h. Combined effects of drought and salinity on photosynthesis have been studie d for a number of agronomic crops but studies on roses have been limited. I n most greenhouse crops a close relationship between total water potential in the root zone (psi (soil)(t)) and in the shoot (psi (shoot)(t)) is found and there are good indications about the plant's ability to make osmotic a djustments in order to lower psi (shoot)(t) and prevent excessive water los ses from the leaves thus maintaining the plant's turgidity. Future studies conducted with roses can provide a better insight into the adaptive process es within the plants when exposed to salt or water stresses. (C) 2001 Elsev ier Science B.V. All rights reserved.