Response of Bedding Plants to Saline Water Irrigation

Bedding plants are extensively used in urban landscapes. As high-quality water supply becomes limited in many parts of the world, the use of recycled water with high salt levels for landscape irrigation is being encouraged. Therefore, information on salt tolerance of bedding plants is of increasing importance. Two experiments were conducted, one in a 25% light exclusion shadehouse in summer (Expt. 1) and the other in a greenhouse in winter (Expt. 2). Plants were irrigated with saline solution at electrical conductivities of 0.8, 2.8, 4.0, 5.1, or 7.4 dS.m(-1) created by adding NaCl, MgSO4, and CaCl2 to tap water to simulate the composition of local reclaimed water. In Expt. 1, shoot dry weight (DW) at the end of the experiments was reduced in all species at 7.4 dS.m(-1) compared with the control (0.8 dS.m(-1)). The magnitude of reduction varied with species and cultivars. The salinity thresholds of irrigation water in which growth reduction occurred were 4.0 dS.m(-1) for angelonia (Angelonia angustifalia) cultivars and ornamental pepper (Capsicum autumn) 'Calico' and 4.0 to 5.1 dS.m(-1) for helenium (Helenium amarum), licorice plant (Helichrysum petiolatum), and plumbago (Plumlago auriculata). Shoot DW and growth index of ornamental pepper 'Black Pearl' and vinca (Catharanthus roseus) 'Rose' decreased linearly as salinity increased. All plants survived in Expt. 1 regardless of treatment, except for ornamental pepper 'Purple Flash'. No visual injuries were observed in Expt. 1 regardless of treatment. Leaf sodium (Na) and chlorine (Cl) concentrations varied with species and treatments. Ornamental pepper 'Black Pearl' had the highest leaf Cl concentrations at higher salinities compared with other species and cultivars. Leaf Na concentrations in licorice plant and plumbago were in the range of 10 to 30 g.kg (1) DW, higher than those in other species. In Expt. 2, shoot DW was reduced by salinity treatments in ornamental pepper 'Black Pearl', plumbago, and angelonia but not in other species. The three ornamental peppers, 'Black Pearl', 'Calico', and 'Purple Flash', exhibited slight foliar injuries on some plants in Expt. 2 as a result of high salinity in the root zone in the highest salinity treatment. Ornamental pepper 'Black Pearl' was most sensitive to salinity stress. In general, the bedding plants tested in this study are moderately tolerant to salt stress and may be irrigated with saline water up to 4.0 d.S.m (1) with little reduction in aesthetical appearance.