Evaluation of salt-tolerant forages for sequential water reuse systems. II. Plant-ion relations

Implementation of the saline drainage water reuse system proposed for the westside of the San Joaquin Valley (SJV) of California requires development of sustainable cropping systems, which incorporate useful, salt-tolerant agronomic species. Ion composition in the saline drainage effluents present in the SJV is unique, with Na+, SO42-, Cl-, Mg2+ and Ca2+ predominating in that order. To identify potentially suitable crops for reuse systems, 10 forages were tested in greenhouse sand cultures located at USDA-ARS George E. Brown Jr. Salinity Laboratory, Riverside, California. Crops were: alfalfa (Medicago sativa L.) cvs. 'Salado' and 'SW 9720'; narrowleaf trefoil (Lotus glaber Greene) cv. 'PI 608022'; broadleaf trefoil (L. ulginosus Schk.) cv. 'Big'; tall wheatgrass (Agropyron elongatum (Host) Beauv.) cv. 'Jose'; alkali sacaton (Sporobolus airoides Torr.); kikuyugrass (Pennisetum clandestinum Hochst. Ex Chiov.) cv. 'Whittet'; paspalum (Paspalum vaginatum Swartz) cvs. PI '299042' and 'Polo'; and bermudagrass (Cynodon dactylon L. Pers.) cv. 'Tifton'. Two saline treatments, 15 and 25 dS m-1, were imposed. Shoots were sub-sampled for mineral ion analysis when the forages were harvested. Patterns of shoot ion concentrations were unique for each species and varied with salinity level, external ion ratios, harvest date and plant age. Plant preference for K+ over Na+ did not appear to be a reliable predictor of salt tolerance. Salt-tolerant crops, such as kikuyugrass and bermudagrass, were highly selective for K+ (SK,Na approximately 200 to 400), whereas selectivity coefficients for the equally salt-tolerant grasses, alkali sacaton and tall wheatgrass, ranged from 20 to 40. The forages were highly selective for K+ over Mg2+ despite unusually low K+:Mg2+ ratios in the external solutions. Total-S in the herbage was high and generally increased as external SO42- increased. Based on sustained nutritional health and lack of visual signs of toxicity or deficiency symptoms, several of the forages tested show promise as suitable candidates for the saline-sodic water reuse systems.