Salinity usually reduces plant growth in terms of height and biomass, but can increase secondary metabolite production. This frequently reported observation in guayule (Parthenium argentatum Gray, Asteraceae) was investigated for possible mechanisms.Osmotic and specific ion effects of four chloride salts (CaCl2, MgCl2, KCl, and NaCl) on leaf anatomical and plant physiological parameters were studied. One-year-old plants of guayule line AZ 2 were grown under two salt concentrations (750 ppm and 1500 ppm) for each salt type (plus a control) in sand culture (semi-hydroponic) for eight weeks under controlled greenhouse conditions in Tucson, Arizona.Growth in height decreased with increasing salt concentration. Shoot dry weight, rubber, and resin contents, however, showed no significant differences between treatments, indicating no effect from either salt concentration or salt type. There was a trend for increasing rubber content with increasing salt concentration, although not statistically significant. At the same time, net CO2 gas exchange rates decreased significantly with increasing salinity.With increasing salt concentration, guayule showed osmotic effects in terms of height, indicating a lower hydraulic conductivity. Although plants of higher salt concentrations utilized significantly less water, they had the same shoot dry weights, rubber, and resin contents. Salt-stressed plants therefore achieved higher water use efficiencies. The diurnal net CO2 gas exchange rates were significantly reduced with increasing salinity; the nocturnal net CO2 gas exchange rates showed no significant difference between the treatments.Anatomically, it was found that the stomata were raised or elevated above the epidermis, and supported by upwardly curving cells. When guayule was grown under salt treatments, the trichomes were found to include deposits of material. Trichomes might act as a detoxification repository for excess ions. Although the physiological significance of raised stomata is unknown, it is hypothesized that the unique combination of raised stomata, indumentum, and multiple layers of palisade parenchyma allows for an overall high photosynthetic capacity and performance. During stress conditions such as salinity or drought, guayule might activate an internal CO2 concentrating mechanism, i.e., bicarbonate/CO2 pump, internal CO2 recycling, or PEP carboxylation activity.