Net photosynthetic rate, radiation use efficiency, chlorophyll (Chl) fluorescence, photochemical reflectance index (PRI), and leaf water potential were measured during a 25-<1 period of progressive water deficit in quinoa plants grown in a glasshouse in order to examine effects of water stress and ontogeny. All physiological parameters except Fv/F m were sensitive to water stress. Ontogenic variations did not exist in Fv/F m and leaf water potential, and were moderate to high in the other parameters. The complete recovery of photosynthetic parameters after re-irrigation was related with the stability in FyIF m. PRI showed significant correlation with predawn leaf water potential, Fm', and midday FyIF m. Thus PRI and Chl fluorescence may help in assessing physiological changes in quinoa plants across different developmental stages and water status.