Plant tolerance of ammonium varies between co-existing Mediterranean species

Previous studies showed that the two main Mediterranean plant functional groups, summer semi-deciduous and evergreen sclerophylls, differ in soil characteristics and nitrate (NO3 (-)) use strategies: even though summer semi-deciduous plants have higher NO3 (-) availability than evergreen sclerophylls, NO3 (-) reduction (i.e., nitrate reductase activity-NRA) is lower, and is not stimulated by substrate (NO3 (-) ) availability. Test if in Cistus albidus plants, a summer semi-deciduous species, ammonium (NH4 (+) ) can inhibit NRA, despite the availability of NO3 (-) , and whether Olea europaea plants, evergreen sclerophyll, are more tolerant of NH4 (+) than the former. One-year-old C. albidus and wild O. europaea potted plants were supplied with both NH4 (+) and NO3 (-) at increasing levels (0.1; 0.2; 0.4; 0.8 and 1.6 % N). Tolerance of NH4 (+) was evaluated using integrative (mortality and biomass accumulation) and plant nitrogen metabolism parameters (in vitro NRA and concentrations of NO3 (-) and NH4 (+)) determined in roots and leaves. C. albidus plants were consistently less NH4 (+) tolerant than O. europaea, displaying: higher mortality; growth and NRA inhibition and NH4 (+) accumulation above 0.2 % NH4NO3-N in the soil. In contrast, O. europaea plants seemed to buffer the full range of tested NH4NO3 levels. C. albidus plants were less NH4 (+) tolerant than O. europaea. The ecological implications of this contrasting NH4 (+) tolerance are discussed.