Plant tolerance of ammonium varies between co-existing Mediterranean species.

Background: Previous studies showed that the two main Mediterranean plant functional groups, summer semi-deciduous and evergreen sclerophylls, differ in soil characteristics and nitrate (NO) use strategies: even though summer semi-deciduous plants have higher NO availability than evergreen sclerophylls, NO reduction (i.e., nitrate reductase activity-NRA) is lower, and is not stimulated by substrate (NO) availability. Aims: Test if in Cistus albidus plants, a summer semi-deciduous species, ammonium (NH) can inhibit NRA, despite the availability of NO, and whether Olea europaea plants, evergreen sclerophyll, are more tolerant of NH than the former. Methods: One-year-old C. albidus and wild O. europaea potted plants were supplied with both NH and NO at increasing levels (0.1; 0.2; 0.4; 0.8 and 1.6 % N). Tolerance of NH was evaluated using integrative (mortality and biomass accumulation) and plant nitrogen metabolism parameters (in vitro NRA and concentrations of NO and NH) determined in roots and leaves. Results: C. albidus plants were consistently less NH tolerant than O. europaea, displaying: higher mortality; growth and NRA inhibition and NH accumulation above 0.2 % NHNO-N in the soil. In contrast, O. europaea plants seemed to buffer the full range of tested NHNO levels. Conclusions: C. albidus plants were less NH tolerant than O. europaea. The ecological implications of this contrasting NH tolerance are discussed. [ABSTRACT FROM AUTHOR]