Mycorrhizae may help plants to thrive in Mediterranean semiarid ecosystems by altering antioxidant enzyme activities. Our objective was to determine the influence of mycorrhizal inoculation with an allochthonous arbuscular mycorrhizal (AM) fungus, Glomus claroideum, Schenck & Smith, or with a mixture of native AM fungi, on the activity of antioxidant enzymes from shoots of Olea europaea L. ssp. sylvestris, Retama sphaerocarpa (L.) Boissier and Rhamnus lycioides L. seedlings afforested in a degraded Mediterranean semiarid soil. One year after planting, shoot biomass of inoculated O. europaea seedlings was about 630%, of non-inoculated plants. Shoot biomass of G. claroideum-colonized R. sphaerocarpa was greater than that of seedlings inoculated with the mixed native AM fungi after 12 months. Inoculation with a mix of native AM fungi was the most effective treatment for increasing shoot biomass and N, P and K contents in shoot tissues of R. lycioides. Both mycorrhizal inoculation treatments increased the nutrient contents in shoots of O. europaea and R. lycioides. In O. europaea plants, the inoculation treatments increased catalase, ascorbate peroxidase and dehydroascorbate reductase activities, but not monodehydroascorbate reductase and glutathione reductase activities. Inoculation with G. claroideum increased the activities of all antioxidant enzymes in R. sphaerocarpa. Monodehydroascorbate reductase, glutathione reductase and superoxide dismutase activities in R. lycioides leaves were preferentially increased by inoculation with the mixture of native AM fungi. This work support the view that increased antioxidant enzyme activities could be involved, at least in part, in the beneficial effects of mycorrhizal colonization on the performance of shrub species grown under semi-arid Mediterranean conditions.

Salt treatment (50 mM NaCl) reduced plant growth of loquat (Eribotria japonica Lindl.) (by up to 40%) but not that of anger (Cydonia oblonga Mill.). Salt stress induced a strong leaf Na+ accumulation in both species. However, the observed increase in leaf Cl– level was higher in loquat (13-fold) than in anger plants (3.8-fold). Addition of Ca2+ (25 mM) significantly reduced Na+ and Cl– concentrations in both salt-treated species. In anger leaves, calcium addition to the nutrient media did not change the leaf calcium contents in salt-treated or untreated plants, this value being lower in salt-treated plants. However, in loquat plants, an increase in leaf Ca2+ was observed after the calcium addition. Surprisingly, an increase in Ca2+ concentration was also observed in salt-treated loquat plants. In general, anger plants had higher constitutive antioxidant enzyme levels in both control and salt-treated plants. Salt stress did not change antioxidant enzyme levels in loquat plants. A similar effect was observed in anger plants, but in this case a 2-fold induction of monodehydroascorbate reductase (MDHAR) activity was observed.In both species, salinity produced an oxidative stress, indicated by an increase in lipid peroxidation, this value being much higher in loquat (83%) than in anger (40%) plants. In salt-treated plants, Ca2+ addition provided some protection to the membranes, because the increases observed in thiobarbituric-acid-reactive substances (TBARS) were not significant.In contrast, in control plants Ca2+ treatments increased glutathione reductase (GR) and decreased catalase activity in anger, but increased MDHAR, dehydroascorbate reductase (DHAR), GR and superoxide dismutase (SOD) in loquat plants. In salt-treated plants, Ca2+ additions decreased catalase (CAT) and ascorbate peroxidase (APX) for anger and raised DHAR, GR and SOD for loquat. However, the mechanism by which Ca2+ regulates antioxidant enzymes remains to be determined.These results suggest that anger plants have a higher capacity to scavenge AOS, both under saline and non-saline conditions. Accordingly, and related to the smaller Cl– increase observed, anger plants are more salt-tolerant, at least partly owing to the higher antioxidant enzyme levels observed.

The activity of antioxidant enzymes in different apricot (Prunus armeniaca L.) cultivars, resistant or susceptible to Plum pox virus (PPV), was analyzed during the years 2002 and 2003. Resistant cultivars showed higher activities of catalase (CAT), ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR) than susceptible cultivars. Only CuZn-SOD isozymes were detected in the apricot cultivars. However, no correlation was observed between this isozyme pattern and the resistance to PPV. On the other hand, PPV-resistant apricot cultivars could have a greater capability for elimination of H 2 O 2 and recycling of ascorbate-glutathione cycle, and they have at least two of these enzymatic activities (CAT, APX and DHAR) over the average. In contrast, this response was not observed in the susceptible cultivars. All these data suggest that the activities of CAT, APX and DHAR could be used as biochemical markers of sharka resistance in apricot.

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