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Nitrogen metabolism was examined in monoxenic cultures of carrot roots (Daucus carota L.) colonized with the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith. Glutamine synthetase and glutamate dehydrogenase activities were significantly increased in mycorrhizal roots for which only the extraradical mycelium had exclusive access to NH4NO3 in a distinct hyphal compartment inaccessible to the roots. This was in comparison with the water controls but was similar to the enzyme activities of non-arbuscular-mycorrhizal (non-AM) roots that had direct access to NH4NO3. In addition, glutamate dehydrogenase activity was significantly enhanced in AM roots compared with non-AM roots. Carrot roots took up 15NH4+ more efficiently than 15NO3-, and the extraradical hyphae transfered 15NH4+ to host roots from the hyphal compartment but did not transfer 15NO3-. The extraradical mycelium was shown, for the first time, to have a different glutamine synthetase monomer than roots. Our overall results highlight the active role of AM fungi in nitrogen uptake, transfer, and assimilation in their symbiotic root association.

In a greenhouse experiment, the effect of arbuscular mycorrhizal (AM) fungus (Glomus intraradices Schenck & Smith) colonization on N assimilation in maize (Zea mays L.) was examined after well‐watered, drought and recovery periods. Seeds of selection cycles C0 (drought‐sensitive) and C8 (drought‐resistant) of the tropical maize cultivar Tuxpeño sequía were used for this study. Maize plants were exposed or not to drought stress for 3 weeks (45‐65 days after sowing, DAS) followed by 3 weeks of recovery (66‐86 DAS) at the preflowering stage. Root and shoot samples harvested at the end of the drought or well‐watered and recovery periods were determined for key enzymes involved in N assimilation (NR, nitrate reductase; NiR, nitrite reductase; GS, glutamine synthetase; GOGAT, glutamate synthase), protein and amino acid concentrations, and total N contents. Drought stress significantly (P ≤ 0.01 or P ≤ 0.001) decreased all the enzyme activities except NiR in the roots and shoots of both cultivars. After 3 weeks of drought, the AM roots of both cultivars had higher activities of NR (C0, 45%; C8, 26%), GS (C0, 76%; C8, 33%) and GOGAT (C0, 41%; C8, 53%) than non‐AM roots and were comparable to well‐watered plants. These enzyme activities were also enhanced in drought‐stressed AM shoots of C0 and C8. Total amino acid concentrations in AM plants of C0 were 4.6 and 1.6 times higher in roots and shoots, respectively, compared to non‐AM plants. The predominant amino acids detected were Ala, Arg, Asn, Asp, Gln and Glu which constituted approximately 56 and 75% of the total pool in roots and shoots, respectively. Soluble proteins and total N contents were also higher in AM plants than non‐AM plants under drought conditions. The enhancement of N‐assimilating enzymes and nitrogenous compounds in maize may indicate a transfer of NO3− through the extraradical mycelium or increased N assimilation due to the AM symbiosis. Our overall results suggest that AM association plays an important role in enhancing N assimilation or N nutritional status which enables the host plant to withstand drought conditions and recover after stress is relieved.

Purple coneflower, Echinacea purpurea (L.) Moench, is an important phytomedicinal species that contains phenolics and alkamides with antipathogenic properties. This study aimed to examine the effect of arbuscular mycorrhizal (AM) colonization on the physiology and biochemistry of E. purpurea. It was hypothesized that AM colonization enhances the growth and secondary metabolism in E. purpurea. In this regard, a 13-week factorial greenhouse experiment was performed with E. purpurea, inoculated (or not) with the AM fungus Glomus intraradices Schenck & Smith. Overall, the results indicated that AM colonization significantly increased the mass of shoots and roots and the concentrations of proteins and most of the phenolics in the roots. Hence, the selected trait of mycorrhiza could play an important role in optimizing the growth of E. purpurea by inducing the production of secondary phytomedicinal metabolites.

RESULTS 95The AM feedback percentages (%) on plant HM concentration (Fig.1a) and plant HM 96 content (Fig.1b) are plotted versus the soil-HM concentration. Their correlation coefficients