Arbuscular mycorrhizae and nitrogen assimilation in maize after drought and recovery

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.