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.