Acquisition of N by external hyphae of an arbuscular mycorrhizal fungus and its impact on physiological responses in maize under drought-stressed and well-watered conditions

This study examined the uptake of nitrogen by external hyphae of an arbuscular mycorrhizal (AM) fungus (Glomus intraradices Schenck & Smith) and its impact on physiological responses in maize plants subjected to well-watered or drought-stressed conditions. Plants were grown in compartmented boxes divided by a nylon mesh (40 mu m) into a root compartment and a hyphal compartment. Maize plants (Zea mays cv. 'Tuxpeno sequia' selection cycle C0) were exposed to 2 weeks of drought 56 days after sowing. A N-15 tracer was applied as K (NO3)-N-15 to the hyphal compartment at a distance of 5 cm from the root compartment. Root and shoot samples were then analyzed for N-15 atom % excess (APE), glutamine synthetase (GS) activity, protein concentration and nutritional status. Evapotranspiration rate and stomatal resistance were monitored daily to determine the degree of drought stress. The APE values for AM shoots and roots were 32% and 33% higher than non-AM shoots and roots, respectively, under drought conditions. This provides clear evidence that the external mycelium of AM fungus transports considerable amounts of (NO3-)-N-15 to the host plant under drought conditions. Drought-stressed AM roots had 28% higher GS activity, possibly as a consequence of higher hyphal acquisition of (NO3-)-N-15 ions. Mycorrhizal colonization significantly increased the host plant P status regardless of soil moisture regime. In addition, the N status of drought-stressed AM shoots and roots was slightly higher than stressed non-AM shoots and roots. The improved nutritional status may assist AM plants to exploit available soil moisture more efficiently and to maintain higher leaf relative water content under moderate drought conditions.