Many C-3 grasses are infected with fungal endophytes.  In at least some of these symbioses, the interaction is a defesive mutualism.  The endophyte produces alkaloid defense compounds, such as, lolines that deter insect herbivores, and receive energy, nutrients and housing from the plant.  Since lolines contain nitrogen, the mutualism may depend upon the availability of nitrogen to the plants. We tested if variation in N application influenced vegetative nitrogen (N), mass, shoot:root ratios, and loline alkaloid concentration in plants, and preference and performance by aphids using tall fescue (Lolium arundinaceum) and meadow fescue (Lolium pratense) infected with Neotyphodium coenophialum and  Neotyphodium uncinatum, respectively.  Four N application treatments (ranging from 0 ppm to 1,000 ppm N) were applied to potted plants grown in the greenhouse. 

Results/Conclusions

Percent total N in plant tissue was higher in plants receiving N fertilizer than in those receiving no fertilizer (tall fescue: F_3,20 = 71.9, p < 0.0001; meadow fescue: H=11.9, df = 3, p < 0.01).  Concentrations of the lolines in tall fescue tended to increase at higher levels of N fertilizer, however the relationship was not strong (R² = 0.455; slope differed from zero: F_1,17 = 14.2, p < 0.01).  Numbers of winged and apterous Rhopalosiphum padi aphids on tall fescue differed among N application treatments with greater numbers found at intermediate N levels (winged: F_3,29 = 4.32, p < 0.05; apterous: F_3,29 = 3.68, p < 0.05).  Aphids showed similar responses to meadow fescue grown at different concentrations of N availability, with greatest reproduction (F_3,72 = 15.6, p < 0.0001) and preference (G_total = 144.1, df = 57, p < 0.05) at intermediate N levels. Our results show the herbivore resistance provided by Neotyphodium to its host is conditional upon N application.