Effects of CO enrichment, nutrient addition, and fungal endophyte-infection on the growth of two grasses.

Increasing atmospheric carbon dioxide (CO) concentration is expected to increase plant productivity and alter plant/plant interactions, but little is known about its effects on symbiotic interactions with microorganisms. Interactions between perennial ryegrass, Lolium perenne (a C3 plant), and purpletop grass, Tridens flavus (a C4 plant), and their clavicipitaceous fungal endophytes (Acremonium lolii and Balansia epichloe, respectively) were investigated by growing the grasses under 350 and 650 μl l 1 CO at two nutrient levels. Infected and uninfected perennial ryegrass responded with increased growth to both CO enrichment and nutrient addition. Biomass and leaf area of infected and uninfected plants responded similarly to CO enrichment. When growth analysis parameters were calculated, there were significant increases in relative growth rate and net assimilation rate of infected plants compared to uninfected plants, although the differences remained constant across CO and nutrient treatments. Growth of purpletop grass did not increase with CO enrichment or nutrient addition and there were no significant differences between infected and uninfected plants. CO enrichment did not alter the interactions between these two host grasses and their endophytic-fungal symbionts.