Does fungal endophyte inoculation affect the responses of aspen seedlings to carbon dioxide enrichment?

Endophytes are microorganisms that live inside plants without causing visible symptoms, at least during some parts of their life cycle. We studied, for the first time, the combined effects of CO2 enrichment (700 ppm) and fungal endophyte inoculation on the growth, the concentrations of low-molecular weight phenolics, and condensed tannins of aspen (Populus tremula) seedlings. As expected, we found that the endophyte strain we inoculated was neutral to plant growth and was able to bypass major plant defences. In addition, CO2 enrichment alone boosted plant growth, but had only minor effects on plant phenolics. Neither did it affect the plant-endophyte relationship. Based on our findings, we suggest that the successful and asymptomatic colonization of endophytes that we found in aspen might be due to the endophytes' special attributes enabling them to thrive inside plant tissues and to avoid or counteract the plant's chemical defences.