Although fungi that asymptomatically inhabit the above-ground tissues of plants are ubiquitous, little is known about the functional effects of these symbionts on their hosts. To test whether foliar fungal endophytes alter their hosts' photosynthetic performance or capacity, we performed a manipulative greenhouse inoculation experiment with clonal cuttings of Populus trichocarpa (black cottonwood). Before and after inoculation with one of three fungal endophyte strains or one of two controls (five treatments in total), we used gas exchange measurements to assay several indices of (i) photosynthetic performance under growth conditions (Anet, gs) and (ii) maximum photosynthetic capacity (Vcmax, Jmax). We compared the measurements (pre- and post-inoculation) in a paired design to test for effects of endophytes on their host while controlling for variation in performance between leaves of the same plant. We also coupled these physiological measurements with electron and light microscopy over an infection time course following in vitro inoculation of leaf disks with the same endophyte strains. This microscopy provided evidence that control leaves from the greenhouse were largely free of fungi, and that all three endophyte strains successfully colonized the interior of asymptomatic Populus leaves, albeit at rates that differed between fungal strains.
Under the greenhouse growth conditions, P. trichocarpa individuals exhibited high leaf-to-leaf variation in both photosynthetic performance and maximum photosynthetic capacity (e.g. Vcmax ranged from 37-115 μmol/m2/s, Jmax from 110-190 μmol/m2/s). Based on the degree of variation within the controls, we estimated that our sampling design (n = 10 plants per treatment) could only detect effects on Vcmax larger than 0.65 (Cohen's d). Observed effect sizes between treatments and controls were lower (0.2 - 0.6) and differed between endophyte treatments. Endophytes did not have effects equal to or larger than this threshold on Anet, gs, Vcmax, or Jmax. At present, we cannot differentiate between the possibilities that a) these endophytes did not significantly impact host photosynthetic performance and capacity under the experimental conditions, vs. b) that endophytes did affect host performance and capacity but with an effect magnitude smaller than 0.65. Work is in progress to differentiate between these two possibilities through the use of additional physiological measurements that are expected to have greater sensitivity than the gas exchange measurements. In either case, a better understanding of the physiological impact of foliar microbial symbionts will contribute towards a better integration of these ubiquitous organisms into synthetic understandings of ecosystem function.