Fungi that asymptomatically inhabit the above-ground tissue of plants--fungal endophytes--exhibit considerable phylogenetic and biogeographic diversity. Despite their ubiquity, and emerging efforts to use these microbial organisms to control plant disease, we still know very little about the functional effects of these symbionts on the physiology of their hosts. In an experiment designed to test whether foliar fungal endophytes can alter their hosts' photosynthetic performance, we performed two related manipulative greenhouse inoculation experiments. Using clonal cuttings of Populus trichocarpa (black cottonwood), we incubated mature leaves and emerging leaves with endophyte inocula from one of three focal strains or a sterile water control. We used gas exchange to measure photosynthetic performance under growth conditions (Anet, gs) and maximum photosynthetic capacity (Vcmax, Jmax) in mature leaves, comparing 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. In emerging leaves, we focused on comparisons between leaves from paired apical meristems of the same individuals, and quantified photosynthetic performance under growth conditions (Anet, gs).
Results/Conclusions
Under the growth conditions, P. trichocarpa individuals in all of the treatments exhibited high leaf-to-leaf variation in both photosynthetic performance and maximum photosynthetic capacity (e.g. Vcmax ranged from 37-115, Jmax from 110-190). We did not find significant effects of inoculation on Anet, gs, Vcmax, or Jmax in mature leaves, but we did find significant differences in Anet between control and endophyte treatments in emerging leaves (Wilcoxon p < 0.05). While these suggest that endophyte colonization may, at least in some cases, positively influence host instantaneous photosynthetic rate if the endophytes colonize leaves early in their development, we have not yet identified the specific developmental or physiological differences that may be responsible for this observed effect. Further experiments are underway to substantiate these findings and determine the mechanisms underlying them.