The ecophysiology of photosynthesis in mixotrophic plants.

The existence and the evolutionary implications of compensatory photosynthesis in partial mycoheterotrophic (mixotrophic) plants was demonstrated by our research team (Bellino et al., 2014; Oecologia 175:875-885) using a mixotrophic orchid, Limodorum abortivum, as a model species. However, the ecophysiology of this process was still largely unexplored. Here we performed the first in-depth study of photosynthesis ecophysiology in L. abortivum, employing chlorophyll fluorescence. Analyses were performed on leaves, stems and ovaries of either plants in which fungal supplies were artificially impaired to trigger compensatory photosynthesis, or untreated plants. The maximum quantum yield (Fv/Fm), electron transport rate (ETR)-irradiance relationships, non-photochemical quenching (NPQ) and dark-light/light-dark transition kinetics were analysed weekly on 42 specimens during 35 days. Ovaries showed higher Fv/Fm and ETRs than leaves and stems, irrespectively of the treatment. Conversely, the ETR-irradiance relationships were similar among the organs and comparable to those of photoautotrophic C3 plants, whereas the transition kinetics showed unusual features. Compensatory photosynthesis in ovaries was quickly triggered by the treatment (enhanced Fv/Fm and ETRs), confirming its importance in buffering fungal carbon limitation and supporting seed development. Our findings strengthen the evolutionary theory we developed for the mixotrophy-mycoheterotrophy transition.