Ectomycorrhizal fungi reduce the light compensation point and promote carbon fixation of Pinus thunbergii seedlings to adapt to shade environments.

We examined the effects of three ectomycorrhizal (ECM) symbionts on the growth and photosynthesis capacity of Japanese black pine (Pinus thunbergii) seedlings and estimated physiological and photosynthetic parameters such as the light compensation point (LCP), biomass, and phosphorus (Pi) concentration of P. thunbergii seedlings. Through this investigation, we documented a new role of ectomycorrhizal (ECM) fungi: enhancement of the survival and competitiveness of P. thunbergii seedlings under low-light condition by reducing the LCP of seedlings. At a CO2 concentration of 400 ppm, the LCP of seedlings with ECM inoculations was 40-70 μmol photons m-2  s-1 , significantly lower than that of non-mycorrhizal (NM) seedlings (200 μmol photons m-2  s-1 ). In addition, photosynthetic carbon fixation (Pn) increased with light intensity and CO2 level, and the Pn of ECM seedlings was significantly higher than that of NM seedlings; Pisolithus sp. (Pt)- and Laccaria amethystea (La)-mycorrhizal seedlings had significantly lower Pn than Cenococcum geophilum (Cg)-mycorrhizal seedlings. However, La-mycorrhizal seedlings exhibited the highest fresh weight, relative water content (RWC), and the lowest LCP in the mycorrhizal group. Concomitantly, ECM seedlings showed significantly increased chlorophyll content of needles and higher Pi concentrations compared to NM seedlings. Overall, ECM symbionts promoted growth and photosynthesis while reducing the LCP of P. thunbergii seedlings. These findings indicate that ECM fungi can enhance the survival and competitiveness of host seedlings under low light.