Endophytes alleviate the elevated CO2-dependent decrease in photosynthesis in rice, particularly under nitrogen limitation.

The positive effects of high atmospheric CO₂ concentrations [CO₂] decrease over time in most C₃ plants because of down-regulation of photosynthesis. A notable exception to this trend is plants hosting N-fixing bacteria. The decrease in photosynthetic capacity associated with an extended exposure to high [CO₂] was therefore studied in non-nodulating rice that can establish endophytic interactions. Rice plants were inoculated with diazotrophic endophytes isolated from the Salicaceae and CO₂ response curves of photosynthesis were determined in the absence or presence of endophytes at the panicle initiation stage. Non-inoculated plants grown under elevated [CO₂] showed a down-regulation of photosynthesis compared to those grown under ambient [CO₂]. In contrast, the endophyte-inoculated plants did not show a decrease in photosynthesis associated with high [CO₂], and they exhibited higher photosynthetic electron transport and mesophyll conductance rates than non-inoculated plants under high [CO₂]. The endophyte-dependent alleviation of decreases in photosynthesis under high [CO₂] led to an increase in water-use efficiency. These effects were most pronounced when the N supply was limited. The results suggest that inoculation with N-fixing endophytes could be an effective means of improving plant growth under high [CO₂] by alleviating N limitations. [ABSTRACT FROM AUTHOR]