Alleviation of CO 2 ‐Induced Reductions in Tomato Photosynthesis Under Deficit Irrigation by Purple Nonsulfur Photosynthetic Bacteria

The stimulatory effect of elevated CO 2 (eCO 2 ) on photosynthesis in most C3 crops under water deficit often declines over time due to photosynthetic acclimation. An exception occurs in plants inoculated with symbiotic nitrogen‐fixing bacteria. Photosynthetic bacteria (PSB), specifically anoxygenic purple nonsulfur bacteria ( Rhodopseudomonas palustris in this study), a group of nitrogen‐fixing bacteria, are effective in enhancing crop photosynthesis. Therefore, this study investigated the synergistic effects of PSB and eCO 2 in alleviating the effects of deficit irrigation and enhancing photosynthetic capacity in tomato plants during prolonged exposure. Our results showed that photosynthetic efficiency was significantly reduced in noninoculated plants under eCO 2 , and this reduction was more pronounced under water deficit. Proteomic analysis revealed that in eCO 2 ‐treated plants, the downregulation of cell wall proteins increased mesophyll resistance to CO 2 diffusion, while the suppression of the photosynthetic apparatus impaired electron transport capacity, ultimately reducing CO 2 assimilation efficiency. In contrast, these negative effects were alleviated by PSB inoculation. PSB promoted the upregulation of proteins involved in photosynthesis under deficit irrigation, as well as proteins related to chlorophyll biosynthesis, components of photosystem I and II, and light‐harvesting complex proteins. These proteins contributed to improved photosynthetic efficiency during deficit irrigation and photosynthetic acclimation. Physiological analyses further confirmed that PSB inoculation enhanced nitrogen content, electron transport capacity, chlorophyll biosynthesis, and overall photosynthetic performance under eCO 2 and deficit irrigation, resulting in improved plant growth. These findings suggest that PSB inoculation is a promising strategy to sustain and enhance the CO 2 fertilization effect on crop productivity under water‐limited conditions.