Beyond the optimum: Excessive application of Bacillus amyloliquefaciens impairs photosynthetic efficiency and yield in soybean

Bacillus amyloliquefaciens is a prominent plant growth-promoting rhizobacterium (PGPR) utilized for the biological control of phytonematodes in soybean ( Glycine max (L.) Merr.). However, the physiological and bioenergetic thresholds governing its application doses remain poorly understood, with current management often assuming a linear positive response while overlooking the risks of high doses. A field experiment was conducted in a randomized complete block design to evaluate the effects of increasing doses of B. amyloliquefaciens strain CEA01: 0 (control – No B. amyloliquefaciens ), 200 (01 dose), 400 (02 doses), 800 (04 doses), and 1600 mL ha −1 (08 doses), alongside a commercial positive control. At the reproductive stage, biometric parameters, photosynthetic pigments, OJIP chlorophyll a fluorescence transience, gas exchange, nematode suppression ( Pratylenchus brachyurus , Helicotylenchus dihystera , and nematode eggs), and crop yield components were determined. Intermediate concentrations (02 and 04 doses) optimized agronomic performance. The 02 doses treatment acted as an efficient biostimulant, maximizing chlorophyll a (26.05 FCI) and b (17.75 FCI) contents and the performance index (PiAbs = 1.00). This bioenergetic stability was associated with higher carboxylation efficiency ( A/Ci ), driving the 04 doses treatment to achieve the maximum dry mass (73.02 g) and grain yield (109.06 bags ha −1 ), while decreasing nematode populations by up to 80.7%. Conversely, the highest dose (08 doses) induced severe physiological stress. Despite a disproportionate increase in plant height (increased of 6.75 cm compared to the positive control), the 08 doses treatment triggered an energetic bottleneck, severely decreasing the maximum quantum yield of PSII (Fv/Fm = 0.64) and forcing non-photochemical dissipation (DIo/RC = 1.43). This bioenergetic reallocation reflects non-stomatal photosynthetic limitations associated with high application rates, leading to a sharp decline in grain yield to 78.47 bags ha −1 . Principal Component Analysis (PCA) supported this dose–response, discriminating the metabolic optimization under intermediate doses from the physiological disturbance caused by elevated application rates and the high nematode pressure in the untreated control. In conclusion, the success of B. amyloliquefaciens management relies on a strict operational window: the concentration must meet a minimum threshold (02 doses) to guarantee rhizospheric biocontrol and root integrity, but must remain below 08 doses to prevent bioenergetic dissipation and yield penalties induced by excessive application rates.