Effects of Bacillus amyloliquefaciens QST713 on Growth and Physiological Metabolism in Cucumber Under Low-Calcium Stress

Soil acidification from excessive nitrogen and potassium fertilization in protected cucumber systems impairs calcium uptake, triggering physiological calcium deficiency and reducing yield. We investigated whether the plant growth-promoting rhizobacterium Bacillus amyloliquefaciens QST713 could mitigate low-calcium stress in cucumber (‘Jinyou No. 4’). Under controlled nutrient solution irrigation (4, 0.4, and 0 mmol/L Ca2+, with or without B. amyloliquefaciens QST713), low-calcium conditions suppressed growth, reduced ion uptake capacity, photosynthetic pigment content, gas exchange (Pn, Gs, Tr), PSII efficiency (ΦPSII, ETR), and decreased carbohydrate (starch, sucrose) accumulation, while disrupting nitrogen balance (decreases in NO3−-N, soluble protein, and amino acids; increase in NH4+-N) and inhibiting key N-assimilation enzymes (NR, GS, GOGAT, GDH). Inoculation with B. amyloliquefaciens QST713 reversed these effects: it enhanced ion acquisition, chlorophyll content, and photosynthetic performance; restored carbohydrate reserves; promoted NO3− uptake and NH4+ assimilation; and upregulated N-metabolizing enzyme activities. Principal component analysis confirmed strong coupling among growth, photosynthesis, and C-N metabolism. In summary, low-calcium stress markedly inhibited cucumber growth, suppressed photosynthetic activity, and reduced the levels of carbon and nitrogen metabolism. Application of B. amyloliquefaciens QST713 effectively alleviated the physiological damage caused by low-calcium stress, enhancing photosynthetic performance and thereby accelerating the synthesis and turnover of carbon- and nitrogen-containing metabolites. These effects collectively improved cucumber tolerance to low-calcium conditions and promoted plant growth and development. This study provides a preliminary theoretical basis for further exploration of the stress-resistance capacity of B. amyloliquefaciens.