Thermal Plasticity and Cotton Production Enhancing Attributes of Phosphate-Solubilizing Bacteria from Cotton Rhizosphere

Phosphate-solubilizing bacteria (PSB) have been known to improve soil fixed phosphorus (P) availability to crop. However, PSB performance under thermal stress is poorly understood. The purpose of this study was to isolate thermo-tolerant PSB from the cotton ( Gossypium hirsutum L.) rhizosphere, cultivated in the alkaline soil of a semi-arid region. Seventeen isolates were identified as efficient PSB on the National Botanical Research Institute’s Phosphate (NBRIP) agar plate. The P-solubilizing activity was observed on NBRIP liquid media and in soil incubated at 30, 40, 45, and 50 °C. Cotton growth-promoting traits of the strains were tested in the laboratory and field conditions. The phosphorus solubilization index (PSI) for isolate TPB11 (2.62) was the highest, followed by TPB19 (2.26), TPB30 (2.26), and TPB8 (2.35). The isolates TPB4, TPB19, and TPB30 significantly improved P-solubilizing activity on NBRIP broth and in soil at 30, 40, 45, and 50 °C. The seed inoculation of PSB significantly increased cotton seedling growth in the laboratory. Isolate TPB30 showed the maximum increase in shoot length (+ 45%), root length (+ 167%), shoot fresh weight (+ 143%), shoot dry weight (+ 54%), and vigor index (+ 116%) over control. While isolate TPB4 showed the maximum increase in root fresh weight (+ 180%) and root dry weight (+ 76%) as compared to control. The isolate TPB19 showed the maximum increase in cotton seed germination (+ 89%) over control. Readings by the Soil Plant Analysis Development (SPAD) meter, leaf area index, photosynthetic rate, seed cotton, and biomass yield of field-grown cotton were significantly improved in comparison to control by the PSB inoculation. Inoculation with TPB30 demonstrated maximum increase in seed cotton yield (+ 26%) and biomass yield (+ 30%) over control. The efficient isolates (TPB4, TPB19, and TPB30) were identified as Bacillus subtilis subsp. subtilis , Bacillus halotolerans , and Bacillus pumilus , respectively, and showed thermal stability. Hence, these isolates can be assumed as potential inoculants to improve cotton production under natural conditions in P-deficient soils of the semi-arid region. The study results suggested that the efficient PSB in the cotton rhizosphere could improve cotton production under high-temperature growing conditions.