The role of ACC deaminase producing bacteria in improving sweet corn (*Zea mays* L. var *saccharata*) productivity under limited availability of irrigation water

Water scarcity is a major constraint to global food security, particularly impacting crop yields in arid and semi-arid regions. Sweet corn (*Zea mays* L. var *saccharata*), a valuable crop with high sugar content and diverse applications (food, forage, biofuel production), is highly susceptible to drought stress. Osmotic stress caused by drought leads to the accumulation of stress ethylene in plants, further hindering growth and yield. Plant growth-promoting rhizobacteria (PGPR), especially those producing 1-aminocyclopropane-1-carboxylate (ACC) deaminase, offer a promising strategy to enhance drought tolerance in crops. ACC deaminase reduces stress ethylene levels by converting ACC, a precursor to ethylene, into less harmful compounds. This study aimed to evaluate the efficacy of four *Pseudomonas fluorescens* strains, individually and in combination, in improving sweet corn productivity under different irrigation regimes, hypothesizing that combined inoculation would be more effective than single-strain inoculation in mitigating drought stress.

Previous research has documented the beneficial effects of PGPR inoculation on crop growth and drought tolerance. Studies have shown that PGPR can improve water stress tolerance by various mechanisms, including enhanced nutrient uptake, improved water status, and reduced ethylene accumulation. For example, Khan et al. (Reference 10) reported that PGPR inoculation decreased the activity of stress-induced enzymes (CAT, POX, APX, SOD) and increased chlorophyll, protein, starch, proline, and carbohydrate content in drought-stressed pea plants. Danish et al. (References 8, 9, 11) demonstrated that certain PGPR strains, including *Pseudomonas aeruginosa*, *Enterobacter cloacae*, *Achromobacter xylosoxidans*, and *Leclercia adecarboxylata*, improved maize growth under water stress by reducing ethylene accumulation and enhancing nutrient uptake. However, studies focusing on the physiological responses and nutrient uptake of sweet corn to *Pseudomonas fluorescens* under drought conditions remain limited.

This field experiment was conducted in Marvdasht, Iran, over two growing seasons (2016 and 2017). Four *P. fluorescens* strains (P₁, P₃, P₅, P₁₄) were characterized for ACC deaminase activity, auxin synthesis, and siderophore production (Table 1). Sweet corn seeds (Chase hybrid) were inoculated with each strain individually, with a combination of all four strains, and with no bacteria (control). Three irrigation levels were applied: 100% (I₁₀₀), 80% (I₈₀), and 60% (I₆₀) of plant water requirement, determined using the Penman-Monteith equation (Equation 1). The experiment followed a completely randomized block design with three replications. At the milk grain stage, leaf samples were collected for analysis of chlorophyll content, chlorophyll fluorescence parameters (F₀, Fₘ, Fv/Fm), proline content, soluble sugars, catalase (CAT) and peroxidase (POX) activity, and nutrient concentrations (N, P, K, Fe, Zn, Mn). Ear yield and canned seed yield were determined at harvest. Data were analyzed using SAS software (v.9.1), and means were compared using LSD at p ≤ 0.05. Soil properties were characterized before the experiment (Table 2), and meteorological data were recorded throughout the growing seasons (Figure 1).

Severe water stress (I₆₀) significantly decreased chlorophyll a, chlorophyll b, and total chlorophyll content, Fv/Fm ratio, and nutrient uptake (N, P, Fe, Zn, Mn), while increasing F₀, Fₘ, proline, soluble sugars, CAT, and POX activity. In contrast, inoculation with the combination of four *P. fluorescens* strains significantly increased ear yield (44%) and canned seed yield (27%) compared to the control under all irrigation treatments. Strain P₁ showed the highest promoting effect in reducing drought stress effects and improving yields. The combined strain treatment had the most positive effect on nutrient accumulation in leaves (N, K, Zn, Mn), and P and Fe content (Figure 2). The combined inoculation also significantly reduced CAT and POX activity (Figure 3), and enhanced chlorophyll content (Figure 4), Fv/Fm ratio, proline, and soluble sugars (Figure 5), especially under water stress conditions. Furthermore, the combined treatment also led to the highest ear and canned seed yield (Figure 6), even under severe water stress (I₆₀).

The results demonstrate the potential of combined *P. fluorescens* inoculation as a biofertilizer to mitigate the adverse effects of drought stress on sweet corn. The improved growth and yield under water-stressed conditions can be attributed to multiple mechanisms: (1) ACC deaminase activity reducing stress ethylene levels; (2) enhanced nutrient uptake by improved root growth and increased nutrient solubilization; (3) enhanced photosynthetic efficiency by improved chlorophyll content and Fv/Fm ratio; and (4) increased osmotic adjustment by higher proline and soluble sugar accumulation. The synergistic effect of the four strains likely contributed to the superior performance of the combined treatment compared to individual strains.

This study conclusively demonstrates that combined inoculation with four *P. fluorescens* strains significantly enhances sweet corn productivity under drought conditions. The improved growth and yield can be attributed to the combined effects of stress ethylene reduction, improved nutrient uptake, enhanced photosynthesis, and osmotic adjustment. This approach presents a sustainable and environmentally friendly strategy for enhancing crop resilience to drought stress. Future research could investigate the underlying molecular mechanisms involved in the interaction between *P. fluorescens* and sweet corn under drought stress and explore the application of this technology in various agricultural settings.

This study was conducted in a specific location with specific soil conditions and sweet corn variety. The results may not be directly generalizable to other environments or cultivars. Further research is needed to validate the findings under different environmental conditions and with a broader range of sweet corn genotypes.