Pot Experiment Methods for Plant Growth-Promoting Bacteria (PGPR): Standards, Procedures, and Evalua

The pot experiment (greenhouse pot trial) is the internationally recognized method for evaluating the plant growth-promoting efficacy of PGPR (plant growth-promoting rhizobacteria) and microbial inoculants under controlled conditions. This article provides a comprehensive technical overview of pot trial methodology, including sample preparation, testing items, inoculation procedures, statistical evaluation, and applicable standards such as GB 20287-2006. It serves as a practical reference for biofertilizer manufacturers, researchers, and accredited third-party laboratories performing efficacy validation.

Technical Overview

A pot experiment for plant growth-promoting bacteria (PGPB/PGPR) is a controlled greenhouse bioassay designed to quantify the ability of a microbial strain or inoculant product to enhance plant growth. By cultivating host plants in pots with a standardized substrate under regulated temperature, light, and moisture conditions, the method isolates the effect of the inoculated microorganism from the uncontrolled variability of field environments. This makes the pot trial the essential intermediate step between laboratory strain screening and large-scale field validation.

In China, the pot experiment is a core technical requirement of GB 20287-2006, the national standard for agricultural microbial inoculants, which mandates greenhouse efficacy data as part of product registration and quality evaluation. Complementary international frameworks, including ISO and ASTM guidelines on soil quality and plant growth testing, provide additional methodological references for experimental design and phytometric measurement. A well-executed pot trial generates reproducible, statistically defensible evidence of growth promotion, supporting product claims, regulatory filing, and research publication.

Testing Samples

Bacterial Cultures and Inoculant Products

Test samples may be submitted as pure cultures (agar slants, lyophilized powder, glycerol stocks) or as finished commercial formulations, including wettable powders, granular carriers, and liquid suspensions. Upon receipt, the laboratory verifies strain identity and purity through morphological observation, Gram staining, and 16S rDNA sequencing where required, followed by revitalization on appropriate agar media. The viable cell concentration is confirmed by plate counting to ensure the inoculum meets the target application dosage.

Host Plant Materials

Seeds of model or target crops, such as maize, wheat, tomato, cucumber, pepper, or Chinese cabbage, are selected as host materials. Seeds must demonstrate high germination energy (typically above 95%) and uniform size to minimize biological variability. Certified seed lots are preferred, and a germination pre-test is performed before the formal trial to establish baseline viability.

Growth Substrates

The growth medium is typically a sieved (2 mm) loam soil, vermiculite, sand, or defined soil–sand mixture. Before use, the substrate is characterized for pH, electrical conductivity, organic matter, and available nitrogen, phosphorus, and potassium. Depending on the study objective, the substrate may be sterilized by autoclaving (121°C, 30 minutes, repeated twice with a 24-hour interval) to exclude native microbial interference, or retained in its natural state to evaluate strain competitiveness and colonization in a realistic rhizosphere.

Testing Items

The pot experiment evaluates a battery of agronomic and microbiological parameters that collectively characterize plant growth-promoting performance. Core testing items include:

• Seed germination rate, emergence rate, and seedling vigor index
• Plant height, stem diameter, leaf number, and leaf area
• Root length, root volume, root surface area, and lateral root density
• Shoot fresh weight and dry weight; root fresh weight and dry weight; root-to-shoot ratio
• Chlorophyll content (SPAD value) as an indicator of photosynthetic capacity
• Total nitrogen, phosphorus, and potassium uptake in plant tissue
• Rhizosphere colonization density (log CFU/g of rhizosphere soil or root)
• Disease incidence reduction or biocontrol index, for strains with antagonistic function

Parameters are selected according to the declared function of the inoculant, such as phosphorus solubilization, nitrogen fixation, phytohormone production, or stress tolerance induction.

Testing Methods

Experimental Design

A randomized complete block design (RCBD) is the standard configuration. Each treatment requires a minimum of four replicate pots, with three to ten plants per pot, yielding at least 30 evaluable plants per treatment for statistical robustness. Typical treatment groups include: a blank control (no inoculant, no fertilizer), a carrier or medium control, a conventional fertilizer control, the inoculant treatment, and, where applicable, an inoculant plus reduced-fertilizer treatment to assess partial fertilizer replacement potential. Pots are randomly arranged on the greenhouse bench and rotated periodically to neutralize positional effects.

Substrate and Seed Preparation

Prepared substrate is filled into pots at a uniform weight (for example, 2 kg per pot) to ensure consistency of root volume and nutrient availability. Seeds are surface-sterilized by immersion in 75% ethanol for 30 to 60 seconds, followed by 2% sodium hypochlorite for 5 to 10 minutes, and then rinsed five to six times with sterile distilled water. Sterilized seeds may be pre-germinated on moist sterile filter paper at 25°C until radicle emergence to synchronize planting.

Inoculum Preparation and Application

The activated strain is cultured in liquid medium to logarithmic phase, and the cell suspension is adjusted to a standardized concentration, commonly 1 × 10⁸ CFU/mL, verified by serial dilution plating. Three application modes are widely used:

• Seed coating: seeds are dressed with the bacterial suspension mixed with a sterile carrier (such as peat or talc) and 0.1% carboxymethyl cellulose adhesive, then shade-dried before sowing
• Soil drenching: the suspension is applied directly to the root zone at sowing or at defined growth stages
• Root dipping: seedling roots are immersed in the suspension for 15 to 30 minutes before transplanting

The application dosage is calculated on a per-seed or per-pot basis according to the product specification or study protocol.

Growth Conditions and Trial Management

Plants are maintained in a greenhouse or growth chamber at 22–28°C during the day and 16–18°C at night, under a 14-hour photoperiod or natural light supplement, with relative humidity of 60–75%. Soil moisture is maintained at 60–70% of field capacity by periodic weighing and irrigation with equal volumes per pot. Cross-contamination between treatments is prevented by spatial separation, dedicated watering equipment, and drainage saucers. The trial duration is typically 30 to 60 days, corresponding to the vegetative evaluation stage of the selected crop.

Data Collection and Statistical Analysis

At harvest, all specified parameters are measured using standardized instruments. Dry biomass is determined after oven drying at 105°C for 30 minutes followed by 75–80°C to constant weight. Results are analyzed by one-way or two-way analysis of variance (ANOVA), with treatment mean separation by Duncan's multiple range test or Fisher's LSD at the p < 0.05 significance level. Growth promotion rate is calculated as: (treatment mean − control mean) / control mean × 100%.

Efficacy Evaluation Criteria

An inoculant is judged effective when at least one key growth indicator (such as dry biomass, plant height, or root development) shows a statistically significant improvement over the appropriate control, commonly an increase of 5–10% or greater, and the result is reproducible across at least two independent trials. Findings are compiled into a technical report documenting experimental conditions, raw data, statistical analysis, and conformity conclusions aligned with GB 20287-2006 requirements.

Applications

PGPR pot experiment data serve multiple technical and commercial purposes across the biofertilizer value chain:

• Regulatory registration and product filing of agricultural microbial inoculants in accordance with GB 20287-2006
• Screening and ranking of candidate strains during research and development of novel bioinoculants
• Formulation optimization, carrier selection, and determination of optimal application dosage
• Batch-to-batch efficacy verification as part of manufacturers' internal quality control systems
• Independent verification of growth-promotion claims by accredited third-party laboratories for product labeling and marketing
• Academic research on plant–microbe interactions, rhizosphere colonization, and induced systemic resistance
• Pre-field validation that de-risks and refines the design of subsequent multi-location field trials

FAQ

What is the difference between a pot experiment and a field trial?

A pot experiment is conducted in a greenhouse or growth chamber under controlled temperature, light, moisture, and substrate conditions, which maximizes sensitivity and reproducibility for detecting microbial effects. A field trial evaluates performance under natural, heterogeneous conditions with native microbiota and weather variability. Pot trials are typically performed first to confirm strain efficacy before investing in field-scale validation.

How long does a standard PGPR pot experiment take?

The typical duration is 4 to 8 weeks (30 to 60 days), depending on the crop species and the growth stage at which evaluation is scheduled. Fast-growing species such as cucumber or Chinese cabbage can be evaluated in about 30 days, while maize or wheat trials may extend to 60 days to capture full vegetative development.

Should the soil substrate be sterilized before the trial?

It depends on the objective. Sterilized substrate is preferred when the goal is to measure the direct physiological effect of the strain on the plant without microbial competition. Non-sterilized soil is used when the objective includes evaluating colonization ability, competitiveness against native microbiota, or performance under realistic soil conditions. Many comprehensive studies include both configurations.

How many replicates are required for reliable results?

A minimum of four replicate pots per treatment with three to ten plants per pot is standard, giving at least 30 evaluable plants per treatment. This replication level provides adequate statistical power for ANOVA and reliable mean separation at the p < 0.05 level.

Which standards govern the pot experiment method?

GB 20287-2006 for agricultural microbial inoculants is the primary reference governing efficacy requirements for registration in China. International guidance on soil quality and plant growth testing from organizations such as ISO and ASTM provides complementary methodological frameworks for experimental setup and phytometric measurement.

What result qualifies as a positive growth-promotion effect?

A positive effect requires a statistically significant improvement (p < 0.05) in at least one key parameter, such as dry biomass, plant height, or root length, relative to the appropriate control, typically with an increase of 5–10% or more, and reproducibility across repeated independent trials. Non-significant trends are not accepted as evidence of efficacy.