Plant Growth Hormone Content Determination: Methods, Standards, and Applications

Plant growth hormone content determination is an essential analytical service for verifying active ingredient levels in agrochemical formulations, screening for undeclared plant growth regulators in fertilizers and biostimulants, and quantifying trace residues in food and agricultural products. Accredited third-party laboratories apply advanced techniques such as HPLC, LC-MS/MS, GC-MS, and ELISA to deliver accurate, standards-compliant results. This article outlines the scope, sample types, testing items, analytical methods, and practical applications of plant growth hormone determination.

Technical Overview

Plant growth hormones, also known as phytohormones, are naturally occurring signal molecules that regulate plant development at very low concentrations. The classical classes include auxins, gibberellins, cytokinins, abscisic acid, and ethylene, while brassinosteroids, jasmonates, salicylic acid, and strigolactones are increasingly recognized as additional hormonal groups. In modern agriculture, synthetic analogues of these compounds, known collectively as plant growth regulators (PGRs), are widely used as rooting agents, fruit-setting agents, growth retardants, ripening agents, and sprout suppressants.

Determining the content of these substances is critical for several reasons: manufacturers must verify the active ingredient content of registered formulations; fertilizer and biostimulant producers must demonstrate that products are free of undeclared regulators; food producers and regulators must confirm compliance with maximum residue limits (MRLs) established under standards such as GB 2763 and the European Union pesticide residue framework; and researchers require precise quantification of endogenous hormone levels in plant tissues. ISO/IEC 17025-accredited third-party laboratories provide these determinations using validated chromatographic, mass spectrometric, and immunochemical methods.

Testing Samples

Fertilizers and Biostimulants

Seaweed extracts, humic and fulvic acid products, amino acid fertilizers, water-soluble fertilizers, and microbial inoculants are analyzed to confirm declared ingredients and to screen for undeclared plant growth regulators, which are restricted or prohibited in many fertilizer categories under regulations such as the EU Fertilising Products Regulation framework.

Agrochemical Formulations

Technical materials, emulsifiable concentrates, suspension concentrates, soluble powders and granules, and tablet formulations containing growth regulators are tested for active ingredient content, uniformity, and label-claim verification during product registration and batch release.

Plant Tissues and Seeds

Leaves, roots, stems, flowers, embryos, and germinating seeds are analyzed for endogenous hormones such as indole-3-acetic acid (IAA), gibberellins, zeatin riboside, and abscisic acid in plant physiology, breeding, and stress-response research.

Fruits, Vegetables, and Processed Foods

Residue monitoring is performed on melons, grapes, kiwifruit, beans, potatoes, onions, tea, and processed products to verify compliance with MRLs for compounds such as forchlorfenuron, paclobutrazol, chlormequat, and maleic hydrazide.

Soil, Water, and Growing Media

Environmental and substrate samples are analyzed to track the persistence and migration of growth regulators in greenhouse and open-field production systems.

Sample Submission Requirements

• Endogenous hormone testing: at least 2–5 g of fresh tissue, snap-frozen in liquid nitrogen, stored at −80 °C, and shipped on dry ice to avoid degradation.
• Residue testing: 100–500 g of homogenized sample, kept refrigerated or frozen in clean containers.
• Formulations and fertilizers: 50–100 g of well-mixed product in sealed, moisture-proof packaging.
• Repeated freeze–thaw cycles, prolonged ambient storage, and incomplete labeling should be avoided to ensure data validity.

Testing Items

Typical analyte groups covered by plant growth hormone determination services include:

• Auxins: indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), 1-naphthaleneacetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), 4-chlorophenoxyacetic acid (4-CPA).
• Gibberellins: gibberellic acid (GA3) and GA4/GA7 mixtures.
• Cytokinins: 6-benzylaminopurine (6-BA), kinetin, zeatin, zeatin riboside, thidiazuron (TDZ), and forchlorfenuron (CPPU).
• Growth retardants and inhibitors: paclobutrazol, uniconazole, chlormequat chloride, mepiquat chloride, daminozide, and maleic hydrazide.
• Ethylene-related and other regulators: ethephon, abscisic acid (ABA), salicylic acid, jasmonic acid, brassinolide, 28-homobrassinolide, diethyl aminoethyl hexanoate (DA-6), and sodium nitrophenolate complexes.

Quantification ranges span from percent-level active ingredients in formulations down to microgram-per-kilogram (ppb) residues and nanogram-per-gram endogenous hormone levels in plant tissue.

Testing Methods

HPLC-UV/DAD

High-performance liquid chromatography with ultraviolet or photodiode array detection is the standard approach for high-content formulations. Separation is performed on C18 reversed-phase columns using water (acidified with 0.1% formic acid or buffered to pH 3) and methanol or acetonitrile gradients, with detection wavelengths selected for each compound class. Typical quantification limits reach 0.01–0.1% of the formulation mass, meeting the requirements of relevant GB, HG/T, and international formulation standards.

LC-MS/MS and UPLC-MS/MS

Liquid chromatography coupled to tandem mass spectrometry is the reference technique for trace residues and endogenous hormones. Electrospray ionization with polarity switching and multiple reaction monitoring (MRM) provides high selectivity in complex matrices. Quantification by isotope dilution, using deuterated internal standards such as d5-IAA and d6-ABA, compensates for matrix effects and recovery losses. Method performance follows the principles of GB 23200 series and GB/T 20769 standards, with limits of quantification typically between 0.001 and 0.01 mg/kg.

GC-MS and GC-FID

Gas chromatography is applied to volatile or derivatizable regulators. Acidic auxins and gibberellins may be methylated or silylated prior to GC-MS analysis, while certain quaternary ammonium retardants can be determined after suitable derivatization. GC-FID remains useful for routine content verification of stable, thermostable actives in formulations.

ELISA Immunoassay Screening

Enzyme-linked immunosorbent assays offer rapid, cost-effective screening of IAA, GA3, ABA, and cytokinins in large sample batches, with sensitivities at the ppb level. As a semi-quantitative technique, positive findings are confirmed by LC-MS/MS.

Sample Preparation and Quality Control

• Endogenous hormones: cryogenic grinding, cold extraction with 80% methanol or isopropanol containing an antioxidant, solid-phase extraction cleanup (C18 or mixed-mode cartridges), and solvent concentration under nitrogen.
• Residues: QuEChERS-style extraction with acidified acetonitrile and dispersive SPE cleanup, using matrix-matched calibration.
• Validation criteria: calibration linearity with R² ≥ 0.99, spiked recoveries of 70–120%, relative standard deviations within laboratory acceptance limits, method blanks, and replicate analyses.
• Non-target screening of unknown regulators in fertilizers is available through high-resolution accurate-mass mass spectrometry with spectral library confirmation.

Applications

Plant growth hormone determination supports a wide range of regulatory, commercial, and scientific objectives:

• Quality control and batch release of registered plant growth regulator formulations.
• Verification that fertilizers, biostimulants, and organic inputs are free of undeclared synthetic regulators.
• Food safety compliance testing against MRLs under GB 2763, Codex, and EU requirements.
• Plant physiology research, including hormone profiling during development, fruit set, ripening, and stress responses.
• Tissue culture optimization, where auxin-to-cytokinin ratios govern morphogenesis.
• Post-harvest management, such as verifying sprout suppressant residues in stored potatoes and onions.
• Market surveillance, trade dispute resolution, and label-claim verification.

FAQ

What sample quantities are required?

For endogenous hormone analysis, 2–5 g of fresh tissue is recommended; for residue testing, 100–500 g of homogenized sample; for formulations and fertilizers, 50–100 g of well-mixed product.

How should samples be preserved and shipped?

Fresh plant tissue should be snap-frozen in liquid nitrogen, stored at −80 °C, and shipped on dry ice. Residue samples can be refrigerated or frozen, while dry formulations should be kept sealed at room temperature away from light and moisture.

What detection limits can be achieved?

LC-MS/MS methods typically achieve limits of quantification of 0.001–0.01 mg/kg for residues and low ng/g levels for endogenous hormones, while HPLC-UV methods are suitable for percent-level content in formulations.

How long does testing take?

Standard turnaround is generally 5–10 working days after sample receipt, depending on the analyte panel, matrix complexity, and confirmation requirements. Expedited services may be arranged.

Can undeclared plant growth regulators in fertilizers be identified?

Yes. High-resolution mass spectrometry enables non-target and suspect screening against comprehensive spectral libraries, allowing identification of unlisted regulators even when their presence is not declared.

What accreditation and reporting apply?

Analyses are performed in ISO/IEC 17025-accredited third-party laboratories following validated methods aligned with GB, GB/T, NY/T, EN, and ISO standards, and test reports are accepted for product registration, regulatory submission, and commercial transactions.