Review on the Analysis and Testing Method of Typical Plant Growth Regulators in Environment

Plant growth regulators (PGRs) are defined as naturally occurring or artificially synthesized compounds. The functions of PGRs mainly include accelerating or delaying seed germination, breaking plant dormancy, stimulating or reducing bud elongation, inducing flowering and fruiting, and affecting the aging process. The application of PGRs has effectively promoted the growth of plants. However, the application concentration of PGRs is more than one millionth. The large amount of abuse and misuse of PGRs in the process of use not only reduces the yield of crops, but also exacerbates their residues in the environment, especially in agricultural products such as fruits and vegetables. So, they are detected in many environmental media like fruits, vegetables, and water. In addition, most PGRs are toxic, and some of them will undergo adsorption, desorption, hydrolysis, photolysis, microbial degradation, and other environmental behaviors after entering the soil. The decomposition products produced by this process are more toxic.In order to comprehensively understand the current status of PGRs pretreatment, analysis and test method, the common pretreatment methods of solid phase extraction and liquid-liquid extraction for typical PGRs in solid substrates such as fruits and vegetables, fertilizers and soil, and in liquid substrates such as water, edible oil and nutrient solution are summarized in this paper, as well as the analysis and testing techniques such as high-performance liquid chromatography (HPLC) and ultra-high performance liquid chromatography (UPLC). Moreover, to reduce the detection limit, high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) is also frequently used. At the same time, considering the strong natural attenuation ability of PGRs and the high toxicity of their intermediates, the advantages, disadvantages and applicability of different testing techniques are systematically summarized, taking into account the special structure and nature of PGRs, in order to fully understand the current status of pretreatment and analytical testing of PGRs and provide literature support for subsequent research on analytical testing, migration transformation, pollution evaluation and treatment of PGRs.

Due to many impurities in the sample that can interfere with the detection, the appropriate pretreatment method can improve the accuracy of the test results. However, the sample pretreatment process accounts for more than 70% of the total analysis and test work, and about 50% of the error in the final test results comes from pretreatment. Therefore, establishing a fast, simple, and stable pretreatment method can effectively improve the efficiency and accuracy of analysis and detection. At present, the forms of environmental media detected for PGRs are mainly divided into two types: solid matrix samples (such as fruits, vegetables, fertilizers and soil, etc.) and liquid matrix samples (water, oil and nutrient solution, etc.). The related pretreatment methods are also mostly targeted at these two different forms of environmental media.

Solid substrates involving PGRs mainly include fruits and vegetables, fertilizers and soil. Among them, fruits and vegetables are the most frequently detected solid substrates of PGRs, and some PGRs have also been detected in fertilizers, soil and other substrates. The pretreatment process of solid matrix samples can be divided into two parts: extraction and purification. Among them, solid phase extraction is the most commonly used extraction technology, and QuEChERS method is the most widely used purification method.

Water, edible oil, and nutrient solution are the most frequently detected liquid substrates of PGRs. At present, liquid-liquid extraction is the most commonly used extraction method for liquid matrix. For the selection of extractants, the octanol-water partition coefficient of PGRs such as gibberellic acid and ethephon is less than 1, which is a strong polar compound and can be extracted by hydrophilic organic solvents such as methanol, ethanol and acetone; the polarity of most other PGRs such as forchlorfenuron and paclobutrazol is relatively weak, but it still belongs to the category of strong polarity compared with other kinds of compounds such as benzene and chloroethane (n-octanol-water partition coefficient>10). Therefore, the extractant can not only use methanol, but also use polar organic solvents such as ethyl acetate and chloroform that are insoluble in water. Although liquid-liquid extraction requires a lot of extractants, the high-water solubility of most PGRs makes them often directly detected by HPLC-MS/MS, which eliminates the complex pretreatment steps such as extraction and purification. In addition, because some PGRs have poor chromatographic characteristics or are not easily detected, the derivatization is also required to convert the components into derivatives suitable for analysis.

Analytical and testing technologies mainly include gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), HPLC/UPLC, HPLC-MS/MS, ion chromatography (IC), spectrophotometry (SP), capillary electrophoresis (CE), enzyme-linked immunosorbent assay (ELISA) and electrochemical sensor method.

At present, the solubility of most PGRs in water (20℃) is 0.50-10g/L, and the octanol-water partition coefficient of most PGRs is 0-4, with strong hydrophilicity. Therefore, HPLC-MS/MS are applicable to the detection of almost all PGRs. However, HPLC-MS/MS are often used to detect the residues of PGRs in fruits and vegetables, followed by soil and fertilizer, while there are few related studies in natural water. This may be due to the rapid natural decay rate of PGRs in the natural environment, resulting in extremely small amounts of residues in natural water bodies such as surface water and groundwater that cannot be directly detected.

GC has the advantages of low cost and easy maintenance, and GC-MS has become a conventional testing technology. However, GC-MS are greatly affected by sample matrix interference and require high pretreatment methods. In addition, due to the influence of the physical and chemical properties of different PGRs (such as abscisic acid and indole acetic acid with a boiling point over 400℃, or forchlorfenuron and cinnamic acid with hydroxyl and carboxyl groups), most PGRs have poor gas chromatographic characteristics and are not easily detected.

The order of detection limits of PGRs is GC>HPLC>chromatography-mass spectrometry, and the lowest instrumental detection limit of chromatography-mass spectrometry is 10^-5mg/kg. However, the higher solubility and the larger natural attenuation rate of most PGRs lead to the lower detection concentrations in complex environmental substrates such as soil and water, so there is still an urgent need to solve the problem of analytical testing of trace PGRs and its intermediates.

In future, PGRs analysis and test method will focus on the analysis and detection of trace PGRs and their intermediates, as well as the development of new materials and technology-based methodologies.