Plant Growth Regulators
Plant Growth Regulators (PGRs) are chemical substances that significantly influence the growth and differentiation of plant cells, tissues, and organs. They are broadly classified into two categories: naturally occurring plant hormones and synthetic substances.
Phytohormones
Plant hormones, or phytohormones, are essential regulators of plant growth and development. They are chemical messengers that influence various physiological processes, helping plants respond to internal and external cues.
Here’s an overview of how different groups of plant hormones impact plant growth:
Auxins are are primarily involved in processes such as cell elongation, apical dominance, root growth, and the differentiation of vascular tissues. The most well-known and naturally occurring auxin is indole-3-acetic acid (IAA).
Gibberellins are particularly important in promoting stem elongation, seed germination, and flowering. The most well-known gibberellin is gibberellic acid (GA3) which was discovered by scientists studying a disease affecting rice, caused by fungus Gibberella fujikuroi .
Cytokinins promote cell division in plant roots and shoots and are essential for the growth and development of plant tissues. High cytokinin levels typically promote shoot formation, while low cytokinin levels favour root formation. Cytokinins work in opposition to auxins in regulating apical dominance. While auxins promote apical dominance, cytokinins promote lateral bud growth, leading to bushier plants. Cytokinins play a role in the mobilization of nutrients within the plant, assisting in the transport of nutrients from older tissues to younger, actively growing tissues.
Abscisic Acid is a growth inhibiting phytohormone which is synthesized particularly in response to environmental stress, as such ABA is often referred to as the "stress hormone" in plants. It is produced in response to environmental stresses such as drought, salt, and cold. ABA helps plants cope with these stresses by initiating protective mechanisms, such as stomatal closure to minimise water loss during periods of drought.
Ethylene is a simple hydrocarbon gas (C2H4) that functions as a plant hormone, playing a crucial role in regulating various aspects of plant growth, development, and stress responses. It is unique among plant hormones because it is a gas and can easily diffuse through plant tissues and the surrounding atmosphere.
Synthetic Plant Growth Regulators
Synthetic PGRs impact plant growth my modifying the balance of phytohormones that exist naturally within a particular plant variety. Typically the goal of PGR application is to control an undesirable aspect of plant growth or to increase desirable crop parameters such as rate of photosynthesis or yield. All of the synthetic PGRs we will discuss in this article disrupt gibberellin synthesis, which results in shorter plants.
Some plants demonstrate increased yield after application of gibberellin inhibiting PGRs, though this effect is not consistent and varies significantly amongst different plant species. Residues of synthetic PGRs on edible plants can pose health risks if consumed. Regulatory bodies have established maximum residue limits (MRLs) to ensure safety. The use of certain synthetic PGRs has raised concerns about potential long-term health effects, including carcinogenicity and endocrine disruption.
Paclobutrazol
Paclobutrazol is a synthetic plant growth regulator belonging to the triazole group. It is commonly used in agriculture and horticulture to control plant size, enhance root development, and improve fruit set and quality. Paclobutrazol works by inhibiting gibberellin biosynthesis, which in turn reduces cell elongation and results in more compact and bushy plants. Paclobutrazol can also improve crop resistance to drought and salinity.
Paclobutrazol has low acute toxicity to humans, but it can cause adverse effects if ingested, inhaled, or absorbed through the skin in significant quantities. Long-term exposure may have more serious health implications. Paclobutrazol is known to produce dangerous nitrogen oxides upon combustion.
Paclobutrazol is relatively persistent in the environment and can affect non-target organisms, including beneficial soil microorganisms, aquatic life, and insects.
Daminozide
Daminozide, also known by its original tradename Alar, is commonly used on ornamental plants to improve marketability and inhibits gibberellin production, thus producing a shorter plant with a bushier growth habit. Daminozide was banned from use on food crops by the US Environmental Protection Agency in 1989 due to concerns that crop residues presented an unacceptable risk of cancer to the consumer, when considered over a lifetime.
Studies have indicated potential health risks, including carcinogenic effects and endocrine disruption. The International Agency for Research on Cancer (IARC) has classified daminozide as a possible human carcinogen (Group 2B).
Due to health and environmental concerns, the use of daminozide is restricted or banned in many countries. For example, both the USA and the European Union have banned its use on food crops, and other countries have implemented stringent regulations to control its application.
Chlormequat chloride
Chlormequat chloride is commonly used on grains and ornamental plants to control plant height. In Australia Chlormequat is approved for use on wheat only, where it is used to control lodging, a condition where excessive internodal spacing causes crops to fall over.
Chlormequat chloride’s persistence in the environment and its potential to contaminate water sources have raised environmental concerns. It may affect non-target organisms, including beneficial soil microorganisms and aquatic life.
Chlormequat chloride and its metabolites can accumulate on edible crops, leading to potential health risks for consumers. Studies have suggested that chlormequat chloride could have endocrine-disrupting properties, potentially affecting hormone regulation in humans and animals.
Due to health and environmental concerns, the use of chlormequat chloride is regulated or banned in some countries. For example, the European Union has restricted its use on certain crops and set stringent regulations on its application to ensure safety. In the US chlormequat chloride is banned from all food crops.
Biostimulants
Biostimulants are substances or microorganisms applied to plants, soils, or growing media to enhance plant growth, nutrient uptake, stress tolerance, and overall plant health without directly providing essential nutrients. Biostimulants exert their beneficial impacts on plant growth by stimulating natural processes within a plant, improving capacity to absorb nutrients, resist stress, and increase yield and quality. Biostimulants are natural alternatives to synthetic plant growth regulators which can be used to target similar plant growth responses.
Coconut water is a rich source of cytokinins, as well as vitamins and minerals such as calcium, magnesium and potassium. Coconut water can promote bushiness and lateral bud growth, whilst providing minerals that are essential to healthy plant cellular function.
Aloe vera extracts contain gibberellins, salicylic acid, vitamins, minerals, polysaccharides and amino acids. Salicylic acid is an invaluable compound for triggering Systemic Acquired Resistance (SAR), one of two complex immune response signalling pathways naturally present in plants, which confers lasting resistance against microbial and fungal pathogens. Aloe vera is also a powerful rooting agent and can be used in place of synthetic rooting hormones.
Monosilicic acid, often represented as , is the most basic and bioavailable form of silicic acid. It is a simple silicon compound in which silicon is bonded to four hydroxyl groups (OH). Monosilicic acid is the only form of silicon that plants can readily absorb and utilise. Silicic acid improves plant resistance to abiotic and biotic stressors, heavy metals and salinity. Silicic acid also increases mechanical strength of stems and the thickness of leaf cuticles.
Triacontanol is a naturally occurring long-chain fatty alcohol with the chemical formula C30H62O. It is known for its role as a potent plant growth regulator (PGR), promoting various beneficial effects on plant growth and development. Triacontanol enhances plant growth, photosynthesis, nutrient uptake, stress tolerance, and overall yield. Triacontanol is found naturally within the waxy cuticle layer on the leaves of some plant species, as well as within beeswax.
References
https://datasheets.scbt.com/sc-236284.pdf