Plant growth regulators (PGRs) represent one of the most significant advances in modern agriculture and horticulture. By precisely targeting biochemical pathways, these compounds have revolutionized plant development and crop product methodologies.
Among the wide range and spectrum of PGRs, Growth Retardants hold a unique position, in that they offer targeted solutions to manage plant growth and optimize their structural characteristics.
Growth Retardants inhibit processes that are specifically designed to control plant height and improve structural integrity. This makes them an invaluable tool in agricultural productivity and ornamental horticulture.
Growth Promoters accelerate growth processes. Inhibitors stop specific developmental stages. Growth Retardants slow down plant cell elongation and division, effectively managing plant height and growth patterns. Thus, Growth Retardants allow for greater control over plant morphology, making them extremely important in activities like commercial farming and landscaping.
Mode of Action
- The fascinating mechanism behind Growth Retardants involves their interaction with plant hormones, particularly gibberellins. These hormones are vital for stem
- When applied, these compounds enter plant tissues and target specific enzymes. This interruption leads to reduced cell elongation while maintaining cell division, resulting in shorter internodes (the stem sections between leaves) but preserving overall plant health.
- The process also triggers secondary effects that enhance plant Treated plants, in some species, exhibit up to 15% higher chlorophyll content, improved photosynthesis, and better stress due to efficient resource use. These complementary effects explain why treated plants often appear darker green and more robust than untreated specimens.
- Growth retardants primarily target cell elongation, but research shows their effects can vary. While compounds like paclobutrazol mainly restrict cell elongation while preserving cell division, others like chlormequat can temporarily affect both processes. This explains why treated plants develop compact, sturdy stems rather than just becoming uniformly smaller.
What are the types of Growth Retardants?
Growth retardants are best categorised based on their chemical structure and mode of action. The following categories are noteworthy:
- Quaternary Ammonium Compounds: Widely used for controlling excessive vegetative growth. For example, chlormequat chloride.
- N-Containing Heterocycles: Effectively reduce internodal Examples include Paclobutrazol and uniconazole.
- Acylcyclohexanediones: Useful in specifically restricting gibberellin pathways. Examples include Ancymidol and flurprimidol.
Fig 1, below, describes the Plant Growth Retardants (PGRts) and their impact on crops during different periods
Fig1
Natural v/s Synthetic Growth Retardants
- Natural growth retardants, originating from plant-based compounds, operate through a variety of mechanisms, including the regulation of growth hormones and modulation of metabolic pathways.
- These compounds often require higher application rates to achieve effective growth inhibition, and their efficacy can fluctuate depending on plant species and prevailing environmental conditions.
- While these substances tend to degrade rapidly, typically within hours or days, they are prized for their environmental advantages. Their biodegradability and eco-friendly nature ensure minimal risk to non-target organisms, making them a sustainable option in plant growth management.
- In contrast, synthetic retardants offer more precise, targeted control. Their molecular design allows for specific enzyme inhibition, often achieving growth reduction with minimal impact on other physiological processes. Modern synthetic compounds demonstrate remarkable efficiency, with some requiring application rates as low as 5-10 grams per hectare for effective control. These compounds typically show longer persistence in plant tissues, with half-lives ranging from 15-30 days compared to natural compounds that may degrade within hours or days.
Applications and Benefits
Prevention of lodging
These compounds address one of farming’s most persistent challenges: lodging (a condition where crops collapse under their weight). This issue, particularly prevalent in cereal crops like wheat, rice, and barley, can reduce harvest yields by up to 40%. Growth retardants strengthen plant stems by promoting increased cell wall thickness and reduced stem length, creating sturdier plants that better withstand environmental stresses.
Uniform & Aesthetic Produce
These compounds help growers to produce uniform, attractive, and desirable plants that meet market demands. For instance, in poinsettia production, growth retardants help maintain the ideal compact form that consumers prefer, while in turf grass management, they reduce mowing frequency by up to 50% while improving density and colour. These compounds also enhance resource efficiency – treated plants typically require 20-30% less water and nutrients due to their compact growth habit, contributing significantly to sustainable farming practices.
Enhanced Seedling Quality
Research has demonstrated that the application of Growth Retardants, such as Paclobutrazol, significantly improves the growth and development of tomato seedlings, leading to enhanced seedling quality.
Reduction in Labor Costs
The compounds can reduce labour costs by up to 30% by eliminating the need for manual pruning and height control. Their use in cereal crops has been shown to increase harvesting efficiency by up to 25% due to more uniform crop height and reduced lodging.
Limitations
Time Restriction
The application of these compounds is constrained by critical timing requirements. For instance, wheat treatments must typically occur between growth stages 30-32 (stem elongation phase) to achieve optimal results.
Need of Optimal Conditions
Errors in timing or dosage can result in undesirable outcomes, such as uneven growth or reduced yield. Additionally, environmental factors like temperature and soil moisture significantly influence the effectiveness of these compounds. Most applications perform best within an optimal temperature range of 15-25°C.
Impact on Beneficial Microbes
While modern Growth Retardants are engineered to break down in soil within 20-60 days, their impact on non-target organisms raises concerns. Beneficial soil
microorganisms can be temporarily affected, although their populations generally recover within one growing season.
Unsuitable for Organic Farming
These compounds are unsuitable for organic farming systems or ecological restoration projects, and in such contexts, require alternative growth control methods.
The Future of Growth Retardants
Growth retardants are evolving, with researchers worldwide developing eco-friendly alternatives and natural compounds to replace conventional options. These innovations, along with breakthroughs in precision application technology and controlled-release formulations, are making growth regulation more efficient and environmentally sound. The focus on biodegradable solutions and microbial-based alternatives particularly supports organic farming practices, pointing to a future where agricultural productivity and environmental stewardship go hand in hand.
Conclusion
References:
ScienceDirect. (n.d.). Gibberellin. ScienceDirect.
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/gibberellin
National Center for Biotechnology Information. (2022). Paclobutrazol: Mechanisms and applications in plant growth regulation. PubMed Central (PMC). https://pmc.ncbi.nlm.nih.gov/articles/PMC8840709/
Zhang, X., Shen, R., & Wang, J. (2014). Environmental impact of plant growth regulators in agriculture. Environmental Science & Technology, 48(10), 5641– 5649. https://pubs.acs.org/doi/10.1021/es500434p
European Journal of Agronomy. (2024). Assessing the efficacy and residual impact of plant growth retardants on crop lodging and overgrowth: A review. Retrieved from ScienceDirect
