Role of growth substances in agri-horticulture

Classification of Plant Growth Regulators

PGRs are broadly classified into five major groups:

• Auxins: Primarily involved in cell elongation, apical dominance, root initiation, and fruit development. Indole-3-acetic acid (IAA) is the most common naturally occurring auxin.
• Gibberellins (GAs): Promote stem elongation, seed germination, flowering, and fruit setting. GA3 is widely used in agriculture.
• Cytokinins: Stimulate cell division, delay senescence, and promote bud formation. Zeatin is a prominent cytokinin.
• Abscisic Acid (ABA): Inhibits growth, promotes dormancy, and helps plants cope with stress. It plays a crucial role in seed maturation and stomatal closure.
• Ethylene: A gaseous hormone that promotes fruit ripening, senescence, and abscission.

Role of PGRs in Agriculture

PGRs play a multifaceted role in enhancing agricultural productivity:

• Seed Germination & Seedling Growth: Gibberellins are used to break seed dormancy and promote uniform germination, particularly in crops like lettuce and barley.
• Vegetative Propagation: Auxins are extensively used to induce rooting in cuttings, facilitating clonal propagation of plants like sugarcane, roses, and grapes.
• Flowering & Fruit Setting: Auxins and gibberellins can induce flowering in pineapple and promote fruit set in tomatoes and cucumbers, even under unfavorable conditions.
• Fruit Development & Ripening: Ethylene is used commercially to accelerate fruit ripening in bananas and tomatoes, enabling synchronized harvesting. Auxins also play a role in parthenocarpic fruit development (fruit without fertilization) in grapes and citrus.
• Weed Control: Synthetic auxins like 2,4-D are used as selective herbicides to control broadleaf weeds in cereal crops.
• Prevention of Premature Fruit Drop: Auxins can prevent premature fruit drop in apples and pears, increasing yield.

Role of PGRs in Horticulture

Horticulture benefits significantly from the precise application of PGRs:

• Ornamental Plant Production: Cytokinins are used to promote branching and bushier growth in ornamental plants, enhancing their aesthetic appeal.
• Post-Harvest Management: Ethylene inhibitors like 1-MCP (1-methylcyclopropene) are used to delay ripening and extend the shelf life of fruits like apples and pears, reducing post-harvest losses.
• Flower Induction & Control: Gibberellins can induce flowering in long-day plants grown in short-day conditions, allowing for year-round production.
• Turf Management: PGRs are used to control the growth rate of turfgrass, reducing mowing frequency and improving turf quality.

Recent Advancements and Challenges

Recent research focuses on developing biostimulants – substances that enhance plant growth without directly acting as PGRs – and utilizing PGRs in combination with other technologies like nanotechnology for targeted delivery. However, challenges remain:

• Optimizing Dosage & Timing: The effectiveness of PGRs is highly dependent on the concentration and timing of application, requiring precise knowledge of plant physiology and environmental conditions.
• Environmental Concerns: The potential environmental impact of synthetic PGRs needs careful consideration, promoting the use of bio-based alternatives.
• Regulatory Framework: A robust regulatory framework is needed to ensure the safe and responsible use of PGRs in agriculture and horticulture.

PGR	Agricultural Application	Horticultural Application
Auxin	Weed control (2,4-D), preventing fruit drop	Rooting cuttings, inducing flowering in pineapple
Gibberellin	Breaking seed dormancy, increasing fruit size	Inducing flowering in long-day plants
Cytokinin	Delaying senescence in leafy vegetables	Promoting branching in ornamental plants
Ethylene	Accelerating fruit ripening (bananas)	Extending shelf life with inhibitors (1-MCP)