Application of Plant Growth Regulators for Canopy Management in Fruit Crops

1. Aim

To study the role of plant growth regulators (PGRs) in canopy management of fruit crops and to understand the effect of traditional and new-generation growth retardants on tree architecture, vegetative growth, flowering, and fruit yield.

2. Principle

Canopy management in fruit crops involves controlling the size, shape, and density of the plant canopy to optimize light interception, air circulation, and photosynthetic efficiency. Proper canopy structure enhances fruit quality, yield, and pest management efficiency.

Plant Growth Regulators (PGRs) are organic compounds that modify plant physiological processes such as cell division, elongation, and differentiation even at very low concentrations.

Exogenous application of PGRs alters the endogenous hormonal balance of plants and allows growers to manipulate:

Shoot growth, Branching pattern, Flower induction, Fruit retention, and Tree size

In fruit orchards, canopy management through PGRs is particularly useful where excess vegetative growth reduces flowering and fruiting efficiency.

3. Scientific Background

Tree canopy architecture determines:

Light interception

Photosynthetic efficiency

Fruit colour and sugar accumulation

Pest and disease incidence

Chemical canopy regulation is now widely practiced using growth retardants such as paclobutrazol, uniconazole, chlormequat chloride, and prohexadione-calcium which inhibit gibberellin biosynthesis, thereby reducing shoot elongation and promoting reproductive growth.

For example:

Paclobutrazol reduces vegetative growth and increases flowering in mango, citrus, and avocado.

Soil drenching of paclobutrazol significantly suppresses shoot elongation and increases fruit yield in mango orchards.

Thus, PGRs are important tools for chemical canopy management in modern orchards.

4. Materials Required

Plant growth regulators: Paclobutrazol, Uniconazole, Prohexadione-calcium, Chlormequat chloride, NAA (Naphthalene acetic acid), GA₃ (Gibberellic acid), CPPU (Forchlorfenuron)

Measuring cylinder,

Weighing balance,

Spray pump,

Distilled water,

Fruit trees (mango, guava, apple, citrus etc),

Field notebook

5. New Generation Plant Growth Regulators

Modern orchard systems use next-generation PGRs that provide precise canopy control.

1. Prohexadione-Calcium

Inhibits late steps in gibberellin biosynthesis

Reduces vegetative shoot growth by 30–50%

Used in high density apple orchards

2. Uniconazole

More potent than paclobutrazol

Controls excessive vegetative growth

Improves branching and canopy compactness

3. Forchlorfenuron (CPPU)

Synthetic cytokinin

Enhances fruit size and canopy productivity

4. Brassinosteroids (emerging class)

Improve stress tolerance

Enhance canopy photosynthesis efficiency

6. Procedure

Step 1: Selection of Trees

Select healthy fruit trees of uniform age and size in the orchard.

Step 2: Preparation of PGR Solution

Prepare PGR solutions at recommended concentrations.

Example: Paclobutrazol solution of 7500 ppm

Calculation: x ppm= mg/ml or g/l

Example: 7500 ppm = 7.5 g/L

Step 3: Method of Application

Two common methods:

1. Soil Drenching

Paclobutrazol is applied around the root zone.

Typical dose:

Mango: 1–20 g active ingredient per tree

This reduces internode length and induces flowering.

2. Foliar Spray

Growth regulators sprayed on leaves.

Examples:

GA₃ – 50 ppm

NAA – 20–40 ppm

Prohexadione-calcium – 125 ppm

Step 4: Observations

Record observations at 15-day intervals:

Shoot length (cm)

Internode length

Number of new shoots

Leaf area index

Canopy volume

Flower initiation

Fruit set percentage

Yield per plant

Table 1: Plant growth regulators used for canopy management in fruit crops

8. Advantages of PGR-Based Canopy Management

Reduces excessive vegetative growth

Improves light penetration inside the canopy

Enhances flowering and fruiting

Improves fruit colour and quality

Facilitates high-density planting systems

Reduces pruning cost

9. Limitations

Over-application may cause phytotoxicity

Residual effect of some PGRs (e.g., paclobutrazol) in soil

Crop-specific dosage requirement

Environmental safety concerns

10. Precautions

Use recommended concentrations only

Avoid spraying during extreme temperatures

Follow protective safety measures

Maintain proper application timing