Fertilization and Parthenocarpy in Horticultural Crops

B. Sc. Agriculture (Hons.) Ist. Semester (Six Deam Commitee of ICAR)

Fertilization and Parthenocarpy in Horticultural Crops

Introduction

The processes of pollination, fertilization, and fruit development are fundamental to the reproductive success of flowering plants.

Fertilization leads to the formation of a zygote and subsequent seed and fruit development.
Parthenocarpy, on the other hand, allows fruit development without fertilization, resulting in seedless fruits.

Both phenomena play a major role in determining the fruit set, yield, and quality in horticultural crops.

Fertilization

Definition
Fertilization is the process of fusion of male and female gametes (nuclei) to form a zygote, which develops into an embryo and subsequently a seed.
It is the final step in sexual reproduction in flowering plants.

Fertilization occurs in several steps after successful pollination.

Pollination; Transfer of pollen grains from the anther to the stigma. Can occur by wind, insects, water, or manual methods.
Pollen Germination; Pollen grains on the stigma absorb moisture and nutrients from the stigma surface. The pollen tube emerges through a germ pore.
Pollen Tube Growth; The pollen tube grows through the style toward the ovary, guided by chemical signals from the ovule. It carries two male gametes (sperms).
Entry into the Ovule: The pollen tube enters the ovule through the micropyle (micropylar entry). Rarely, it may enter through chalaza (chalazogamy) or integuments (mesogamy).
Double Fertilization (Unique to Angiosperms): One male gamete fuses with the egg cell → forms zygote (2n) → develops into embryo. The second male gamete fuses with the two polar nuclei → forms primary endosperm nucleus (3n) → develops into endosperm (nourishment tissue).
Post-Fertilization Changes

- Ovule → Seed
- Ovary → Fruit
- Ovary wall → Pericarp
- Integuments → Seed coat
- Ovary attachment → Fruit stalk (pedicel)

Conditions for Successful Fertilization

Importance	Explanation
1. Seed formation	Zygote develops into embryo → seed.
2. Fruit development	Fertilized ovary enlarges to form fruit.
3. Genetic recombination	Combines traits from both parents.
4. Crop improvement	Basis of hybridization and breeding programs.
5. Productivity	Determines fruit set percentage and yield.

Parthenocarpy

Definition
Parthenocarpy is the phenomenon of fruit development without fertilization of the ovule.
Such fruits are generally seedless because no zygote or embryo is formed.

Type	Description	Examples
1. Natural (Genetic) Parthenocarpy	Occurs naturally due to genetic factors without external influence.	Banana, Pineapple, Seedless guava, Seedless grape.
2. Induced (Artificial) Parthenocarpy	Brought about by the use of plant growth regulators or environmental manipulations.	Tomato, Citrus, Apple (GA₃, NAA, 2,4-D).
3. Vegetative Parthenocarpy	Fruit develops without pollination or fertilization.	Banana, Fig, Pineapple.
4. Stimulative Parthenocarpy	Pollination occurs but fertilization does not; pollen stimulates fruit development.	Tomato, Eggplant, Grape.

Causes of Parthenocarpy

Hormonal imbalance — high levels of auxins or gibberellins stimulate ovary growth.
Genetic factors — certain cultivars have genes for natural parthenocarpy.
Environmental stress — unfavorable temperature or absence of pollinators triggers fruit set without fertilization.
Growth regulator application — use of synthetic hormones like NAA, GA₃, or 2,4-D.

Role of Plant Growth Regulators in Induced Parthenocarpy

Plant Growth Regulator	Concentration	Effect / Example
NAA (α-naphthaleneacetic acid)	20–40 ppm	Induces seedless fruits in citrus and tomato.
GA₃ (Gibberellic acid)	25–100 ppm	Induces parthenocarpy in grapes and cucumber.
2,4-D	10–20 ppm	Promotes fruit set in apple and citrus.
IAA (Indole-3-acetic acid)	25–50 ppm	Promotes ovary enlargement in tomato.

Advantages of Parthenocarpy

Advantage	Explanation
1. Seedless fruits	Preferred by consumers (banana, grape).
2. Uniform fruit size	Because fruit set is not dependent on pollination.
3. Higher market value	Seedless fruits fetch premium prices.
4. Useful under adverse pollination conditions	When pollinators are absent or weather is unfavorable.
5. Regular cropping	Reduces dependence on pollination success.

Disadvantages of Parthenocarpy

Disadvantage	Explanation
1. Lack of seeds	No natural propagation through seeds.
2. Dependence on vegetative propagation	Increases cost of propagation and management.
3. Limited genetic diversity	Difficult to breed new varieties without seeds.
4. Hormonal control required	Artificial induction may be inconsistent.

Crop	Type of Parthenocarpy	Remarks
Banana	Natural	Seedless due to sterility.
Pineapple	Natural	Fruit develops from unfertilized ovary.
Citrus (seedless lime)	Natural or Induced	Consumer preferred.
Grapes (seedless varieties)	Natural or GA₃ induced	Common in ‘Thompson Seedless’.
Tomato	Induced	Hormone-treated for off-season production.
Cucumber	Induced	Commercial use for uniform seedless fruits.
Fig	Vegetative	Develops from parthenocarpic flower parts.

Differences Between Fertilization and Parthenocarpy

Feature	Fertilization	Parthenocarpy
Definition	Fusion of male and female gametes to form zygote.	Fruit development without fertilization.
Pollination required	Yes.	Not always necessary.
Seed formation	Seeds formed (fertile fruit).	No seeds (seedless fruit).
Zygote formation	Occurs.	Absent.
Hormone involvement	Natural balance of auxin, GA, cytokinin.	High auxin or GA induces ovary growth artificially.
Examples	Mango, Apple, Guava.	Banana, Pineapple, Seedless grape, Tomato (GA₃).

Process	Description	Result
Pollination	Transfer of pollen to stigma.	Leads to fertilization (in normal fruit set).
Fertilization	Fusion of gametes inside ovule.	Seeded fruit (normal).
Parthenocarpy	Fruit development without fertilization.	Seedless fruit (natural or induced).

Note: Some fruits (e.g., cucumber, tomato) can develop either through fertilization or parthenocarpy, depending on conditions.

Applications of Parthenocarpy in Fruit Industry

Banana industry: All commercial varieties are parthenocarpic and sterile.
Seedless citrus and grapes: Market demand for fresh consumption and juice industry.
Off-season fruiting: Induced parthenocarpy allows fruit set in unfavorable weather.
Vegetable crops: Parthenocarpic cucumbers and tomatoes are valuable for greenhouse production.

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