Unit 5: Reproductive biology of fruit trees

1. Bearing Habits of Fruit Trees

Understanding the bearing habit—the specific location and type of wood on which fruit buds are formed—is critical for pruning and training operations. Improper pruning can remove fruit-bearing wood, resulting in yield loss.

A. Classification Based on Position of Fruit Buds

Fruit buds may be terminal (at the tip of the shoot) or lateral (in the axils of leaves).

1. Terminal Bearing: Flower buds form at the tip of the shoot.
   • Examples: Mango (panicle), Loquat, Avocado.
2. Lateral (Axillary) Bearing: Flower buds form in the axils of leaves along the shoot.
   • Examples: Guava, Fig, Peach, Plum.

B. Classification Based on Age of Wood

This is the most practical classification for pruning purposes.

1. Bearing on Current Season's Growth

Flowers are borne on shoots that grow in the current spring/season. Pruning is usually done to encourage new vegetative growth.

• Examples:
  • Grape: Flowers on current season shoots arising from one-year-old canes.
  • Guava: Flowers on current season shoots.
  • Pomegranate, Citrus, Ber, Phalsa.

2. Bearing on One-Year-Old Wood

Flowers are produced on the shoots that grew during the previous season.

• Examples:
  • Peach: Fruit buds are borne laterally on previous season's growth.
  • Plum (Japanese).

3. Bearing on Old Wood (Spurs)

Flowers are borne on specialized, short, stubby shoots called spurs. Spurs grow very slowly and may bear fruit for many years.

• Examples:
  • Apple: Bears on spurs formed on wood 2 years or older.
  • Pear, Sweet Cherry, Apricot, Almond.

2. Juvenility and Flower Bud Differentiation

A. Juvenility

The juvenile phase is the physiological state of a seedling plant during which it cannot be induced to flower, regardless of environmental conditions. It is the vegetative lag phase before the reproductive phase begins.

• Characteristics of Juvenile Phase:
  • Rapid vegetative growth.
  • Morphological differences (e.g., thorny nature in citrus/pear seedlings, deeply lobed leaves).
  • High rooting ability of cuttings (due to lower phenol content and higher auxin sensitivity).
• Duration: Varies significantly.
  • Papaya: 6–8 months.
  • Mango/Citrus: 5–8 years.
  • Beech tree: 30–40 years.
• Transition: The shift from juvenile to adult is called Phase Change.

B. Flower Bud Differentiation (FBD)

FBD is the process where a vegetative meristem transforms into a reproductive (floral) meristem.

1. Stages of FBD

1. Induction: Chemical/hormonal signals trigger the change (invisible change).
2. Initiation: Morphological changes begin; the apical meristem flattens.
3. Differentiation: Formation of floral parts (sepals, petals, stamens, pistil).
4. Development: Growth of the bud until anthesis (flower opening).

2. Factors Affecting FBD

• C/N Ratio (Carbon/Nitrogen Ratio): Based on Kraus and Kraybill's hypothesis.
  • High C/N Ratio: Favors flowering (accumulation of carbohydrates).
  • Low C/N Ratio: Favors vegetative growth (high nitrogen supply).
• Hormonal Control:
  • Florigen: The hypothetical flowering hormone.
  • Gibberellins (GA): generally inhibit flowering in fruit trees (promote vegetative growth).
  • Cytokinins: generally promote flowering by mobilizing nutrients.
  • Ethylene: Promotes flowering in Pineapple and Mango.
• Environmental Factors:
  • Photoperiod: Short-day vs. Long-day plants (less critical in tropical trees than annuals).
  • Temperature: Vernalization (chilling requirement) is essential for temperate fruits (Apples, Peaches) to break bud dormancy and differentiate.

3. Pollination: Pollinizers and Pollinators

A. Pollination Defined

The transfer of pollen grains from the anther (male part) to the stigma (female part) of a flower.

B. Types of Pollination

1. Self-Pollination (Autogamy): Pollen from the same flower or same cultivar/variety.
   • Examples: Apricot, Peach, Citrus, Grape (mostly).
2. Cross-Pollination (Allogamy): Pollen from a different cultivar or species. Required for self-incompatible fruits.
   • Examples: Apple, Pear, Almond, Sweet Cherry, Mango, Avocado.

C. Distinction: Pollinizers vs. Pollinators

This is a common point of confusion in horticulture exams.

D. Pollinizer Placement

For self-incompatible orchards (e.g., Apple, Sweet Cherry), pollinizer varieties must be interplanted.

• Ratio: typically 1 pollinizer tree for every 8–9 main variety trees.
• Design: Usually planted as every third tree in every third row.

4. Fertilization and Parthenocarpy

A. Fertilization

The fusion of the male gamete (pollen nuclei) with the female gamete (egg cell) in the ovule.

• Process: Pollen germination on stigma Pollen tube growth through style Entry into ovule (usually via micropyle) Double Fertilization (in Angiosperms).
  1. Male nuclei + Egg cell = Zygote (Embryo).
  2. Male nuclei + Polar nuclei = Endosperm (Food storage).

B. Parthenocarpy

The development of fruit without fertilization. These fruits are seedless.

Types of Parthenocarpy:

1. Vegetative Parthenocarpy:
   • Fruit sets without the stimulus of pollination.
   • Example: Banana, Pineapple, Fig (common type).
2. Stimulative Parthenocarpy:
   • Requires the stimulus of pollination (pollen landing on stigma) to set fruit, but fertilization does not occur.
   • Example: Black Corinth Grape.
3. Stenospermocarpy:
   • Fertilization takes place, and the embryo begins to develop but subsequently aborts. The fruit matures, appearing seedless (sometimes with small traces of seeds).
   • Example: Thompson Seedless Grape, Sindhu variety of Mango.

5. Unfruitfulness in Fruit Crops

Unfruitfulness refers to the state where a tree fails to set fruit or fails to mature fruit to harvest.

A. Internal Causes (Evolutionary/Genetic)

1. Evolutionary Tendencies:
   • Monoecy: Male and female flowers on separate locations of the same plant (Walnut, Pecan).
   • Dioecy: Male and female flowers on separate plants (Papaya, Date Palm). Requires planting male trees.
2. Dichogamy: Maturation of male and female sex organs at different times.
   • Protandry: Pollen sheds before stigma is receptive (Walnut, Sapota).
   • Protogyny: Stigma receptive before pollen sheds (Avocado, Custard Apple).
3. Heterostyly: Styles and filaments are of different lengths, preventing self-pollination (Pomegranate, Litchi).
4. Self-Sterility/Incompatibility:
   • Pollen is viable but fails to fertilize the egg of the same variety due to genetic barriers (Apple, Pear, Cherry, Almond).
   • Gametic sterility: Defective pollen or egg.
5. Embryo Abortion: The embryo dies after fertilization due to malnutrition or genetic lethal factors.

B. External Causes (Environmental/Nutritional)

1. Nutrient Status:
   • Excess Nitrogen leads to excessive vegetative growth (low C/N ratio), reducing flowering.
   • Boron deficiency impacts pollen germination.
2. Weather Conditions:
   • Frost: Kills blossoms (common in stone fruits).
   • High Temp/Wind: Desiccates stigmas, reducing receptivity.
   • Rains during bloom: Washes away pollen and prevents bee activity.
3. Alternate Bearing (Biennial Bearing):
   • Tendency of trees to bear heavily one year (On-year) and poorly the next (Off-year) due to hormonal imbalances and resource exhaustion (Mango, Apple).

C. Remedies for Unfruitfulness

1. Use of Pollinizers: Interplanting compatible varieties for self-incompatible crops (e.g., planting 'Santa Rosa' plums with other Japanese plums).
2. Beehives: Introducing 2–4 hives/hectare during bloom.
3. Plant Growth Regulators (PGRs):
   • NAA / 2,4-D: Prevent fruit drop.
   • Paclobutrazol: Used in Mango to check vegetative growth and induce flowering.
   • Gibberellic Acid: Induce fruit set in seedless grapes.
4. Nutrient Management: Balancing N-P-K. Avoiding heavy nitrogen application right before flowering.
5. Ringing/Girdling: Removing a strip of bark to stop downward translocation of carbohydrates, increasing C/N ratio in the upper canopy to promote flowering (Grape, Mango).
6. Top Working: Grafting a pollinizer variety onto a branch of a main tree if pollinizers were forgotten during planting.