Unit5 - Subjective Questions

Bearing Habit refers to the specific position on the tree where fruit buds are formed and the type of wood (shoot or spur) on which they are borne. Understanding this is crucial for pruning and training operations.

Classification based on Position of Fruit Buds:

1. Terminal Bearing:

   • Flower buds differntiate at the apex (tip) of the shoot or spur.
   • Examples: Mango (Panicle), Loquat.

2. Lateral/Axillary Bearing:

   • Flower buds differentiate in the axils of leaves along the shoot.
   • Examples: Guava, Fig, Peach.

3. Mixed Bearing:

   • Some crops may exhibit both habits depending on the variety or environmental conditions, though distinct classifications are usually maintained.

Difference:

• Pollinizer: It is the plant source (tree/variety) that provides compatible pollen for cross-pollination. It is the 'father' parent in the pollination process.
• Pollinator: It is the agent (insect, wind, water, bird) that physically transfers the pollen from the anther of the pollinizer to the stigma of the recipient flower. The honey bee (Apis mellifera) is the most common pollinator in fruit crops.

Characteristics of a Good Pollinizer:

1. Bloom Synchrony: Its flowering period must overlap significantly with the commercial variety requiring pollination.
2. Compatibility: It must be cross-compatible with the main variety.
3. Pollen Abundance: It should produce a large quantity of viable pollen.
4. Regular Bearing: It should not be an alternate bearer; it must flower annually.
5. Commercial Value: Ideally, the fruit of the pollinizer should also have some market value.

Definition:
Juvenility is the physiological state of a seedling plant during which it is incapable of flowering, even if environmental conditions are favorable. It represents the vegetative growth phase preceding the reproductive phase.

Physiological Characteristics:

• Inability to Flower: The plant does not respond to flower-inducing stimuli.
• Morphological Differences: Often entails presence of thorns (e.g., Citrus, Pear), different leaf shapes (heterophylly), and vigorous vegetative growth.
• Rooting Ability: Juvenile tissues usually root more easily than mature tissues.

Methods to Shorten Juvenility:

1. Grafting: Grafting scion wood from a mature tree onto a seedling rootstock bypasses the juvenile phase.
2. Girdling: Ringing the bark to accumulate carbohydrates.
3. Growth Retardants: Application of chemicals like Paclobutrazol or CCC.
4. Environmental Manipulation: Optimizing light and nutrition to accelerate growth.

The C:N Ratio Theory, proposed by Kraus and Kraybill, suggests that the balance between Carbohydrates () and Nitrogen () in the plant tissue governs flowering and fruiting.

Four Conditions:

1. High N, Low C: Vigorous vegetative growth, succulent stems, no flowering. (Plant is starved of carbohydrates).
2. Moderate N, Moderate C: Good vegetative growth, but limited flowering.
3. Moderate N, High C: Ideal condition for flowering. Accumulation of carbohydrates favors flower bud differentiation.
4. Low N, High C: Weak vegetative growth, yellowish leaves, poor flowering (Starvation condition).

Mechanism:
Flower bud differentiation generally requires a high level of stored carbohydrates. Nitrogen promotes vegetative growth. By restricting nitrogen or increasing carbohydrate accumulation (via photosynthesis or ringing), the ratio increases, shifting the plant from a vegetative to a reproductive phase.

Unfruitfulness is the state where a fruit tree fails to set or mature fruit, or produces crops that are structurally or economically non-viable.

Classification of Causes:

1. External (Environmental) Factors:

   • Nutritional Status: Imbalance in NPK or micronutrients (e.g., Boron deficiency).
   • Climate: Frost during bloom, high temperatures, strong winds, rain washing away pollen.
   • Pests and Diseases: Floral blast, mango hoppers, powdery mildew.

2. Internal (Physiological/Genetic) Factors:

   • Evolutionary tendencies: Diceciousness (male/female separate).
   • Impootent pollen: Non-viable pollen grains (e.g., Grape var. 'Brighton').
   • Incompatibility: Self-incompatibility or Cross-incompatibility.
   • Sterility: Structural defects in reproductive organs.
   • Embryo Abortion: Death of the embryo after fertilization.
   • Dichogamy: Non-synchronization of male and female organ maturity.

Dichogamy is a condition in hermaphroditic (bisexual) flowers where the male (anthers) and female (stigma) reproductive organs mature at different times, preventing self-pollination.

Types of Dichogamy:

1. Protandry:

   • The anthers (male) mature and shed pollen before the stigma is receptive.
   • Examples: Walnut, Pecan, Coconut, Sapota.

2. Protogyny:

   • The stigma (female) becomes receptive before the anthers shed pollen.
   • Examples: Banana, Fig, Avocado, Custard Apple.

Impact on Unfruitfulness:
Since self-pollination is impossible due to the timing mismatch, if a compatible pollinizer variety is not present nearby to provide pollen at the correct time, fertilization will not occur, leading to unfruitfulness. This necessitates the planting of mixed varieties in an orchard.

Self-Incompatibility is the inability of fertile pollen to fertilize the same flower or other flowers on the same plant due to genetic mechanisms, despite the pollen and ovule being functional.

1. Gametophytic System:

• Mechanism: The outcome is determined by the genotype of the pollen grain (microgametophyte) itself ().
• Interaction: If the -allele in the pollen matches one of the alleles in the pistil, pollen tube growth is arrested in the style.
• Examples: Apple, Pear, Cherry, Citrus.

2. Sporophytic System:

• Mechanism: The outcome is determined by the genotype of the sporophyte (the parent plant) producing the pollen ().
• Interaction: Inhibition usually occurs on the surface of the stigma. If the parent plant shares alleles with the recipient pistil, germination is inhibited.
• Examples: Mango, Aonla, Cocoa.

Significance: It promotes cross-pollination and genetic diversity but requires careful planning of pollinizers in orchards.

Parthenocarpy is the formation and development of fruit without fertilization. These fruits are generally seedless.

Types of Parthenocarpy:

1. Vegetative Parthenocarpy:

   • Fruit development occurs without any external stimulation (pollination is not required).
   • Examples: Banana, Pineapple, Fig (common type).

2. Stimulative Parthenocarpy:

   • Requires the stimulus of pollination (without fertilization) to initiate fruit set. The pollen tube may grow but fails to fertilize, or simply the presence of pollen on the stigma triggers hormonal changes.
   • Examples: Black Corinth Grapes, some Watermelons.

3. Stenospermocarpy (Seed abortion):

   • Technically fertilization occurs, but the embryo aborts at an early stage. The fruit develops, but the seeds are trace or soft.
   • Examples: 'Thompson Seedless' Grapes, Sindhu variety of Mango.

Fertilization is the fusion of male and female gametes to form a zygote.

Process of Double Fertilization:

1. Pollination: Pollen grain lands on the stigma and germinates.
2. Pollen Tube Growth: The tube grows down the style, entering the ovary and then the ovule through the micropyle.
3. Release of Gametes: The pollen tube releases two male gametes () into the embryo sac.
4. Syngamy (First Fertilization):
   • One male gamete () fuses with the egg cell ().
   • Result: Formation of a Diploid Zygote (), which develops into the Embryo.
5. Triple Fusion (Second Fertilization):
   • The second male gamete () fuses with the two polar nuclei (secondary nucleus, ) in the center.
   • Result: Formation of the Triploid Primary Endosperm Nucleus (), which develops into the Endosperm (providing nutrition).

This simultaneous occurrence of Syngamy and Triple Fusion is unique to flowering plants.

Heterostyly is a morphological condition where flowers of the same species have styles of different lengths relative to the stamens.

Mechanism as a Barrier:
It prevents self-pollination physically by spatial separation of anthers and stigmas.

1. Distyly (Dimorphic Heterostyly):
   • Pin type: Long style and short stamens (Stigma protrudes beyond anthers).
   • Thrum type: Short style and long stamens (Anthers are above the stigma).
   • Effective pollination usually occurs only between Pin Thrum or Thrum Pin (Legitimate pollination).

Examples:

• Pomegranate: Has heterostyly regarding style length.
• Litchi: Exhibits functional male and female flowers with structural differences.
• Almond: Some varieties show spatial separation.

Remedies for Unfruitfulness:

1. Planting Pollinizers: Ensuring 10-15% of the trees in the orchard are pollinizer varieties (e.g., planting 'Golden Delicious' with 'Red Delicious' apples) to ensure cross-pollination.
2. Introduction of Pollinators: Placing beehives (Apis mellifera) in the orchard during bloom (approx. 3-5 hives per hectare) to facilitate pollen transfer.
3. Top Working: If an established orchard is unfruitful due to self-incompatibility, some trees or branches can be grafted (top worked) with a compatible pollinizer variety.
4. Nutritional Management: Correcting the ratio. Nitrogen application for weak trees, or root pruning/ringing for overly vigorous trees to induce flowering.
5. Use of Plant Growth Regulators (PGRs): Spraying Auxins (NAA) or Gibberellins () to prevent flower/fruit drop or induce parthenocarpy (e.g., in grapes and citrus).

Flower Bud Differentiation (FBD) is the physiological and morphological process by which a vegetative meristem transforms into a reproductive (floral) meristem.

Key Phases:

1. Induction: Chemical or hormonal changes occur within the tissue (increase in C:N ratio, hormonal balance) signaling the plant to flower. This is not visible externally.
2. Initiation: The first visible morphological change occurs. The flat meristem becomes dome-shaped.
3. Differentiation: The specialized floral parts develop in sequence—sepals, petals, stamens, and finally the pistil.
4. Development: The flower parts grow in size until anthesis (flower opening).

Timing: FBD usually takes place season before flowering in temperate fruits (summer/autumn for spring bloom) and shortly before flowering in many tropical fruits.

Abortive Flowers refer to flowers that have imperfectly developed or degenerated reproductive organs (pistils or ovules), rendering them incapable of setting fruit.

Characteristics & Causes:

• Pistil Abortion: The pistil remains rudimentary or underdeveloped. These flowers function only as males (staminate).
• Nutritional Stress: Late-emerging panicles or flowers in shaded areas often have high abortion rates due to lack of photosynthates.
• High Fruit Load: Heavy bearing in the previous year can exhaust reserves, leading to poor flower quality and abortion in the current year.

Examples:

• Mango: A high percentage of perfect (hermaphrodite) flowers is required for good yield. Many varieties produce a high ratio of staminate (male) flowers due to pistil abortion.
• Olive: Produces many staminate flowers due to ovarian abortion.

Role of Hormones in Reproductive Biology:

1. Gibberellins (e.g., ):

• Pollen Germination: Necessary for pollen tube growth.
• Parthenocarpy: Application of can induce fruit set without fertilization. It is commercially used in grape production (e.g., Thompson Seedless) to increase berry size and induce seedlessness.
• Fruit Retention: helps in preventing early fruit drop.

2. Auxins (e.g., IAA, NAA):

• Fruit Set: Endogenous auxins produced by seeds signal the plant to retain the fruit. If fertilization fails, auxin levels drop, and the fruit falls.
• Prevention of Abscission: Synthetic auxins (NAA, 2,4-D) are sprayed to prevent the formation of the abscission layer, thereby reducing pre-harvest fruit drop (e.g., in Citrus and Apple).

Environmental conditions during the blooming period are critical for successful pollination and fertilization.

1. Temperature:

   • Low Temp: Can inhibit pollen germination and slow down pollen tube growth. Frost can kill the stigma.
   • High Temp: Desiccates the stigmatic fluid, making it non-receptive, and reduces pollen viability.

2. Wind:

   • Strong winds can hinder insect activity (bees do not fly in high wind).
   • Dry winds dry out the stigma.

3. Rain:

   • Washes away pollen grains from the anthers/stigma.
   • Dilutes stigmatic fluid.
   • Restricts bee flight activity.

4. Light:

   • Cloudy weather reduces bee activity.
   • Poor light penetration inside the canopy can lead to poor flower quality.

Comparison:

Desirability in Fruit Production:
Cross-pollination is generally more desirable for most commercial fruit crops (Apple, Almond, Pear, Sweet Cherry, Mango) because:

1. Overcoming Incompatibility: Many fruits are self-incompatible.
2. Hybrid Vigor: Cross-pollination often leads to better fruit set.
3. Fruit Quality: Metaxenia effects (influence of pollen on fruit tissues) can sometimes improve size and sugar content.
4. Adaptability: Results in seeds with better adaptability (though fruits are propagated vegetatively, the crop yield depends on the seed set signaling fruit retention).

Stenospermocarpy:
It is a biological mechanism where pollination and fertilization take place normally, but the embryo aborts at an early stage of development. The fruit continues to grow due to sufficient hormonal stimulus generated before abortion, but the seeds remain undeveloped or vestigial (soft seeds).

Difference from Vegetative Parthenocarpy:

• Fertilization:

  • Stenospermocarpy: Fertilization occurs.
  • Vegetative Parthenocarpy: Fertilization does not occur.

• Stimulus:

  • Stenospermocarpy: Requires pollination and fertilization trigger.
  • Vegetative Parthenocarpy: Requires no external stimulus (genetic).

• Examples:

  • Stenospermocarpy: 'Thompson Seedless' Grapes.
  • Vegetative Parthenocarpy: Banana.

Alternate Bearing is the tendency of fruit trees to carry a heavy crop in one year ('On' year) followed by a very light or no crop the succeeding year ('Off' year).

Causes:

1. Hormonal Inhibition: Developing seeds in the 'On' year produce Gibberellins which inhibit flower bud differentiation for the next season.
2. Carbohydrate Depletion: Heavy fruiting exhausts stored food ( ratio drops), leaving insufficient resources for differentiation of new flower buds.
3. Genetic Factors: Common in Mango (var. Alphonso, Dashehari) and Apple.

Control Measures:

1. Thinning: Removing flowers or young fruits in the 'On' year to save resources.
2. Pruning: Regular pruning to maintain vegetative vigor.
3. Paclobutrazol: Application of Paclobutrazol (Cultar) in Mango inhibits gibberellin synthesis and promotes flowering in the 'Off' year.
4. Nutrution: Heavy nitrogen application during the 'On' year.

1. Apple Bearing Habit:

• Wood Type: Apples bear fruit mostly on spurs (short, compressed shoots).
• Position: They are terminal bearers on these spurs.
• Implication: Pruning should be done carefully to avoid removing spurs, as they remain productive for many years (Spur pruning vs. renewal pruning).

2. Grape Bearing Habit:

• Wood Type: Grapes bear fruit on canes (1-year-old mature wood arising from 2-year-old wood).
• Position: Flower buds are borne laterally (usually from the 3rd to 5th node, depending on variety).
• Implication: Heavy pruning is required annually. The cane must be cut back to a specific number of buds (e.g., 3-4 buds for spur pruning varieties like Muscat, or longer canes for Thompson Seedless) to ensure fruitfulness.

Xenia:

• The immediate effect of pollen on the seed and endosperm characteristics (color, shape, size) in the current generation.
• Example: In Maize, pollen from a purple-grained variety pollinating a white-grained variety results in purple grains.

Metaxenia:

• The direct effect of pollen on the maternal tissues of the fruit (skin, pulp, shape, maturation time), essentially tissues outside the embryo sac.
• Example: In Date Palm, the pollen source can influence the size and ripening time of the date fruit. In Apple, pollen can influence fruit shape.

Relation to Fertilization:
Both phenomena indicate that fertilization does not just trigger development but that the genetic constitution of the male gamete (pollen) actively influences the phenotypic expression of the resulting fruit and seed.