Fruit Development and Seed Germination

Week 9 Overview

  • Key Topics:

    • Fruit growth

    • Fruit ripening:

    • Changes in fruit during ripening

    • Climacteric and non-climacteric fruit

    • Extending storage life

    • Seed testing

    • Seed dormancy

    • Treatments to break dormancy

    • Germination stimulants

    • Light sensing by seeds

    • Direction of seedling growth:

    • Gravitropism

    • Thigmotropism

    • Phototropism

Fruit Development

  • Fruit Growth:

    • Double Sigmoid Curve:

    • Fruit growth often follows a double sigmoid pattern.

    • Phase 1: Cell division is most active, particularly in the pericarp (fruit wall).

    • Phase 2: Growth is primarily due to the enlargement of already existing cells.

  • Fruit Ripening:

    • Key Changes During Ripening:

    • Peak in respiration rates.

    • Increased production of ethylene, a plant hormone that promotes ripening.

    • Flesh softening, altering texture for consumption.

    • Change in color to indicate ripeness.

    • Increase in volatiles that contribute to flavor.

    • Decreased detachment force, making it easier for fruits to fall or be harvested.

    • Climacteric vs Non-Climacteric Fruits:

    • Climacteric Fruits:

      • Exhibit a significant rise in respiration and ethylene production at the onset of ripening.

      • Example: Kiwifruit.

    • Non-Climacteric Fruits:

      • Do not follow the respiratory pattern; ripening is influenced by factors other than ethylene.

  • Extending Storage Life:

    • Techniques to slow down plant metabolism:

    1. Temperature Control:

      • Specific temperature ranges facilitate better storage, e.g., apples at 0-4°C, avocados at 7-10°C.

    2. Heat Treatment:

      • Pre-cooling treatment for fruits like persimmons to prolong shelf life.

    3. Humidity Control:

      • Maintain humidity levels close to 100% to prevent moisture loss from fruits.

    4. Controlled Atmosphere Storage:

      • Adjusting levels of gases (CO2 and O2) within storage environments to extend shelf life.

    5. Use of Ripening Inhibitors:

      • Compounds such as aminovinylglycine, methylcyclopropene, and silver thiosulfate to inhibit ethylene synthesis or action, delaying ripening.

Seed Development

  • Seed Germination:

    • Seeds must undergo appropriate conditions for germination, including adequate moisture, temperature, and sometimes light.

  • Seed Longevity:

    • Varies significantly among species; some seeds can remain viable for many years (e.g., Acacia, Eucalyptus) while others may last only a short time (e.g., Schefflera).

  • Seed Storage Types:

    • Orthodox Seeds:

    • Best stored at low water content (<10%) and cool temperatures (<10°C). Examples: Eucalyptus, Brassica.

    • Recalcitrant Seeds:

    • Sensitive to desiccation; prefer high water content (30%) and warmer temperatures (>15°C). Examples: Citrus, Mango.

  • Seed Testing:

    • Natural Germination Unit:

    • Different types of seeds (Pittosporaceae, Orchidaceae, Asteraceae, etc.) require specific testing approaches.

    • Imbibition Testing:

    • Determines water uptake; some seeds have water-impermeable coats leading to dormancy. Testing involves gradual increase of mass indicative of water absorption.

    • Initial Germination Testing:

    • Conducted in labs and nurseries continuously to assess viability.

Seed Dormancy

  • Types of Seed Dormancy:

    • Physical Dormancy (PY):

    • Impervious seed coats; require mechanical scarification to germinate.

    • Physiological Dormancy (PD):

    • Seed coats are permeable but the embryo’s growth is inhibited by mechanical or chemical barriers.

    • Morphological Dormancy (MD):

    • Embryos are not fully developed at time of dispersal.

    • Combinational Dormancy:

    • Mixed forms of dormancy, which may include elements of PY, PD, or MD.

  • Treatments to Break Dormancy:

    • Scarification:

    • Mechanical methods to nick or scratch seed coats (e.g., Solanum orbiculatum).

    • Hot Water Treatment:

    • Soaking seeds in hot water (85-99°C for 15s - 5 min) to break dormancy.

    • Concentrated Acid Treatment:

    • Applying concentrated acids (e.g., 90% H2SO4) for a duration to alter seed coat properties.

    • Stratification:

    • Moist chilling or warming over periods of weeks/months at specified temperatures.

    • After-ripening:

    • Period of drying at warm temperatures to increase seed viability.

  • Germination Stimulants:

    • Gibberellic Acid (GA):

    • Commonly used to trigger germination in otherwise dormant seeds (e.g., Helianthus annuus).

    • Smoke:

    • Stimulates germination, particularly in fire-adapted species. Karrikinolide (KAR1) is a key compound from smoke.

Light Sensing and Seedling Growth

  • Light Sensing by Seeds:

    • Phytochrome detects red light, informs seeds about environmental conditions (e.g., closed canopy).

  • Seedling Tropisms:

    • Growth responses optimized towards environmental stimuli:

    • Gravitropism: Roots may show positive response while shoots show a negative response to gravity.

    • Thigmotropism: Responses to mechanical stimuli, such as bending around supports.

    • Phototropism: Growth toward light source, mediated by differential distribution of auxins depending on light exposure.