AGRI 103: Agronomy - Plant Growth and Development pt. 1/2 lecture 6

Definition: The irreversible increase in weight or size of an organism due to an increase in the number and size of cells.
Key Points:
- A portion of the carbon compounds produced during photosynthesis is used for growth.
- Growth can be measured in different ways:
- Fresh weight
- Dry weight: Most common due to fluctuation in water content.
- Linear dimensions such as height.
- Leaf area.
- Growth is associated with development: As plants grow larger, they transition through different stages of differentiation.
- Dry matter accumulation: Increase in dry weight of the plant over time changes with growth.

Imbibition: The absorption of water by the seed through the seed coat.
- Cells become rehydrated.
- Seed coat gets ruptured.
Initiation of metabolic processes in the embryo:
- Respiration.
- Activation of enzymes that break down stored food from either cotyledons or endosperm.
Growth of embryo:
- Cell enlargement, cell division, and cell differentiation.
- Radicle emerges first, followed by the shoot.
- Germination is the first step in the development of a plant from a seed.

Water:
- Necessary for physical and chemical processes that take place in germinating seeds.
- Before planting, most seeds contain less than 15% moisture; they need to absorb a significant amount of water during imbibition.
- After imbibition: If water is lacking, the germinating seed can dehydrate and die.
Oxygen:
- Many plant metabolic processes require oxygen, most importantly respiration.
- Oxygen is not usually a limiting factor in soils unless soils are flooded or compacted.
Temperature:
- Affects metabolic processes and reactions, especially the activation of enzymes that break down stored energy and the absorption of water and oxygen.
- Different plants have different minimum, optimum, and maximum temperatures for germination, depending on their origin:
- Cool season, temperate species: Minimum temperature is 4-10°C.
- Warm season, tropical species: Minimum temperature is 10-16°C.
- Recommended minimum average soil temperature at seeding depth (Celsius):
- Peas: 5
- Lentils: 5
- Chickpeas – Kabuli: 10
- Chickpeas – Desi: 7
- Faba beans: 3-5
- Dry beans: 12
- Soybeans: 10
- Wheat: 4
- Barley: 3-5
- Oats: 5
- Canola: 5
- Mustard: 5
- Flax: 5
- Canaryseed: 5.

Definition: The failure of a seed to germinate when all required environmental conditions are met.
Functions:
- Prevents seed germination while the seed is mature and still on the plant.
- Ensures that plants will become established under favorable growing conditions (more common in wild plants).
- Enables weed seeds to persist and remain viable in the soil for a long time.
- Breeding of commercial seeds has selected against dormancy for this purpose.

Various factors induce seed dormancy:
- Physical factors: Seed coat is a physical barrier that can be impermeable to air and water and needs to be disrupted.
- Naturally: Abrasion from soil movement or decomposition by fungi and insects.
- Commercially: Scarification (seed coat is damaged mechanically by shaking seed with sharp material or sand or chemically using acid).
- Chemical factors: Some seeds contain germination inhibitors that must be leached out by water.
- Physiological factors: Some forms of seed dormancy involve further development of the embryo, requiring enzymatic and biochemical changes that are not well understood (referred to as after-ripening).
Environmental cues required to break dormancy:
- Some seeds need specific cues to break dormancy such as:
- Seeds need to be dried before germination occurs (to prevent germination in fleshy fruit).
- Require exposure to low temperatures for extended periods (2-3 months) = stratification.
- Light/photoperiod: Germination is activated when red light exposure converts Pr to Pfr in the phytochrome.

Epigeal emergence:
- Radicle breaks through the seed coat first.
- The hypocotyl (the portion of the embryonic plant between the radicle and cotyledons) lengthens and “pulls” the cotyledons above the soil surface.
- After cotyledons emerge, the first true leaves develop, and cotyledons begin to wither.
- Cotyledons develop chlorophyll and conduct photosynthesis briefly until true leaves develop.
Hypogeal emergence:
- Radicle breaks through the seed coat first.
- The cotyledons remain below the soil surface while the epicotyl (the region above the cotyledon) elongates and emerges from the soil.
- Once the epicotyl breaks through the soil surface, the first true leaves will develop and grow.
- Cotyledons decompose after food reserves are used up.

Coleorhiza: Emerges first, allowing the radicle to come through and develop into a small, temporary primary root.
Coleoptile: Encloses the embryonic shoot which emerges from the seed and protects the growing point as it pushes to the surface.
First foliage emerges after the coleoptile.
The scutellum and endosperm remain underground, while adventitious roots develop from the lower nodes of the stem.

Recommended seeding depth (inches):
- Peas: 1-3
- Lentils: 1-3
- Chickpeas – Kabuli: 1.5-2.5
- Chickpeas – Desi: 1.5-2.5
- Faba beans: 2-3
- Dry beans: 2-2.5
- Soybeans: 0.75-1.5
- Wheat: 1.5-2.5
- Barley: 1.5-1.75
- Oats: 1-2
- Canola: 0.5-1
- Mustard: 0.5-1
- Flax: 1-1.5
- Canaryseed: <2.5.
Factors to consider when choosing seeding depth:
- Seed size: Larger seeds have greater energy reserves to reach the soil surface before developing photosynthetic tissue.
- Soil moisture: Ensures the seed is deep enough to maximize contact with soil water appropriate for the seed size.
- Soil texture: Affects the crusting, depth, and retention of soil moisture. Epigeal emergence can be more hindered by crusting of the soil surface.