Explain growth process in plants (cell cycle, growth patterns, seed dormancy and viability, and primary and secondary growth)

The cell cycle is the sequence of events that occur between one cell division and the next. It consists of interphase (Growth Phase I, Synthesis, and Growth Phase II) and the mitotic phase.

1.1 Interphase

Interphase is the period where the cell prepares for division. It accounts for about 90% of the cell cycle.

• G₁ Phase (Growth Phase I): After mitosis, the cell grows rapidly. All cell organelles (except mitochondria and chloroplasts) are synthesized. Metabolic rate is high, and the cell increases in size.

• S Phase (Synthesis): DNA replication occurs. Each chromosome is duplicated into two sister chromatids. Histone proteins are synthesized and bind to DNA. The chromosome number doubles (becomes 4n).

• G₂ Phase (Growth Phase II): Mitochondria and chloroplasts divide. Energy stores accumulate for mitosis. Chromosomes begin to condense, and the mitotic spindle starts forming.

1.2 Mitosis

Mitosis produces two daughter nuclei with identical chromosome sets to the parent cell. It occurs in meristematic tissues of plants.

The Four Phases of Mitosis:

1. Prophase: Chromosomes become short and thick. In animal cells, centrioles migrate to opposite poles and form asters. The nucleolus and nuclear envelope disintegrate. Spindle fibers form.

2. Metaphase: Chromosomes align at the cell equator (the metaphase plate), attached to spindle fibers at their centromeres.

3. Anaphase: Sister chromatids separate at the centromeres. Spindle fibers pull them to opposite poles of the cell.

4. Telophase: Chromatids reach the poles and uncoil to form chromatin. Nuclear membranes reform, nucleoli reappear, and spindle fibers disappear.

1.3 Cytokinesis

Cytokinesis is the division of the cytoplasm. In plant cells, Golgi vesicles fuse at the equator to form a cell plate, which develops into the primary cell wall. This differs from animal cells, where the membrane pinches inward from the periphery.

Plants exhibit different growth patterns that can be limited or unlimited.

2.1 Sigmoid Growth Curve (Limited Growth)

Annual plants like beans and maize show a sigmoid (S-shaped) growth curve:

1. Initial decline: At germination, dry mass decreases as stored food in cotyledons is oxidized for energy.
2. Exponential phase: After leaves develop, photosynthesis exceeds respiration, and dry mass increases rapidly.
3. Plateau phase: At maturity, anabolism equals catabolism, so no net growth occurs.
4. Senescence: Catabolism exceeds anabolism; the plant loses mass as leaves die and seeds disperse.

2.2 Indeterminate Growth

Perennial plants like mango trees and woody plants exhibit unlimited growth. Their growth curve consists of repeated sigmoid curves, with growth rate varying seasonally—rapid in wet seasons and slow in dry seasons. These plants continue to increase in height and diameter throughout their lives.

3.1 Seed Dormancy

Dormancy is the state where seeds fail to germinate even when conditions (water, oxygen, temperature) are favorable. It ensures germination occurs under favorable conditions.

Types of Dormancy:

• Primary Dormancy: Present at seed dispersal. Causes include:

  • Immature embryo
  • Hard seed coat (impermeable to water and gases)
  • Presence of growth inhibitors (e.g., abscisic acid)

• Secondary Dormancy: Develops when seeds are exposed to unfavorable conditions after dispersal.

Methods to Break Dormancy:

3.2 Seed Viability

Viability is the seed's ability to germinate under suitable conditions. Factors affecting viability include:

• Embryo maturity: A fully developed embryo is necessary.
• Enzyme activity: Enzymes must be active for germination processes.
• Storage conditions: Seeds lose viability over time; most remain viable for about one year.
• Disease absence: Infected seeds lose viability.

Viability Test: The Tetrazolium Chloride (TTC) test stains living tissues red. Dead tissues remain unstained (white).

Primary growth results from the activity of apical meristems located at the tips of roots and shoots. It increases the length of the plant (elongation).

4.1 Meristematic Tissues

Meristematic cells are characterized by:

• Thin, extensible cell walls
• Dense cytoplasm
• Small vacuoles
• Numerous ribosomes and mitochondria
• High metabolic rate

4.2 Zones of Primary Growth

In roots and shoots, three overlapping zones exist:

1. Zone of Cell Division: Contains apical meristems; cells undergo mitosis.
2. Zone of Elongation: Vacuoles fuse and enlarge; turgor pressure stretches the cell wall, causing elongation.
3. Zone of Maturation: Cells differentiate into permanent tissues (epidermis, cortex, vascular tissues).

4.3 Products of Apical Meristems

The apical meristem produces three primary meristematic tissues:

• Protoderm → Epidermis
• Procambium → Primary vascular tissues (xylem and phloem)
• Ground Meristem → Cortex and pith

Secondary growth results from lateral meristems and increases plant girth (diameter). It occurs in woody dicotyledons but not in monocots or herbaceous dicots.

5.1 Lateral Meristems

Two types contribute to secondary growth:

1. Vascular Cambium:

   • Located between primary xylem and phloem
   • Produces secondary xylem (wood) inward and secondary phloem outward
   • Contains fusiform initials (produce longitudinal tissues) and ray initials (produce ray parenchyma)

2. Cork Cambium (Phellogen):

   • Produces phellem (cork) outward and phelloderm (secondary cortex) inward
   • Replaces ruptured epidermis as stem diameter increases

5.2 Secondary Growth Process

1. Vascular cambium divides mitotically, producing more secondary xylem than phloem.
2. Secondary xylem accumulates as wood, adding strength and conductivity.
3. As diameter increases, the epidermis ruptures.
4. Cork cambium develops and produces protective cork tissue.
5. Annual rings form in secondary xylem due to seasonal growth variation.

Worked Example: A student observes a cross-section of a 5-year-old dicot stem. How can the age of the stem be determined?

Answer: By counting the annual rings in the secondary xylem. Each ring represents one year's growth—a light-colored spring ring (wide vessels for rapid water conduction) and a dark-colored autumn ring (narrow vessels).

In Tanzania, understanding plant growth processes directly benefits farming practices. For instance, knowing that rice seeds may exhibit dormancy after harvest helps farmers store seeds properly or use methods like soaking before planting to ensure uniform germination. Similarly, coffee farmers in Mbeya benefit from understanding secondary growth when pruning coffee bushes to increase yield—removing apical buds stimulates lateral branch development, which produces more flowering nodes. This knowledge translates into better crop management, higher yields, and improved food security for smallholder farmers across the country.