Chapter Notes

Introduction to Plant Development

Have you ever wondered how a tiny seed grows into a massive tree with roots, stems, leaves, and flowers, all appearing in a specific order? This entire process, from a fertilized egg (zygote) to a mature plant that eventually dies, is called development.

Development is the sum of two key processes:

1. Growth: The increase in size.
2. Differentiation: The process where cells become specialized for different functions.

The journey of a plant begins with seed germination, which is the first step in its growth. A seed will only germinate when conditions like water, oxygen, and temperature are just right. If conditions are unfavorable, the seed enters a state of rest or dormancy. Once favorable conditions return, it resumes its metabolic activities and begins to grow.

The development of a plant is controlled by two types of factors:

• Intrinsic (Internal) Factors: These include genetic instructions and chemical signals within the plant.
• Extrinsic (External) Factors: These are environmental cues like light, temperature, water, and nutrients.

Growth

Growth is one of the most visible characteristics of a living thing. It is defined as an irreversible permanent increase in the size of an organ, its parts, or even a single cell. This process requires energy and involves metabolic activities (both building up and breaking down substances). For example, the expansion of a leaf is a form of growth.

Plant Growth is Generally Indeterminate

A unique feature of plants is their ability to grow throughout their lives. This is possible because they have specialized regions of actively dividing cells called meristems.

• Cells in the meristems can divide continuously.
• Some of the new cells lose the ability to divide and become part of the permanent plant body, while others remain in the meristem to continue dividing.
• This continuous addition of new cells is called the open form of growth.

There are two main types of growth based on the meristems involved:

1. Primary Growth: This is the increase in the length of the plant along its axis. It is carried out by the apical meristems located at the tips of roots and shoots.
2. Secondary Growth: This is the increase in the girth or thickness of the plant. It occurs later in the life of dicots and gymnosperms and is caused by lateral meristems, such as the vascular cambium and cork-cambium.

Growth is Measurable

At the cellular level, growth is mainly an increase in the amount of protoplasm (the living substance inside a cell). Since measuring protoplasm is difficult, we measure growth using other parameters that are proportional to it, such as:

• Increase in fresh weight
• Increase in dry weight
• Increase in length, area, or volume
• Increase in cell number

• A single maize root apical meristem can produce over 17,500 new cells per hour. Here, growth is measured by the increase in cell number.
• A cell in a watermelon can increase in size by up to 350,000 times. Here, growth is measured by the increase in cell size.
• The growth of a pollen tube is measured by its length, while the growth of a flat leaf (dorsiventral leaf) is measured by its surface area.

Phases of Growth

The period of growth can be divided into three distinct phases, which can be easily observed at the tip of a root:

1. Meristematic Phase:

   • This occurs at the very tips of roots and shoots.
   • It is a zone of constant cell division.
   • Cells in this phase are rich in protoplasm, have large nuclei, and thin, cellulosic primary walls.

2. Elongation Phase:

   • This region is located just behind the meristematic zone.
   • Cells here enlarge rapidly, vacuoles increase in size, and new cell wall material is deposited.
   • This phase is responsible for the increase in length of the plant organ.

3. Maturation Phase:

   • This zone is further away from the tip, behind the elongation phase.
   • Cells in this phase reach their final, maximum size.
   • They undergo structural modifications to their cell walls and protoplasm to become specialized tissues. Most of the different tissues you see in a plant are in this phase.

Growth Rates

The growth rate is the measure of increased growth per unit time. Plant growth can follow two main patterns: arithmetic and geometric.

Arithmetic Growth

In this type of growth, after a cell divides (mitosis), one daughter cell continues to divide while the other stops dividing and starts to differentiate and mature.

• A root elongating at a constant rate is a good example.
• When you plot the length of the organ against time, you get a straight, linear curve.
• The mathematical formula for arithmetic growth is: $L_t = L_0 + rt$ Where:
  • $L_t$ = length at time ‘t’
  • $L_0$ = length at time ‘zero’
  • $r$ = growth rate (elongation per unit time)

Geometrical Growth

In most biological systems, growth starts slow, then accelerates rapidly, and finally slows down as resources become limited.

• In this pattern, both daughter cells produced after mitosis retain the ability to divide.
• This results in a characteristic S-shaped or sigmoid growth curve.
• Lag Phase: Initial growth is slow.
• Log Phase (Exponential Phase): Growth is very rapid.
• Stationary Phase: Growth slows down and stops, often due to limited nutrient supply.
• This S-curve is typical for cells, tissues, and organs growing in a natural environment.
• The formula for exponential growth (the log phase) is: $W_1 = W_0 e^{rt}$ Where:
  • $W_1$ = final size (weight, height, number, etc.)
  • $W_0$ = initial size at the beginning
  • $r$ = relative growth rate
  • $t$ = time of growth
  • $e$ = base of natural logarithms

The term r is called the relative growth rate and is a measure of the plant's ability to produce new material, also known as the efficiency index.

Absolute and Relative Growth Rates

We can compare growth quantitatively in two ways:

• Absolute Growth Rate: The measurement of total growth per unit time.
• Relative Growth Rate: The growth of a system per unit time, expressed relative to its initial size.

• Their absolute growth rate is the same ($5 \text{ cm}^2$/week).
• However, the relative growth rate of Leaf A is much higher because the $5 \text{ cm}^2$ increase is a much larger proportion of its small initial size compared to the large initial size of Leaf B.

Conditions for Growth

For a plant to grow, several essential conditions must be met:

• Water: Needed for cell enlargement, maintaining turgidity (which helps in extension growth), and as a medium for enzymatic reactions.
• Oxygen: Required for respiration to release the metabolic energy needed for growth activities.
• Nutrients: Macro and micro elements are the building blocks for synthesizing protoplasm and also act as an energy source.
• Temperature: Every plant has an optimal temperature range for growth. Deviations can be harmful.
• Light and Gravity: These environmental signals influence certain stages and directions of growth.

Differentiation, Dedifferentiation and Redifferentiation

Cells produced by meristems go through a process of specialization to perform specific functions. This involves changes in their structure and function.

• Differentiation: This is the process where cells mature and become specialized. It involves major structural changes in both the cell walls and protoplasm.

  • Example: To form a tracheary element (part of xylem for water transport), a cell loses its protoplasm and develops a strong, lignified secondary wall to handle water transport under tension.

• Dedifferentiation: This is an interesting phenomenon where living, differentiated cells that have lost the ability to divide can regain this capacity under certain conditions.

  • Example: The formation of secondary meristems like the interfascicular cambium and cork cambium from fully differentiated parenchyma cells.

• Redifferentiation: This occurs when the cells produced by dedifferentiated tissues (like the cambium) divide and then once again lose the ability to divide, maturing to perform specific functions.

  • Example: The secondary xylem and phloem tissues formed by the cambium are products of redifferentiation.

Development

Development is a broad term that includes all the changes an organism goes through during its life cycle, from the germination of a seed to aging and death (senescence). It is the combined result of growth and differentiation.

Plasticity

Plants often form different kinds of structures in response to their environment or different phases of life. This ability to change is called plasticity.

A key example of plasticity is heterophylly, which means having different forms of leaves on the same plant.

• Developmental Heterophylly: The leaves of a juvenile plant are different in shape from those on the mature plant. Examples include cotton, coriander, and larkspur.
• Environmental Heterophylly: The shape of leaves depends on the environment. For example, in buttercup, leaves produced in the air have a different shape from those produced underwater.

Plant Growth Regulators

Plant growth and development are controlled by intrinsic chemical substances known as Plant Growth Regulators (PGRs) or phytohormones. These are small, simple molecules with diverse chemical compositions.

Characteristics

There are five major groups of PGRs:

1. Indole compounds: e.g., Indole-3-acetic acid (IAA)
2. Adenine derivatives: e.g., Kinetin
3. Derivatives of carotenoids: e.g., Abscisic acid (ABA)
4. Terpenes: e.g., Gibberellic acid ($GA_3$)
5. Gases: e.g., Ethylene ($C_2H_4$)

PGRs can be broadly divided into two functional groups:

1. Plant Growth Promoters: These are involved in growth-promoting activities like cell division, cell enlargement, flowering, and fruiting.
   • Examples: Auxins, Gibberellins, Cytokinins.
2. Plant Growth Inhibitors: These are involved in responses to stress and inhibiting activities like dormancy and abscission (shedding of leaves, flowers, or fruit).
   • Example: Abscisic acid (ABA).
   • Ethylene can be considered both a promoter and an inhibitor, but it is largely an inhibitor of growth activities.

The Discovery of Plant Growth Regulators

Interestingly, the discovery of each of the five major PGRs was accidental.

• Auxins: Discovered through the work of Charles Darwin and his son Francis, who observed that the coleoptiles (protective sheaths) of canary grass bent towards light (phototropism). They concluded that a transmittable influence from the tip caused the bending. F.W. Went later isolated this substance, naming it auxin.
• Gibberellins: Discovered from the "bakanae" (foolish seedling) disease in rice, where infected seedlings grew abnormally tall. E. Kurosawa found that a substance from the fungus Gibberella fujikuroi caused these symptoms. This substance was later identified as gibberellic acid.
• Cytokinins: F. Skoog and his colleagues found that callus (undifferentiated cells) from tobacco stems would only proliferate if the medium was supplemented with auxin plus extracts like coconut milk or DNA. Miller et al. later crystallized the active substance promoting cell division (cytokinesis) and named it kinetin.
• Abscisic Acid (ABA): In the mid-1960s, three different researchers independently isolated three growth inhibitors: inhibitor-B, abscission II, and dormin. They were all later found to be the same chemical, which was named abscisic acid.
• Ethylene: H.H. Cousins confirmed in 1910 that a volatile substance released from ripened oranges caused stored unripe bananas to ripen faster. This substance was later identified as ethylene.

Physiological Effects of Plant Growth Regulators

Auxins

• Natural Auxins: Indole-3-acetic acid (IAA), Indole butyric acid (IBA).
• Synthetic Auxins: Naphthalene acetic acid (NAA), 2, 4-D (2, 4-dichlorophenoxyacetic).
• Production Site: Growing tips of stems and roots.
• Functions and Applications:
  • Initiate rooting in stem cuttings, used for plant propagation.
  • Promote flowering (e.g., in pineapples).
  • Prevent early fruit and leaf drop, but promote the falling of older leaves and fruits.
  • Cause apical dominance, where the apical (main) bud inhibits the growth of lateral (axillary) buds. Removing the shoot tip (decapitation) allows lateral buds to grow, a practice used in tea plantations and for making hedges.
  • Induce parthenocarpy (fruit development without fertilization), e.g., in tomatoes.
  • Used as herbicides. 2, 4-D is widely used to kill broad-leaved (dicot) weeds without harming mature monocot plants like grasses, making it useful for creating weed-free lawns.
  • Control xylem differentiation and help in cell division.

Gibberellins

• Over 100 types exist ($GA_1, GA_2, GA_3$, etc.). Gibberellic acid ($GA_3$) is the most studied. All are acidic.
• Functions and Applications:
  • Increase the length of the main axis, used to increase the length of grape stalks.
  • Cause fruits like apples to elongate and improve their shape.
  • Delay senescence (aging), allowing fruits to be left on the tree longer to extend the market period.
  • $GA_3$ is used to speed up the malting process in the brewing industry.
  • Spraying sugarcane with gibberellins increases stem length, boosting the yield by up to 20 tonnes per acre.
  • Hasten maturity in juvenile conifers, leading to early seed production.
  • Promote bolting (internode elongation just before flowering) in plants with a rosette habit, like beets and cabbages.

Cytokinins

• Discovered as kinetin (a modified adenine), but kinetin does not occur naturally in plants. Zeatin is a natural cytokinin isolated from corn kernels and coconut milk.
• Production Site: Synthesized in regions of rapid cell division, such as root apices, developing shoot buds, and young fruits.
• Functions and Applications:
  • Have specific effects on cytokinesis (cell division).
  • Help produce new leaves and chloroplasts.
  • Promote lateral shoot growth and adventitious shoot formation.
  • Help overcome apical dominance (acting against auxins).
  • Promote nutrient mobilization, which helps delay leaf senescence.

Ethylene

• A simple gaseous PGR, synthesized in large amounts by aging tissues and ripening fruits.
• Functions and Applications:
  • Causes horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings.
  • Promotes senescence and abscission of leaves and flowers.
  • Highly effective in fruit ripening. It enhances the respiration rate during ripening, a phenomenon called the respiratory climactic.
  • Breaks seed and bud dormancy and initiates germination (e.g., in peanut seeds).
  • Promotes rapid internode/petiole elongation in deep-water rice plants, helping leaves stay above water.
  • Promotes root growth and root hair formation, increasing the absorption surface.
  • Used to initiate flowering and synchronize fruit-set in pineapples and mangoes.
  • Ethephon is a compound widely used in agriculture as a source of ethylene. It is absorbed by the plant and releases ethylene slowly. Ethephon hastens fruit ripening (tomatoes, apples) and promotes thinning of cotton, cherry, and walnut. It also promotes female flowers in cucumbers, increasing yield.

Abscisic Acid (ABA)

• Acts as a general plant growth inhibitor and an inhibitor of plant metabolism.
• Functions and Applications:
  • Regulates abscission and dormancy.
  • Inhibits seed germination.
  • Stimulates the closure of stomata during water stress, helping plants tolerate stressful conditions. For this reason, it is also called the stress hormone.
  • Plays a key role in seed development, maturation, and dormancy, helping seeds withstand desiccation.
  • In most situations, ABA acts as an antagonist to gibberellins (GAs).

• Synergistic: Working together to enhance an effect.
• Antagonistic: Having opposite effects (e.g., ABA and GAs in dormancy). These internal controls (PGRs and genes) work alongside external factors like light and temperature to regulate all stages of a plant's life.