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Plant Growth and Development shows the different changes that plants go through from seed to fully grown plants.
- Growth can be defined as an irreversible permanent increase in the size of an organ or its parts or even that of an individual cell.
- For growth in plants, the meristem cells which are responsible for growth, divide and self-perpetuate.
- The meristem cells are located in different parts of the plant.
- There are stages in the growth of a plant right from the zygote stage to a fully grown plant due to an orderly succession of development.
- There is differentiation in each stage of growth thus development is the sum of growth and differentiation.
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Key Terms: Meristem, protoplasm, dicotyledons, gymnosperms, mitotic division, cell wall, nuclei, daughter cells, photoperiodism, vernalisation, mitotic division, plant growth hormones
Growth in Plants
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Plants have the unique ability to grow throughout their life.
- The meristem cells present in the roots and shoot apical lead to the primary growth of the plant.
- The primary growth which occurs at the tips of the stem and roots of the plant contributes to the elongation of the plant along its axis.
- In the later stages of growth in plants, the growth in dicotyledonous, gymnosperms, vascular cambium, and cork-cambium are also caused by the meristems present in them.
- These intercalary meristems contribute to increasing the girth of the organs of the plant in which they are active.
- Such a type of growth is known as secondary growth.
Primary Growth
Many parameters such as volume, fresh weight, area, cell number, dry weight, length, etc are used to measure the growth in plants as the increase in the amount of protoplasm is responsible for the growth in plants.
Secondary Growth
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Phases of Growth
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The three phases of growth are as follows.
Meristematic (formative phase)
In this phase, the cells at the root and stem apex are rich in protoplasm.
- They are constantly dividing and multiplying.
- They have large conspicuous nuclei.
- The cells in that region have abundant plasmodesmatal connections along with thin and cellulosic cell walls, primarily in nature.
Elongation
The cells just beside the apex, the meristematic zone are in the elongation phase.
- They undergo cell enlargement, increased vacuolation, and new cell wall deposition.
- As the name of the phase suggests, this part undergoes elongation away from the apex.
Phases of Growth
Maturation
This phase follows the elongation phase.
- The part undergoing maturation is in the proximity of the elongated part.
- The cells in this part have gained their maximum size.
- They have thicker walls and protoplasmic modification.
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Rate of Growth
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Growth rate is defined as increased growth per unit time.
- This makes it easier to understand mathematically in two ways, arithmetic and geometric.
- Arithmetic growth is when cell differentiation follows mitotic division.
- Here only the daughter cell continues to divide while the other differentiates and matures.
- In geometric growth, both the progeny cells follow mitotic division.
- All the cells thus continue to divide.
- At a point when the nutrition starts to lack, the growth starts slowing down.
| Type of Growth | Mathematical Representation |
|---|---|
| Arithmetic Growth
| Lt = L0 + rt Where, Lt = length at time ‘t’ L0 = length at time ‘zero’ r = growth rate/elongation per unit time. |
| Geometric Growth
| W1 = W0 ert Where, W1 = final size (weight, height, number etc.) W0 = initial size at the beginning of the period r = growth rate t = time of growth e = base of natural logarithms (progeny cells = daughter cells) |
Factors Affecting Plant Growth
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The various factor that affects the growth of plants are
- Water: useful for turgidity of cells, medium for enzyme activities
- Oxygen: useful in releasing metabolic energy
- Nutrients: used for the synthesis of protoplasm and source of energy
- Sunlight: useful in photosynthesis and in other stages of plant life
- Temperature: different plants have a different range of temperatures suitable for their growth
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Differentiation, Dedifferentiation, and Redifferentiation
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Differentiation, Dedifferentiation, and Redifferentiation are discussed below.
Differentiation
The act of cells from root apical and shoot-apical meristems and cambium differentiate and mature to perform specific functions is termed differentiation.
- The cell’s protoplasm and cell walls undergo changes.
- As they mature, the cells develop strong, lignocellulosic secondary cell walls in order to survive extreme tension while transporting water to long distances.
Trichome means small outgrowth from the epidermis of the plants
Dedifferentiation
The phenomenon where the living differentiated cells, which by now have lost the capacity to divide can regain the capacity of division under certain conditions is known as dedifferentiation.
Differentiation
The above diagram shows both the phases after differentiation.
- Dedifferentiation is the intermediate phase.
- The diagram marks it as a transient phase.
- The term chromatic decondensation is used as this interphase leading to mitosis causes further condensation and individualization of chromosomes discreetly.
- These chromosomes will now be carried forward to other daughter cells if mitosis is continuous.
- Cell death happens if hormones are not provided to the cell.
Redifferentiation
The dedifferentiated cells that perform this phenomenon divide and produce new cells, but they lose the ability to divide further instead they mature to perform certain functions.
Plant Growth Regulators
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Similar to human hormones, plants have simple chemicals naturally released that regulate their growth and development.
- They are also known as plant growth hormones, plant hormones, or phytohormones.
- Based on the changes that these hormones bring about in a plant’s body they are divided into two groups such as growth promoters and growth inhibitors.
Plant Hormones Types
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Growth Promoters
These hormones promote cell division, flowering, cell enlargement, fruiting, and seed formation.
Auxins
Auxins promote flowering and initiate rooting in stem cut.
- They prevent the early fall of leaves & fruits and promote the abscission of old fruits & leaves.
- Auxins also helps in cell division & control differentiation of the xylem.
Some of the types of auxins are:
- Indole-3-acetic acid (IAA)
- Indole butyric acid (IBA)
- Naphthalene acetic acid (NAA)
- 2,4-dichlorophenoxyacetic (2, 4-D) IAA & IBA are natural auxins & other two are synthetic auxins.
Auxins
Gibberellins
Gibberellins increase axis length and delay senescence in fruits. It also helps fruits (e.g. apples) elongate & improve their shape. There are over 100 varieties, all are acidic & denoted as GA1, GA2, etc. Some of the applications of Gibberellins are as follows.
- The brewing industry uses GA3 to speed up the malting process.
- Increase sugarcane yield by lengthening the stem.
- Hasten maturity period in young conifers & promote early seed production.
- Promote bolting in beet & cabbage.
Cytokinins
Cytokinins aid in the formation of new leaves & chloroplast.
- They promote lateral shoot growth & adventitious shoot formation.
- Cytokinins help overcome apical dominance and delay leaf senescence by promoting the mobilization of nutrients.
Some of the types of Cytokinins are as follows.
- Natural – Zeatin
- Synthetic cytokinin
Cytokinins
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Growth Inhibitors
Growth Inhibitor hormones prevent, inhibit growth, and promote dormancy and abscission in plants. Some of the growth inhibitors are discussed below.
Abscisic Acid
Abscisic acid regulates abscission and dormancy and inhibits plant growth, metabolism, and seed germination. It increases stress tolerance and helps in seed development & maturation.
Abscisic acid induces seed dormancy & helps withstand critical growth factors.
Functions of Abscisic Acid
Ethylene
Ethylene hastens fruit ripening and affects horizontal growth of seedlings & swelling of the axis in dicot seedlings.
- It promotes abscission and senescence and enhances respiration rate during the ripening of fruits (respiratory climactic).
- Ethylene also increases root growth & root hair formation.
Some of the applications of Ethylene are as follows.
- Breaks dormancy of seed & bud.
- Promotes sprouting.
- Boosts rapid petiole elongation.
- Initiates flowering and synchronizes fruit set.
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Photoperiodism
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The time duration to which a plant is exposed to sunlight also makes a difference in inducing flowering in different plants.
- Some plants require comparatively long hours of sunlight while others require less period of time to flower; and take less time than the former group.
- Thus the plant group that takes a longer duration is known as long-day plants while the other group is known as short-day plants.
- Some plants do not have any particular correlation between the duration of exposure to light and the resultant flowering
- Such plants are known as day-neutral plants.
Photoperiodism
It can be said that flowering in certain plants depends not only on a combination of light and dark exposures but also their relative durations. This response of plants to periods of day/night is termed photoperiodism.
Vernalisation
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The phenomenon where plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature is termed vernalisation. It is particularly for stimulation in flowering in cold temperatures.
Vernalisation
Things to Remember
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- Plant growth is dependent on many factors such as plant hormones, photoperiodism, vernalisation, conditions for growth, the presence of meristems, etc.
- Its development and growth are equally dependent on extrinsic and intrinsic factors.
- Plants show plasticity in development.
- The meristems in the plant body keep on adding new cells thus promoting an open form of growth.
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Sample Questions
Ques. What are the extrinsic conditions required for plant growth? (2 marks)
Ans. The extrinsic conditions required for plant growth are –
- Water
- Oxygen
- Nutrients
- Light
- Temperature
Ques. Write the structural features of
a) Meristematic cells near the root tip
b) The cells present in the root’s elongation zone (3 marks)
Ans.
a) Meristematic cells near the root tip are characterized by the following:
- Big conspicuous nucleus
- Rich protoplasm
- The cell wall is cellulosic and thin – basic in nature
- Less number of vacuoles
- Large numbers of mitochondria
- Plentiful plasmodesmata
b) The cells present in the root’s elongation zone are characterised by:
- Extended dimension
- Increased vacuolation
- New cellulosic cell walls are collected
Ques. Compare Auxin and Gibberellin growth promoter hormones (3 marks)
Ans.
| Auxin | Gibberellin |
| It is a plant hormone which is responsible for elongation of cells in shoots, regulating the plant growth | It is a plant hormone that stimulates stem elongation, germination, and flowering |
| It is found in higher plants | It is found in some higher plants and fungi |
| Can promote the growth of shoot segments | Only promotes the growth of the intact shoot |
| Has little effect on leaf growth | Enhances leaf growth |
| Leads to apical dominance | Does not cause apical dominance |
Ques. What are the functions of Auxins in plant growth? (2 marks)
Ans. Auxins are one of the most important plant hormones. Being growth promoting hormones, auxins are found where growth occurs distinctly, in the root and stem tips. In all plants where tissues distribute resources throughout the plant, auxins play a vital role in:
- Cell division.
- Plant propagation.
- Promoting flowering in plants.
- Initiation of roots and stem cuttings.
- Prevent premature dropping of fruits and leaves.
Ques. What is Abscisic Acid? Explain in brief. (2 marks)
Ans. Abscisic Acid is a growth inhibitor. It promotes dormancy and abscission in plants. It has the following effects on plants –
- Regulates abscission and dormancy.
- Inhibits plant growth, metabolism, and seed germination.
- Increases stress tolerance.
- Helps in seed development & maturation.
- Induces seed dormancy & helps withstand critical growth factors.
Ques. What are the differences between primary and secondary growth in plants? (3 marks)
Ans.
| Dedifferentiation | Redifferentiation |
|---|---|
| The phenomenon where the living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain conditions is known as dedifferentiation. | The dedifferentiated cells which perform this phenomenon divide and produce new cells, but they lose the ability to divide further instead they mature to perform certain functions. |
| It is the reversal of differentiation | It is the reversal of dedifferentiation |
| Acts as meristematic tissue including interfascicular vascular cambium, cork cambium, and wound meristem | Serves as the functionally – specialised tissue |
| Allows the plant body to produce new cells at a particular location | Important to perform a function specific to a particular part of a plant. |
| Eg. The formation of the interfascicular cambium and cork cambium from fully differentiated parenchyma cells | Eg. Secondary xylem and secondary phloem |
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