Plant Growth

Plant Growth

  • Growth is a fundamental characteristic of living organisms, referring to a permanent increase in size. 
  • It can occur at the level of individual cells, organs, or the entire organism. 
  • Growth is irreversible, meaning it leads to a permanent change in size. 
  • It involves both anabolic (building up) and catabolic (breaking down) metabolic processes. 
  • Energy is required for growth, obtained through metabolic activities. 
  • Expansion of a leaf, resulting in an increase in its size, is an example of growth. 
  • This growth is driven by cellular processes such as cell division and enlargement. 
  • When a piece of wood is placed in water, it absorbs water through its cell walls. 
  • This absorption causes the wood cells to swell, leading to an increase in the overall size of the wood. 
  • However, unlike living organisms, this process is not considered growth in the biological sense since wood lacks metabolic activities associated with living cells.

 

Indeterminate Growth in Plants:

 

  • Plant growth is unique because it's generally indeterminate, meaning plants can grow continuously throughout their life. 
  • This ability is maintained by meristems, regions of actively dividing cells located at certain points in the plant body. 
  • Meristematic cells have the ability to divide and produce new cells, perpetuating growth. 
  • As new cells are added, older cells lose their ability to divide and contribute to the plant body's structure. 
  • In open growth, new cells are continually added to the plant body by meristematic activity. 
  • This ensures that plants can grow in size and complexity throughout their lifespan. 
  • If meristems cease to divide, the plant would stop growing in length or girth, depending on the type of meristem affected. 
  • This situation is rare because the plant's survival depends on continuous growth for adaptation and reproduction. 
  • Root apical meristem and shoot apical meristem are responsible for primary growth, elongating the plant along its axis. 
  • In dicotyledonous plants and gymnosperms, lateral meristems like vascular cambium and cork cambium appear later and contribute to secondary growth. 
  • Secondary growth increases the girth of organs by adding layers of new cells, allowing plants to become thicker and more structurally complex over time. 

 

Measuring Growth:

 

  • Growth at a cellular level primarily results from an increase in the amount of protoplasm, the living substance within cells. 
  • Directly measuring protoplasm increase is challenging, so growth is often assessed through various parameters that are related to it. 
  • Growth can be measured using parameters like increase in fresh weight, dry weight, length, area, volume, and cell number. 
  • Each parameter provides insight into different aspects of growth, such as overall size, cell proliferation, or cellular expansion. 
  • Examples of Growth Measurement:

- In maize, the root apical meristem can generate over 17,500 new cells per hour, demonstrating growth in terms of cell number. 

- Conversely, watermelon cells can increase in size up to 350,000 times, illustrating growth through cellular enlargement. 

  • Growth in structures like pollen tubes is measured by length, indicating elongation. 
  • In dorsiventral leaves, growth is expressed as an increase in surface area, reflecting expansion in two dimensions. 

 

Necessary Conditions for Growth:

 

  • Water, oxygen, and nutrients are essential elements for plant growth. 
  • Water is crucial for cell enlargement and turgidity, which aid in extension growth. 
  • It also serves as a medium for enzymatic activities required for growth. 
  • Oxygen is necessary for releasing metabolic energy essential for growth processes. 
  • Nutrients, both macro and micro essential elements, are required for synthesizing protoplasm and providing energy.

 

 

Optimum Temperature:

 

  • Each plant organism has an optimum temperature range best suited for its growth. 
  • Deviations from this range can be detrimental to the plant's survival. 

 

Environmental Signals:

 

  • Environmental signals such as light and gravity also influence certain phases or stages of growth. 
  • Light is crucial for photosynthesis and can affect the timing of flowering and other developmental processes. 
  • Gravity influences root orientation and growth direction, aiding in proper anchorage and nutrient uptake.

 

Phases of Growth

  • Growth in plants is typically divided into three phases: meristematic, elongation, and maturation. 
  • These phases are observed in structures like root tips, where different regions exhibit distinct characteristics.

 

Meristematic Phase:

 

  • The meristematic phase is characterized by actively dividing cells found at both root and shoot apices. 
  • Cells in this phase are rich in protoplasm, have large nuclei, and possess thin primary cell walls with abundant plasmodesmata connections.

 

Elongation Phase:

  • Cells proximal to the meristematic zone enter the elongation phase. 
  • During this phase, cells undergo vacuolation, enlarge in size, and deposit new cell walls, leading to elongation of the structure.

 

Maturation Phase:

  • Further away from the apex, cells enter the maturation phase. 
  • Cells in this phase reach their maximal size with thickened cell walls and protoplasmic modifications. 
  • Many tissues and cell types studied in earlier classes represent this phase, as they have completed growth and specialized for specific functions.

 

 

 

 

Growth Rate

  • Growth rate refers to the increased growth per unit time, expressed mathematically. 
  • Growth rates can vary between arithmetic and geometrical patterns.

 

Arithmetic Growth:

  • In arithmetic growth, one daughter cell continues to divide while the other differentiates and matures. 
  • It's exemplified by a root elongating at a constant rate, resulting in a linear curve when plotting length against time. 
  • Mathematically, it is expressed as

Lt = L0 + rt

Where Lt = length at time ‘t’,

L0 = length at time ‘zero’

r = growth rate / elongation per unit time.

 

 

 

Geometrical Growth:

  • Geometrical growth starts slowly (lag phase), then accelerates rapidly (log or exponential phase), and finally slows down in the stationary phase due to limited nutrient supply. 
  • When plotting growth against time, a sigmoid or S-curve is obtained, typical for living organisms growing in a natural environment.

 

Exponential Growth Formula:

  • Exponential growth can be expressed as

W1 = W0 ert

 

  • Where W1 is the final size, W0 is the initial size, r is the growth rate, t is the time, and e is the base of natural logarithms. 

 

 

 

Relative Growth Rate (RGR):

  • RGR measures the ability of the plant to produce new plant material and is also referred to as the efficiency index. 
  • The final size depends on the initial size, with higher RGR indicating faster growth.

 

Absolute vs. Relative Growth Rate:

  • Absolute growth rate measures the total growth per unit time, while relative growth rate measures growth per unit time expressed relative to an initial parameter.

 

 

 

  • In the comparison between two leaves, A and B, both show absolute increase in area over time, but one may have a higher relative growth rate due to starting from a smaller initial size, thus achieving relatively more growth. 

 

 

Differentiation

  • Differentiation is the process by which unspecialized cells transform into specialized cells with specific structures and functions. 
  • It is a fundamental process in the development and growth of multicellular organisms, including plants. 
  • During differentiation, cells undergo structural and functional changes to fulfill specific roles within the organism.

 

Cell Specialization:

  • Unsuitable cells undergo modifications in their structure, such as changes in cell wall composition and protoplasm content. 
  • These modifications enable cells to perform specialized functions necessary for the organism's survival and growth.

 

Examples of Differentiation:

  • In plants, differentiation leads to the formation of various cell types, such as xylem and phloem cells for water and nutrient transport, respectively. 
  • Tracheary elements develop strong lignocellulosic secondary cell walls to provide structural support and conduct water efficiently.

 

Importance of Differentiation:

  • Differentiation ensures that different parts of the organism have the necessary cells and tissues to perform their specific functions. 
  • It contributes to the overall complexity and organization of the organism, allowing it to adapt to diverse environmental conditions.

 

Regulation of Differentiation:

  • Differentiation is regulated by genetic factors and environmental cues. 
  • Signals from neighboring cells and external stimuli influence the fate of undifferentiated cells, guiding them towards specific developmental pathways.

 

Consequences of Differentiation:

  • Once cells become specialized, they typically lose the ability to divide further. 
  • However, some tissues retain the potential for dedifferentiation and redifferentiation under certain conditions, allowing for regeneration and repair in plants.

 

Dedifferentiation

  • Dedifferentiation is the process by which specialized cells revert to a less specialized or undifferentiated state. 
  • It involves the loss of specialized features and the reactivation of genes associated with cell division and growth.

 

Reversion to Undifferentiated State:

  • Dedifferentiated cells return to a state similar to that of stem cells or meristematic cells, which have the ability to divide and differentiate into various cell types.

 

Occurrence in Plants:

  • Dedifferentiation commonly occurs in plants, especially in tissues with meristematic potential, such as cambium and callus tissues. 
  • Under certain conditions, fully differentiated cells can dedifferentiate and regain the capacity for cell division.

 

Role in Regeneration:

  • Dedifferentiation plays a crucial role in regeneration and repair processes in plants. 
  • It allows damaged or wounded tissues to regenerate new cells and tissues, promoting healing and recovery.

 

Examples of Dedifferentiation:

  • In plant tissue culture, dedifferentiated cells form callus tissues, which can then be induced to differentiate into roots, shoots, or other structures. 
  • Dedifferentiated cells can also give rise to meristematic tissues, such as cambium, which contribute to secondary growth in woody plants.

 

Regulation of Dedifferentiation:

 

  • Dedifferentiation is regulated by various factors, including hormonal signals, environmental conditions, and genetic factors. 
  • Signals from neighboring cells or injury sites can trigger dedifferentiation in nearby cells, initiating regeneration processes.

 

Potential Applications:

  • Understanding dedifferentiation mechanisms is important for applications in tissue engineering, regeneration, and plant biotechnology. 
  • Dedifferentiated cells can be manipulated and induced to differentiate into specific cell types, offering potential for crop improvement and medical research.

 

Redifferentiation

  • Redifferentiation is the process by which dedifferentiated cells regain their specialized structures and functions. 
  • It involves the re-establishment of cellular characteristics associated with a specific cell type. 

 

Return to Specialized State:

  • After dedifferentiation, cells undergo redifferentiation to adopt a specialized fate once again. 
  • This process involves the reactivation of genes and cellular pathways associated with the desired cell type.

 

Role in Regeneration:

  • Redifferentiation is essential for tissue regeneration and repair in plants. 
  • Dedifferentiated cells, such as those found in callus tissues, can redifferentiate into various cell types to replace damaged or lost tissues.

 

Examples of Redifferentiation:

  • In plant tissue culture, dedifferentiated cells within callus tissues can redifferentiate into roots, shoots, or other structures under specific culture conditions. 
  • Redifferentiation also occurs in meristematic tissues, such as cambium, where dedifferentiated cells regain their ability to differentiate into xylem, phloem, or other vascular tissues during secondary growth.

 

Regulation of Redifferentiation:

  • Redifferentiation is regulated by complex signaling networks involving hormones, environmental cues, and genetic factors. 
  • Signals from neighboring cells or developmental cues trigger redifferentiation in dedifferentiated cells, guiding them towards specific developmental pathways.

 

Importance in Plant Development:

  • Redifferentiation contributes to the maintenance of tissue homeostasis and the formation of specialized structures during plant growth and development. 
  • It ensures that plants can regenerate and repair damaged tissues, allowing them to adapt to changing environmental conditions and stressors.

 

Applications in Biotechnology:

  • Understanding redifferentiation mechanisms has practical applications in plant biotechnology and tissue engineering. 
  • Researchers can manipulate redifferentiated cells to produce desired traits or structures in plants, offering potential for crop improvement and regeneration techniques.

 

Development in plants

  • Development encompasses all the changes that occur in an organism throughout its life cycle, from seed germination to aging and senescence. 
  • It includes the processes of growth, differentiation, and the formation of various structures and organs.

 

Plasticity in Plants:

  • Plants exhibit plasticity, the ability to follow different pathways in response to environmental cues or different phases of life, leading to the formation of diverse structures. 
  • Examples include heterophylly, where leaves of the same plant may have different shapes depending on their stage of development or environmental conditions.

 

Heterophylly:

  • Heterophylly is a phenomenon where leaves of the same plant exhibit different shapes or forms at different stages of development or under different environmental conditions. 
  • For instance, in some plants like cotton and coriander, juvenile leaves may differ in shape from mature leaves, showcasing developmental plasticity.

 

Relationship Between Growth, Differentiation, and Development:

  • Growth, differentiation, and development are closely related events in the life of a plant. 
  • Growth refers to the increase in size or mass of an organism, while differentiation involves the specialization of cells into specific types. 
  • Development is considered the sum of growth and differentiation, encompassing the overall changes that occur in the plant's structure and function over time.

 

 

 

Control of Development:

  • Development in plants is regulated by both intrinsic and extrinsic factors. 
  • Intrinsic factors include genetic mechanisms within the plant cells, while extrinsic factors encompass environmental cues such as light, temperature, water, nutrients, and chemicals like plant growth regulators.