Plant Growth and Development

Definition of Growth

Growth is defined as “an irreversible permanent increase in size of an organ or its part or even of an individual cell.”

In other words, Growth is the most fundamental and conspicuous characteristics of living beings and is accompanied by several metabolic processes that occurs at the expense of energy. These metabolic processes may be catabolic or anabolic. In case of plants, seed germinates, develops into seedling and later it takes the shape of an adult plant are different stages of growth. Plants displays indefinite growth.

On the other hand, animals show uniform and fixed growth.
 

Characteristics of Growth

• Plant Growth is generally Indeterminate – Plants possess the ability of growth throughout their life. This is due to the presence of meristems at certain locations in their body and these meristems have the ability to divide and self –perpetuate.

• Growth is Measurable – At cellular level, Growth is the consequence of increase in protoplasm and this increase is difficult to measure. Growth, in plants, is measured via different methods like increase in dry weight, volume, cell number, volume or increase in fresh weight.

The following diagram represents the location of root apical meristem, shoot apical meristem and vascular cambium. The arrows display the direction of growth of cells and organs. 

The Growth of Plants has three phases:

• Formative Phase – Cell division is the basic event in the growth of plant. All cells are the result of division of pre-existing cells. Mitosis is the type of cell division that happens during growth and includes both quantitative and qualitative division of cells. This division is carried out in two steps – Division of Nucleus, which is referred as Karyokinesis and division of cytoplasm referred as Cytokinesis. In case of higher plants, an increase of cells is carried out in meristematic region, whereby some daughter cells retain this meristematic activity while some enter in the next phase of growth, i.e. the phase of cell enlargement.

• Cell Enlargement and Cell Differentiation – At this stage, the size of tissues and organs is increased and this enlargement occurs by forming Protoplasm, Hydration (absorbing water), developing vacuoles and then adding new cell wall to make it permanent and thicker.

• Cell Maturation – At this stage, the enlarged cells acquire specific size and forms as per their location and role. Thus, several cells are differentiated from simple and complex tissues which perform different functions.

Experiment to Study Phases of Growth

In order to study the phases of Growth, Germinate few seeds of peas in moist saw dust. Select the couple of seedlings with 2 – 3 cm of length, wash them and blot the surface water. Then, mark the radicles from tip to base with 10 – 15 point at interval of 2 mm via water proof ink. After drying of ink, place those seedlings on moist blotting paper and allow them to grow for 1 – 2 days. Finally measure the intervals between the marks and we can clearly observe the different phases of growth.

Following diagram shows the phases of growth in root. A is the marked radicle of seedling at the beginning of experiment and B is the condition of seedling after 48 hours. We can clearly identify zone of cell formation, cell elongation, cell differentiation and zone of matured cells. 

“The increased growth per unit time is termed as Growth Rate. Thus, the rate of growth is expressed mathematically.” An organism can produce cells in several ways and display Geometric as well as Arithmetic Growth. Following diagram shows both types of growth in plants:

The following diagram displays the various stages of embryo development showing both Geometric and Arithmetic Phases. Here dark blue blocks represent the cells capable of division while light blue blocks represents the cells that have lost the capacity to divide:

Thus, in Arithmetic Growth, only one daughter cell continues to divide while other differentiates and matures. The following graph represents the length of an organ against time, whereby a linear curve is obtained. We can clearly observe the constant linear growth against time t.

In Mathematical Terms, Growth Rate is expressed as:

L_t = L_o + rt

Where, L_t = length at time “t”

L_o = length at time “zero”

r = growth rate or elongation per unit time.

Now focusing on Geometrical Growth, In majority of systems, Initial Growth is slow and is referred as lag phase. Then, it increases rapidly at an exponential rate referred as log phase or exponential phase. The growth of plant slows down in cases of limited nutrient supply and results in stationery phase. When we plot the growth against time, it results in S-Curve or Sigmoid Curve. Following graph represents an idealized sigmoid growth curve typical of cells in culture and many higher plants and plant organs.

The above sigmoid curve is the characteristic of living organism growing in natural environment and is typical for all cells, tissues and organs. The exponential growth in expressed as:

W₁ = W_oe^rt

Where,

W₁ = final size (weight, height, number etc.)

W_o = Initial size at the beginning of the period

r = relative growth rate and the measure of the ability of plant to produce new plant material

t = time of growth

e = base of natural logarithms
 

Types of Growth

There are five types of Growth:

• Primary and Secondary Growth: “The mitotic divisions of meristematic cells present at the root and shoot apex increases the length of the plant body. This is referred as Primary Growth and the Secondary meristem that results in an increase in diameter of the body of plant is called as Secondary Growth.”

• Unlimited Growth: This is the stage, when root and shoot of plant continuously grow from germination stage to death and throughout the entire lifespan.

• Limited Growth: This is the stage, when fruits, leaves and flowers stop growing after attaining certain size. It is also called determinate type of Growth.

• Vegetative Growth: The Growth of Plant before flowering in called Vegetative Growth. This Growth includes producing of stems, leaves and branches.

• Reproductive Growth: At this stage, plants start flowering, which is the reproductive part of the plant. 
   

Factors Affecting Plant Growth

• External Factors: The Growth of Plant primarily depends on habitat in which it is growing. Along with this, external factors also play an integral role in the growth of plants. It includes availability of Oxygen, Water and Nutrients followed by Temperature and Light.

  • Temperature plays important role in the growth of plants. The minimum, optimum and maximum temperature varies and from species to species. As the temperature increases above minimum, growth is accelerated until the optimum temperature is attained, when the growth gets slower and is completely retarded. Effect of duration for which a plant is exposed to certain temperature also varies amongst different species. For Example: The plant shows good growth when it is exposed to 86°F for a short duration and the same temperature has negative impact if maintained for longer duration.

  • Light also affect the growth and development of plant. Several factors of light like light intensity, duration of light and quality of light influences several physiological processes like movement of stomata, chlorophyll synthesis, temperature of aerial organs, formation of anthocyanin, absorption of minerals streaming of protoplasm and rate of transpiration. Intensity of light also influences plant growth and the variation in intensity has significant impact on growth pattern. Most ornamental plants and crops, such as Peas, Corn, Tobacco and Peas makes stocky and vigorous growth will full sun and thus, is also called “Sun Plant.”

  • Difference in wave length of light also effects the growth of plant. Several experiments have proved that plants that has full spectrum of visible light shows proper development and increase in dry weight. Plants grown in violet and blue light tend to dwarf, while plants in red light are taller and spindly.

  • Duration of light also affects the plant growth as it affects the rate of photosynthesis. For instance, during winters when days are short, the growth is very slow, while, it increases during summers when the days are longer.

  • The plants with lesser availability of oxygen show retarded growth while it is vice versa in the presence of ample of oxygen. It is important to note that plants in flooded areas, results in deficiency of soil aeration which on the other hand, results in poor plant growth.

  • Water is very important for plants and inadequate water results in poor growth. Plants grow well only in the presence of optimum water. Plants respond to deficiency of moisture as well. For instance, peppers, spinach and radishes wilt and cease to grow when the percentage of water in soil is lower.

  • Soil nutrients, their quantity and nature also affect the growth of plant. For Luxuriant Growth, it is important to have adequate amount of nutrients.

• External Factors: The Growth of Plant primarily depends on habitat in which it is growing. Along with this, external factors also play an integral role in the growth of plants. It includes availability of Oxygen, Water and Nutrients followed by temperature and light.These factors include growth regulators, C/N ratio and genotype and genetic factor.

  • There are several classes of growth regulators. Some promote the growth like Auxins, Florigen, Cytokinins, Gibberellins, etc., while some are growth inhibitors like ethylene, abscisic and chlorocholine chloride.

  • The ratio of carbohydrates and nitrogen also govern the growth of plants. Presence of more carbohydrates as compared to nitrogen facilitates vegetative growth, fruiting and flowering while presence of more nitrogenous compounds results in poor vegetative growth.

  • Following diagram shows the percentage contribution of various factors in the growth of plants. According to it, the percentage of mineral particles is 45% and air & water is 25%.

  • Genotypes are responsible for controlling all the metabolic activities, growth and development of plant. Expression of genes in the correct sequence is controlled by two things, i.e. environment and genes. These genes are located in chromosomes and transcribe information to m-RNA that translates in enzyme and structural protein.

Differentiation, Dedifferentiation and Redifferentiation

“The cells derived from root apical and shoot – apical meristems and cambium differentiate and mature to perform specific function and this act leading to maturation is termed as differentiation.” During this process, several structural changes are carried out in cells and protoplasm. For instance, In order to form a tracheary element, cells would lose protoplasm and develop elastic, strong and lignocellulosic secondary cell walls in order to transport water even in extreme tensions.

In plants, we can study another interesting phenomenon, i.e. dedifferentiation. In this Phenomenon, the living differentiated cells that have lost the capacity to divide regain it under certain conditions. For Example: Formation of Meristems – cork cambium and interfascicular cambium form fully differentiated parenchyma cells and in such condition, tissues and meristems are able to divide and produce cells even after losing the capacity to divide. 

“While the product of dedifferentiated cells or tissue which lost the ability to divide are called redifferentiated cells/ tissue and this event is referred as redifferentiation.” 
 

Development

Development is the term that includes all the changes an organism goes throughout its life cycle right from germination of seeds to attaining senescence.

Following diagram shows the sequence of the development process in a plant cell. This process is also applied to Tissues or Organs. 

Plants follow several pathways or phases of life in response to environment in order to attain different kind of structures and this ability is referred as plasticity, such as heterophylly in coriander, cotton and larkspur. In these plants, leaves of juvenile plant are entirely different in shape as compared to the matured plant. Added to this, there is a vast difference in shape of leaves of plant produced in air and water.

Following diagram shows heterophylly development because of environment, in which

(a) Represents larkspur and  (b) is buttercup.

Thus, it can be said that growth, development and differentiation are three concepts which are closely related with the events of life. To summarize, development is the sum total of growth and differentiation and is under the control of several extrinsic and intrinsic factors.
 

Plant Growth Regulators

“Plant growth regulators function as chemical messengers for intercellular communication.” These regulators work in coordination with each other to enforce growth and development of cells. The discovery of PGR is entirely accidental. All this started with the observation of Charles Darwin and Francis Darwin (son). They observed the process of photoperiodism in the tip of coleoptile, whereby the canary grass responded to unilateral illumination by growing towards source of light.

Following diagram shows the experiment to demonstrate the tip of coleoptile is the source of auxin and arrows indicate the direction of light. a, b, c, d are the different stages of grass:

Example of Plant Growth Regulators

Auxins, Cytokinins, Gibberellins, Ethylene and Abscisic Acid (ABA)
 

Characteristics of Plant Growth Regulators

• These are small and simple molecules with diverse chemical composition. These are described as Plant Growth Substances, Phytohormones or Plant Hormones.

• These could be adenine derivatives; indole compounds derivatives of carotenoids or terpenes.

• PGR are divided in two groups on the basis of functions in a Living Plant Body:

  • First Group is involved in growth promoting activities like cell division, Enlargement, Tropic Growth, Fruiting, Pattern Formation, Flowering and Formation of seed. These are also referred as plant growth promoters such as gibberellins, auxins and cytokinins.

  • Second Group responses to wounds and stresses of abiotic and biotic origin. These are involved in growth inhibiting activities such as abscission and dormancy.
     

Physiological Effect of Plant Growth Regulators

• Auxins: This was first isolated from the urine of human beings and is applied to indole – 3 – acetic acid (IAA) and several other synthetic and natural compounds possessing growth regulating properties. Auxins are produced by growing apices of roots and stems and are used extensively in horticultural and agricultural practices. Auxins initiate rooting in stem cutting and promote flowering. Auxins are used in inducing parthenocarpy and are widely used as herbicide. It also used to prepare weed free lawns and control the differentiation of xylem.

• Gibberellins: These are another kind of PGR and more than 100 gibberellins are widely reported in different organisms. These are denoted as GA₁, GA₂, GA₃ and so on. Amongst these, GA₃ was the first gibberellins to be discovered. These are acidic in nature and possess ability to cause an increase in length of axis. It causes fruits to elongate and improve shape and also delay senescence. Gibberellins results in an increase in the length of stem, promotes bolting and fastens the maturity period.  

• Cytokinins: These were discovered as kinetin and it does not occur in plants naturally. Natural cytokinins are formed in those regions where cell division occurs rapidly. These help in overcoming apical dominance and promote nutrient mobilization.

• Ethylene: It is PGR in gaseous form which is synthesized in large amount by tissues undergoing senescence and ripening of fruits. It is highly effective in ripening of fruits and improves rate of respiration referred as respiratory climactic. Ethylene breaks bud and seed dormancy, sprouting of potato tubers and germination of peanut.

• Abscisic Acid (ABA): It acts as general growth inhibitor as it inhibits seed germination. It stimulates closure of stomata and increase tolerance of plants in response to various kinds of stresses, therefore also referred as stress hormone.
   

Photoperiodism
 

Definition of Photoperiodism

“Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in both animals and plants it can also be defined as the developmental responses of plants to the relative lengths of light and dark periods.”

Thus, the term photoperiodism is coined to explain the ability of plants to flower in response to changes in the relative length of day and night. It is observed that there are several plants which require periodic exposure to light in order to induce flowering.

Plants are grouped as per their response to the length of day, in the following manner:

• Long Day Plants: These plants begin flowering when they are exposed to longer days. Below the critical photoperiod, these plants show only vegetative growth. The critical photoperiod varies from species to species and plants to plants. Some of the common examples of long day plants are Radish, Barley, Spinach, Onion, Carrot and Henbane.

• Short Day Plants: These plants flower when the length of day is shorter than their critical photoperiod. When these plants are exposed to more than the critical period, it shows vegetative growth. Some of the common examples of short day plants are Tobacco, Soybean, Sugarcane and Cock – Lebur.

• Day Neutral Plants: These plants flower only after completely the period of vegetative growth irrespective of the duration of day and night. Some of the common examples include Tomato, Maze, Cucumber, Cotton, some varieties of Pea, etc.

Following diagram shows short day and long day plants. Here short day plants flower when the length of day is shorter and on the contrary, long day plants flower when the length of day is longer. 

Vernalization

There are several plants in which flowering are qualitatively or quantitatively dependent on the exposure to low temperature. This phenomenon is referred as vernalization.

In other words, temperature plays an integral role in metabolic activities of plants, germination of seeds and their flowering. Plants of temperate zone, germinate at relatively low temperature while plants in tropical areas germinate in higher temperature. There are several plants that do not flower before they experience low temperature. This dependency on the temperature to flower is referred as vernalization.
 

Example of Vernalization

Food plants like barley, wheat rye are of two varieties, i.e. winter and spring varieties. In this, spring variety is planted in spring and winter variety in winters. These plants grow in their respective seasons, flower and produce grains before the end of the growing season.

Another example can be observed in biennial plants. These are monocarpic plants that normally flower and die in second season. Some of the common biennials are Carrot, Cabbages and sugerbeet.
 

Frequently Asked Questions(FAQs)
 

Q1. Differentiate between Photoperiodism and Vernalization

Sol.

Q2. Comment, “Both growth and differentiation in higher plants are open.”

Sol. The higher plants possess the ability for unlimited growth and this ability is due to the presence of meristems at several locations of their body. These meristems have the ability to self – perpetuate and divide and therefore, growth in higher plants is open. Added to this, some of the cells in such plants always undergo differentiation after some rounds of cell division and therefore, differentiation is also open.
 

Q3. Would a defoliated plant respond to photoperiodic cycle? Why?

Sol. No, the defoliated plants do not respond to photoperiodic cycle because such plants lack the site of perception of light and dark duration. These sites are leaves and therefore, they do not respond to light. 
 

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