Saturday, 14 September 2019

Plant Growth Stages

Plant Growth Stages

Plant Growth Stages


An organism starts its life as a single cell. Unicellular organisms grow and reproduce without increasing number of body cells but multicellular organisms undergo a far more complex process of growth and development.

GROWTH

•In lower plants, growth is diffused, i.e., every cell can divide and enlarge but in higher plants, special body regions called meristems cause body growth. contributed by root and shoot apical meristem and intercalary meristem.
• Growth is a quantitative or measurable phenomenon. 

Phases of Growth
• Plant growth takes place in three phases :
(i) Formative phase: It is the phase of cell division. As the formation of new cells require biosynthetic activity, the respiration rate of cells in this phase is high.
(ii) The phase of enlargement: In this phase, the newly formed cells, produced informative phase undergo enlargement. Cell enlargement may occur in all directions, e.g., isodiametric parenchymatous cells.
(iii) Phase of differentiation or maturation: In this phase, the enlarged cells develop into special or particular type of cells by undergoing structural and physiological differentiation.

Growth Curve
• Growth curve refers to the graphic representation of total growth against time.
• When total growth is plotted against time, an S-shaped or sigmoid curve is obtained.

Growth Curve

Growth Rate
• The growth rate is defined as increase in growth per unit time and can be expressed mathematically.
• The rate of growth can be measured by estimating the increase in size or area of an organ of the plant-like leaf, flower, fruit, etc., in a unit time. The rate of growth is also called as efficiency index.
• For measurement and comparison between growth of various systems, absolute and relative growth rates are considered. The total growth per unit time is called the absolute growth rate.
• The growth of a given system per unit time expressed per unit initial parameter is called relative growth rate.

Relative growth rate =   Growth in a given time period
Measurement at the start of time period
• Suppose, two leaves have grown by 5 cm 2 in one day. Initial size of leaf A was 5 cm 2 while that of leaf B was 50 cm 2 .

Types of Growth
(Based upon the growth rate)

Types of Growth (Based upon the growth rate)

• Practically, in living organisms, the geometric growth curve is not observed because growth depends upon nutrition and it does not show a steady increase. Geometric growth cannot be sustained for long. Limited nutrient availability causes slowing
down of growth and some cells may die. It leads to a stationary phase. There may be actually a decline. As a result, sigmoid growth curve is obtained.
• The S-shaped growth curve is typical of most living organisms in their natural environment. It is also applicable to cells, tissues, and organs of plants.

Factors Affecting Growth

Nutrients
Act as raw materials for the synthesis of protoplasm as well as a source of energy. Both types of nutrients, i.e., macro and micronutrients should be available for proper growth.

Temperature
The optimum temperature of 28-30°C is required for the proper growth of plants. A temperature below this range inactivates enzymes while high temperature hinders growth.

Other factors
Excess of salt, mineral deficiency and other stress factors also have detrimental effects on growth.

Oxygen
It is essential for aerobic respiration and hence energy.

Water
It is essential for cell elongation, maintenance of turgidity of growing cells and for providing medium for enzyme action.

Light
It is essential for tissue differentiation, synthesis of photosynthetic pigments and photoperiodism.

Gravity
Gravity determines the direction of the shoot and root growth.

Differentiation, Dedifferentiation and Redifferentiation

• Differentiation: It refers to the permanent qualitative changes in structure, chemistry, and physiology of cell wall and protoplasm of cells, tissues and organs.

• Dedifferentiation: It is the process of de specialization of differentiated living cells so that they regain the capacity to divide and form new cells. A dedifferentiated tissue can act as meristem, e.g., interfascicular vascular cambium, cork cambium, and wound cambium. In culture experiments, parenchyma cells dedifferentiate to produce a mass of dividing cells called callus.

• Redifferentiation: Structural, chemical and physiological specialization of cells derived from dedifferentiated meristematic cells are called redifferentiation. It is similar to the differentiation of cells and tissues formed by primary meristems.

DEVELOPMENT

sequence of developmental processes

• Development is the sequence of changes that occur in the structure and functioning of an organism, organ, tissue or cell involving its formation, growth, differentiation, maturation, reproduction, senescence and death. A plant passes through developmental stages of seed germination, seedling, juvenile phase, maturation, flowering, seed formation, and senescence.
• Conversion of one phase into next is also development, e.g., leaf initiation to leaf expansion, vegetative phase to flowering phase, etc. Development may even occur at the sub-cellular level, e.g., the appearance of chloroplasts in cells exposed to sunlight. The development ultimately leads to senescence and then death.
• Development in plants is not always straightforward. Sometimes, different structures may develop in different phases of growth as well as in response to the environment.
• Therefore, development is under control of both intrinsic, i.e., genetic factors and growth regulators and extrinsic factors, i.e., light, temperature, water, oxygen, and nutrition.

Plasticity
It is the ability to change under the influence of internal or external stimuli. The intrinsic plasticity is observed in the juvenile stage of plants such as cotton, coriander, larkspur, etc., whereas environmental plasticity is found in emergent hydrophytes, e.g., Ranunculus (buttercup). In both the types of plasticity, plants show heterophylly.
Heterophylly refers to the presence of different types of leaves on the same plant habitually in different phases of growth or under different environmental conditions.


No comments:

Post a comment