PLANT GROWTH AND DEVELOPMENT
INTRODUCTION :
Growth is a characteristic feature of all living organisms.
Growth is a vital process, which brings about permanent and irreversible change in any plant or its part.
Growth in plants means increase in shape, size, weight and volume of a plant or plant part.
Growth leads to increase in fresh weight, dry weight, length, area, volume and cell number. All these are controlled externally (by environmental factors) and internally (by Genetics).
Growth is diffused in animals, but in plants growth is localised & irregular (nail in plant stem, occupies same height till several years of growth).
Seed germination is the first step of plant growth. Almost all the plants face a period of suspended growth.
If the suspension of growth is due to exogenously controlled factors (environmental factors) then it is called quiescence. Development is a part of growth.
When the suspension of growth is due to the endogenously controlled factors (hormonal, genetic) then it is termed as dormancy.
Weight increased during growth but exceptions are potato & seed germination, where weight decrease.
PHASES OF GROWTH
(1) Phase of cell division or cell formation :- Number of cells is increases by cell division.
(2) Phase of cell enlargement or cell elongation :- Size of cells increases due to vacuolization & TP (turgor pressure).
(3) Cell maturation or differentiation phase :- (Also called as morphogenetic, organogenic phase) Development or qualitative change is important feature of this phase.
Pattern or course of plant growth : (growth curve)
The pioneering work on growth was done by Von Sachs.
He plotted a growth curve between time & growth, which is known as sigmoid curve or S–curve or GP (Grand
period)–curve.
Growth pattern of cell, organisms is uniform under favourable conditions. Thus following phases of growth are
recognized.
(1) Lag phase :- In lag period the growth is slow.
e(2) Log phase :- Also called as exponential phase. During this phase growth is maximum & most rapid.
(3) Steady or stationary phase :-
Time taken in growth phases (mainly log phase) is called as "grand period of growth".
Definite direction in relation to gravity, (root cap percept stimulus).
(c) Chemotropism :- Ex.Pollen tubes & fungal hyphae
Definite direction in relation to chemicals.
(d) Thigmotropism (haptotropism) :- Ex : Tendrils, haustoria of Cuscuta.
Definite direction in response to contact or support.
(e) Hydrotropism :- Ex : Roots of seedlings
(ii) Nastic movement (External stimulus but diffused type or nondirectional) :-
(a) Nyctinasty :- Ex : Flowers, leaves, stomata, daily movement (Sleep movements)
Due to rhythemicity of day and night.
(b) Thigmonasty or haptonasty :- Tentacles of insectivorous plants
(c) Chemonasty :- Ex : Tentacles of insectivorous plants
(d) Seismonasty :- Ex : Mimosa (touch me not plant) turgor change in pulvinus leaf base
KEY POINTS
Many plant parts specially leaves exhibits nastic movement and involves differential growth, this type of movement is known as movement of gwowth. This movement, is caused due to unequal growth in plant organs.
Ex : Epinasty, hyponasty, Nutation.
Epinasty & hyponasty :- Ex : Leaves, flower (petal) opening & closing respectively.
Epinasty - More growth on upper surface of plant parts.
Hyponasty - More growth on lower surface of plant parts.
Both epinasty & hyponasty are example of autonomic growth movements.
Nutation :- Zig zag growth of plant organ mainly shoot, is called as nutation
Circumnutation :- Spiral growth of plants in tendrils
Portulaca is known as compas plant.
Rhizomes diageotropic (90° to gravitation force)
Clinostat :- used for nullifying geotropism
Xerochasy :- Due to loss of water
Hygroscopic movement
Ex :- Dehiscence of legume fruits
Development is the sum of two processes -Growth and differentiation
Growth is irreversible permanent increase in size of an organ or it's parts or even of an individual cell.
Growth is accompanied by metabolic processes-both catabolic and anabolic, that occur at the expense of energy.
Plants show open form of growth -new cells are always being added to the plant body
Apical meristem -contribute primary growths
Lateral meristem (vascular cambium and cork cambium) contribute lateral growth and causes increase in girth.
Growth at cellular level is principally a consequence of increase in the amount of protoplasm.
Growth is measured by a variety of parameters like -increase in fresh weight, dry weight, lengths, areas, volume and cell number
Period of growth is generally divided into three phases -“Meristematic“Elongation“Maturation“Cells in meristematic
phase are rich in protoplasm, possess large conspicuous nuclii.“Their cell walls are primary“Phase of elongation charaterised by increased vacuolation, cell enlargement and new cell wall deposition
Cell of maturation phase attain theirs maximal size in terms of wall thickening & protoplasmic modification.
The increased growth per unit time is termed as growth rate.
The growth rate shows an increase that may be arithmetic or geometrical.
In Arithmetic growth only one daughter cell continuous to divide while other differentiate and matures. Root elongation at constant rate is the expression of arithmetic growth
It proved linear growth and can be expressed as Lt = Lo + rt
In Geometrical growth -both daughter cells continuously divide. It attain sigmoid curve if space and food is limited.trees with seasonal activities show sigmoids curve.
Exponential growth (log phase of sigmoid curve) can be expressed as -
Here r is referred as efficiency index
Quantitative comparisons between the growth of living system cans also be made in two ways -
(1) Absolute growth rate measurement and comparision of total growth per units time
(2) Relative growths rate per unit initial parameter. increase/initial parameter × 100
Water, oxygen and nutrients are essential for growth
Plant growth and furthers development is intimately linked to the water status of plant
Water helps in -Turgidity, enzyme activation
Oxygen helps in releasing metabolic energy
Nutrients helps in synthesis of protoplasm and act as source of energy.
Optimum temperature range and environmental signals such as light and gravity also affect growth.
Plant growth is open - can be determinate (Plant organs) or Indeterminate (Shoot & root apices).
Plant differentiation is also open - Same meristem have differents structures at maturity
Final structure at maturity of cell/tissue is also determined by location of cell within.
Development includes all changes from seed germination to senescence.
Plant follow different pathway in response to environment or phases of life to form different kinds of structure, this ability called Plasticity
Heterophylly in Butter cup, cotton, coriander and larkspur is an examples of plasticity
Development in plants is under control of extrinsic (light, Temperature, water, oxygens & nutrition) and Intrinsic (Intracellular - genetic/intercellular - PGR) factors
Plant growth regulators are small, simple molecules of diverse chemical composition.
Comments
Post a Comment