Class 11 Biology Chapter 15 Plant Growth and Development Notes

Plant Growth and Development

Abscission : Shedding of plant organs like leaves, flowers and fruits etc. from the mature plant.

Apical dominance : Suppression of the growth of lateral buds in presence of apical bud.

Dormancy : A period of suspended activity and growth usually associated with low metabolic rate.

Photoperiodism : Response of plant to the relative length of day and night period to induce flowering.

Phytochrome : A pigment, which control the light dependent developmen- tal process.

Phytohormone : Chemicals secreted by plants in minute quantities which influence the physiological activities.

Senescene : The last phase of growth when metabolic activities decrease.

Vernalisation : A method of promoting flowering by exposing the young plant to low temperature.

Growth : An irreversible permanent increase in size of an organ or its parts or even of an individual.

Abbreviations

IAA -  Indole acetic acid

NAA - Naphthalene acetic acid

ABA - Abscissic acid

IBA -  Indole-3 butyric acid

2.4D - 2.4 dichlorophenoxy acetic acid

PGR - Plant growth regulator

Measurement of growth : Plant growth can be measured by a variety of parameters like increase in fresh weight, dry weight, length, area, volume and cell number.

Phases of growth : The period of growth is generally divided into three phases, namely, meristamatic, elongation and maturation.

1. Meristematic zone : New cell produced by metotic division at root-tip and shoot tip thereby show increase in size. Cells are rich in protoplasm and nuclei.
2. Elongation zone : Zone of elongation lies just behind the meristematic zone and concerned with enlargement of cells.
3. Maturation zone : The portion lies proximal to the phase of elonga- tion. The cells of this zone attain their maximum size in terms of wall thickning and protoplasmic modification.

Growth rate : The increased growth per unit time is termed as growth rate.

The growth rate shows an increase that may be arithmetic or geometrical.

Growth Mathematical expression		Curve
In Arithematic growth : Only one daughter cell continues to divide mitotically while other differentiates and matures. In geometrical growth :The initial growth is slow (lag phase) and increase rapidly there- after at an exponential rate (log phase). Both the progeny cells divide mitotically and continue to do so. However, with limited nutrient supply, the growth slow down leading to stationary phase.	Lt = L0 + rt Lt = Length at time t L0 = Length at time zero r = growth rate W1 = W0e rt W1 = Final size W0 = Initial size r = growth rate t = time of growth e = base of natural logarithms	Linear curve Sigmoid or S-curve

Differentiation : A biochemical or morphological change in meristemic cell (at root apex and shoot apex) to differentiate into permanent cell is called differentiation.

Dedifferentiation : The phenomenon of regeneration of permanent tissue to become meristematic is called dedifferentiation.

Redifferentiation : Meristems/tissue are able to produces new cells that once again lose the capacity to divide but mature to perform specific functions.

PHYTO HORMONE OR PLANT GROWTH-REGULATOR

Growth promoting hormones : These are involved in growth promoting activities such as cell division, cell enlargement, flowering, fruiting and seed formation. e.g., Auxin, gibberellins, cytokinins.

Growth inhibitor : Involved in growth inhibiting activities such as dor- mancy and abscission. e.g., Abscisic acid and Ethylene.

Hormones	Functions
Auxins	Apical dominance, cell elongation, prevent premature leaf and fruit falling, initiate rooting in stem cutting, as weedicide, induce parthenocarpy.
Gibberellins	Delay senescence, speed up malting process, in crease in length of axis (grape stalk), increase in length of stem (sugarcane), bolting in beet, cab bages and many plants with rosette habit.
Cytokinins	Promote cell division, induce cell enlargment, re duce apical dominance, induce growth in auxilary bud, chlorophyll preservation, lateral shoot growth, adventitious root formation.
Ethylene	Promotes senescence and abscission of leaf and fruits, promotes ripening of fruits,
	break seed and bud dormancy, initiate germina tion in peanut, sprouting of potato tuber, promotes root growth and root hair formation.
Abscisic acid	Inhibit seed germination, stimulate clouser of stomata, increase tolerance to various stress, induce dormancy in seed and bud, promotes ageing of leaf (senescence).