Plant Growth and Development

it includes all the changes that an organism goes through during its life cycle from germination of seed to senescence it is also applicable to tissues or organs

plants follow different pathways in response to environment or phases of life to form different kind of structures.This ability is called plasticity Eg:Heterophylly in cotton, coriander,larkspur,and buttercup leaves of juvenile plant are different in shape from those in mature plant(cotton, coriander and larkspur) shape are different in leaves produced in air and water (buttercup)-Ranunculus

Development = Growth + Differentiation , Development in plants is under the control of intrinsic and extrinsic factors.
• Intrinsic → intracellular [genetic] or
intercellular [PGR]
• extrinsic → light, temperature, water,
nutrition, etc

PGR: They are small, simple molecules of diverse chemical composition
They can be, Indole compounds [Indole-3-acetic acid-IAA]
Adenine derivatives N6- furfurylamino purine - Kinetin]
Derivatives of carotenoids [Abscisic acid - ABA]
Terpenes [Gibberellic acid -GA3
Gases [Ethylene -C2H4

purine (adenine--> cytokinin) indole compound (auxin) terpenes(gibberellins) carotenoid (ABA) Gas(ethylene)

Plant Growth Promoters
• They promote growth promoting activities such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting & seed formation

Eg: Auxins, gibberellins, cytokinins Plant Growth Inhibitors• They involve various growth inhibiting activities such as dormancy and abscission

• They play important role in plant responses to wounds and stresses of biotic and abiotic origin

Eg: ABA

• Ethylene (80% inhibitor 20% promoter)

• Auxin - Observed by Charles Darwin & Francis Darwin
• Isolated by F.W. Went [from tip of coleoptile of oat seedlings
• Darwin and his son observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source [phototropism]
• Gibberellic acid - Discovered by E. Kurosawa
• In rice plants
• Bakanae disease [foolish seedling] caused by Gibberella fujikuroi
• Cytokinin - observed by F. Skoog and co-workers (Tobacco)
• Identified and crystallised by Miller
• Ethylene - H.H. Cousins - from ripened oranges
• The bakanae (foolish seedling) disease of rice seedlings, was caused by a fungal pathogen Gibberella fujikuroi. E. Kurosawa reported the appearance of symptoms of the disease in rice seedlings when they were treated with sterile filtrates of the fungus. The active substances were later identified as gibberellic acid
• F. Skoog and his co-workers observed that from the internodal segments of tobacco stems, the callus (a mass of undifferentiated cells) proliferated only if, in addition to auxins the nutrient medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA.
• Miller et al (1955), later identified and crystallised the cytokinesis promoting active substance that they termed kinetin
• During mid-1960's, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin
Later all the three were proved to be chemically identical. It was named abscisic acid (ABA)
Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas.
Later this volatile substance was identified as ethylene, a gaseous PGR

[Auxin:- Auxein - means - to grow]
It was first isolated from human urine
Generally produced by growing apices of stem and root, from where they migrate to the region of their action.
Types of Auxin:
1. Natural auxin
Auxin which are present naturally in plants
Eg: IAA, IBA
2. Synthetic auxins
Artificially synthesised
Eg: NAA (Naphthalene acetic acid)
2, 4 - D [2, 4 - Dichlorophenoxyacetic acid]
• Auxins are extensively used in agricultural and horticultural practices

• Initiate rooting in stem cuttings - widely used for plant propagation
• Promote flowering - Eg: Pineapples
• Prevent fruit and leaf drop at early stage
• Promote the abscission of older mature leaves and fruits
• Induce parthenocarpy Eg: Tomatoes
• Widely used as herbicide Eg: 2, 4-D used to kill dicotyledonous weeds & does not affect mature monocotyledonous plants [Grasses-Monocots]
• Used to prepare weed free lawns by gardeners
• Controls xylem differentiation
• Help in cell division
• Promote apical dominance
Apical dominance
• In higher plants, growing of apical bud inhibit the growth of lateral (axillary) buds, this phenomenon is called apical dominance
• To overcome apical dominance decapitation is used [Removal of shoot tip]. It is widely used in
• > Tea plantations
• > Hedge making