Auxins are of two types :
(1) Natural auxins : These are naturally occuring auxins, isolated from plants and are regarded as phytohormones e.g. IAA. NAA etc.
(2) Synthetic auxins : These are synthesized by artificially e.g. IBA 2,4-D etc.
(A) Photoperiodism . It is the response of plants to periods of day/night. The flowering in some plants depends not only on a combination of light and dark exposures but also their relative durations. The plants that need a longer duration for a period exceeding a well defined critical duration, while the short day plants must be exposed to light for a period less than this critical duration before the flowering is initiated in them.
Significance : Photoperiodism helps in studying the response of flowering in various crop plants with respect to the duration of exposure to light. It is the phenomenon which helps in flowering out of season.
(B) Vernalization : Vernalisation refers specially to the promotion of flowering by a
period of low temperature. It is the treatment of cold (3-15°C) to arrest vegetative growth and initiate flowering.
Significance of vernalization :
1. It shortens the vegetative phase.
2. It makes the plant resistant to cold and fungal pathogens.
3. It prevents precocious reproductive development late in the growing season, and enables the plant to have sufficient time to reach maturity.
Abscisic acid is known as the stress hormone because of the following reasons :
1. It helps plant to cope with adverse environmental conditions or stresses.
2. It causes temporary closure of stomata due to which there is reduction in the rate of transpiration. Therefore, it acts as antitranspirant and reduces water loss.
3. It acts as a general growth inhibtor and inhibitor of plant metabolism
4. It inhibits seed germination.
5. It induces dormancy to escape the unfavourable conditions.
The flowering in plants is in response to duration period of light or relative length of light. The long day plants need a longer duration of light whereas the short day plants need short duration of light.
If the long day plant and the short day plants gets the required duration of the light necessary for their flowering they may produce flower simultaneosly in a given plave. If the long day plants is planted before the short day one such that the long day plants gets a longer duration of light , both the short day plant and a long day plant can produce flower simultaneously in a given place
Growth : Growth may be defined as permanent or irreversible change in the size of a cell, organ or whole organism usually accompanied by an increase in dry weight.
Differentiation : Differentiation is the process whereby cells undergo few to major structural changes that lead to the maturation. The cells mature to perform specialized functions.
Development: Development includes all the series of changes which an organism goes through during its life cycle.
Dedifferentiation : It is the process by which the living differentiated cells which had lost the capacity to divide regain the capacity to divide under certain conditions.
Redifferentiation : Redifferentiation is defined as maturation or differentiation of dedifferentiated tissues.
Determination growth . The ability of a cell, tissue or the organism to grow for a limited period is called determinate growth.
Meristem : Meristem is a group of cells which have power of division and add new cells to the body.
Growth rate : It is the increased growth per unit time.
The main five groups of plant growth regulators (PGRs) are :
5. Abscisic acid.
Physiological functions of auxins are:
1. They help to initiate and enhance rootings in the stem cuttings.
2. They promote flowering in certain plants like pineapple.
3. They prevent the leaf and fruit drop at early stages. They promote abscission of older or mature leaf and fruit.
4. They induce apical dominance.
5. They are responsible for tropic movements.
1. They induce parthenocarpy in plants like tomatoes..
2. They are used as weedicides that kill dicotyledenous weeds.