(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.
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.
Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedling.