Biology PHOTOPERIODISM AND VERNALISATION

### KEY TOPICS

star Photoperiodism
star Vernalisation

### PHOTOPERIODISM

● It has been observed that some plants require a color{violet}"periodic exposure to light" to color{violet}"induce flowering".

● It is also seen that such plants are able to measure the color{violet}"duration of exposure to light".

● For example, some plants require the color{violet}"exposure to light" for a period exceEding a well color{Brown}"defined critical duration", while others must be exposed to light for a color{violet}"period less than" this critical duration before the color{violet}"flowering is initiated" in them.

● The former group of plants are called color{Brown}"long day plants" while the latter ones are termed color{Brown}"short day plants".

● The color{violet}"critical duration" is different for different plants.

● There are many plants, however, where there is color{violet}"no such correlation" between exposure to color{violet}"light duration and inducation" of flowering response; such plants are called color{Brown}"day-neutral plants"

● It is now also known that not only the color{violet}"duration of light period" but that the color{violet}"duration of dark period" is also of equal importance.

● Hence, it can be said that color{violet}"flowering in certain plants" depends not only on a color{violet}"combination of light and dark" exposures but also their relative durations.

● This color{violet}"response of plants" to periods of day/night is termed color{Brown}"photoperiodism."

● It is also interesting to note that while color{violet}"shoot apices modify themselves" into flowering apices prior to flowering, they (i.e., shoot apices of plants) by themselves color{violet}"cannot perceive photoperiods".

● The color{violet}"site of perception" of light/dark duration are the color{violet}"leaves".

● It has been hypothesised that there is a color{violet}"hormonal substance(s)" that is responsible for flowering.

● This hormonal substance migrates from color{violet}"leaves to shoot apices" for inducing flowering only when the plants are exposed to the color{violet}"necessary inductive photoperiod".

### VERNALISATION

● There are plants for which flowering is either color{violet}"quantitatively or qualitatively dependent" on exposure to color{violet}"low temperature".

● This phenomenon is termed color{Brown}"vernalisation".

● It prevents color{violet}"precocious reproductive development" late in the growing season, and enables the plant to have color{violet}"sufficient time" to reach maturity.

● Vernalisation refers specially to the color{violet}"promotion of flowering" by a period of color{violet}"low temperature".

● Some important food plants, color{violet}"wheat, barley, rye" have two kinds of varieties: color{Brown}"winter and spring varieties". The ‘spring’ variety are normally color{violet}"planted in the spring" and come to flower and produce grain before the end of the growing season.

● color{violet}"Winter varieties", however, if planted in spring would normally color{violet}"fail to flower "or produce mature grain within a span of a flowering season.

● Hence, they are color{violet}"planted in autumn".

● They color{violet}"germinate, and over winter" come out as small seedlings, resume growth in the spring, and are color{violet}"harvested usually around" color{violet}"mid-summer".

● Another example of vernalisation is seen in color{violet}"biennial plants".

● Biennials are color{violet}"monocarpic plants" that normally color{violet}"flower and die" in the second season.

● color{violet}"Sugar beet, cabbages", color{violet}"carrots" are some of the common biennials.

● Subjecting the growing of a color{violet}"biennial plant to a cold treatment" stimulates a subsequent color{violet}"photoperiodic flowering response".