Plant - Growth & Development Test - 2

The effect of photoperiods or daily duration of light hours (and dark periods) on the growth and development of plants, especially flowering, is called photoperiodism. Photoperiodism was first studied by Gamer and Allard (1920) in 'Maryland mammoth' a variety of Tobacco.
Long Day Plants (LDPs) flower when they receive long photoperiods or light hours which are above a critical length, e.g., henbane (Hyoscyamus Nget), wheat oat sugar beet, spinach (Spinacia oleracea), radish, barley, larkspur, lettuce.

Phytochrome is a bright blue or bluish green pigment which was first of all isolated from plasma membrane of alga Mougeotias. Phytochrome is a photo receptor pigment that controls flowering. It exists in two interconvertible  forms: P_fr  or P₇₃₀ (absorbs far red light) and P_r or P₆₆₀ (absorbs red light).
By absorbing red light, P_r is converted into P_fr repidly. P_fr on absorbing far-red light is converted into P_r rapidly. P_fr  is physiologically active form: P_r  is inactive.

Experimental evidences have indicated that the photoperiodic stimulus in plants is perceived by pigment phytochrome. Phytochrome is a photoreversible pigment that absorbs light and the flowering is a phytochrome-mediated process.

Plants which flower after exposure to light above critical photoperiod are called long day plants. Plants which flower when exposed to light below critical photoperiod are called short day plant. Plants which flower after vegetative maturity are called day neutral plant.

A short day plant is the one that flowers on photoperiods shorter than the critical day length. As the critical duration of darkness in the Maryland mammoth tobacco is 10 hours i.e., it requires a minimum period of darkness of 10 hours to flower. Thus under the conditions shown in option (a), the plant will not flower.

A long day plant (LDP) is the one that flowers on photoperiods longer than critical day length. As the critical duration of light in Sedum is given to be 13 hours i.e., it required a minimum period of light of 13 hours to flower. Thus, the plant will flower only under the conditions shows in option (c).

A short day plant (SDP), which has a critical period of 14 hours, will flower only if it is provided a continuous period of darkness of 14 hours; which has been given to the potted plant-I only. In rest of the potted plants, more amounts of light periods are being given and thus the dark periods have been reduced to below 14 hours.

Vernalisation is the process of shortening of the juvenile or vegetative phase and hastening flowering by a previous cold treatment. In vernalization by cold treatment, winter varieties are transferred into spring or summer varieties. The site of vernalization is apical meristem or all the meristematic cells e.g., shoot tip, embryo tips, root apex etc.  The low temperature required for vernalization is usually 0^o − 5^oC. As a result of vernalization, a flowering hormone called vernalin is formed (reported by Melchers), but vernalin has never been isolated.

The low temperature required for vernalization is usually 0^o − 5^oC. In nature, the plants requiring vernalization are commonly biennials (e.g., cabbage, sugar canes), which complete their life cycle in two years. They germinate and grow vegetatively in the first year and produce flowers in the second year of growth. These plants fulfill their cold requirement during winters. However, such-biennial plants can be made to flower in one growing season by providing low-temperature treatment (i.e.,0^o − 5^oC temperature) to young plants or moistened seeds.

Vernalisation or low temperature treatement of some plants can be replaced by gibberellins.