Question

1. Identify and explain photoperiodic induction for flowering in angiosperms. a) Diagram a model for the phytochrome pigment (Pr/Pfr) discussed in the notes from Monday(or use WWW search engine) to show its role in flowering of long-day and short-day plants. Include: mechanism for far red reversion, dark reversion, active form, etc. Using this diagram explain "h" and "C” below: b) Give the results and an explanation for your results of a long day plant (spinach) being exposed to 8 hr. light, 16 hr. dark, daily cycle and a second spinach plant exposed to 16 hr dark, 8 hr. light cycle. Use diagram above to explain. c) Give the results and an explanation for your results of a long day plant (spinach) being exposed to 8 hr. light, 8 hr. dark cycle in a growth chamber. Use diagram above to explain.

Answer 1

1) The response to the length of light and dark period in plants is called as Photoperiodism. Normally angiosperms or flowering plants have a protein that can identify the changes in light. Every plant differs in the length of photoperiod to flower and reproduce. Plants are divided normally into three categories - short day plants, long day plants and day-neutral plants.

Long day plants normally flower during summer time when the days are longer. In this type of plants, there will be light for 15 hours or longer duration. Example - oat. Short day plants flower when the nights occur for a longer period of time. In this type of plants, the light period occurs for 9 hours or less. Example - rice, cotton. Day-neutral plants are not dependent on the photoperiod to flower.

In equator region, the light and dark period are constant since the sunrise and sunset occur almost the same for the entire year. There are also intermediate day length flowering plants which require light duration between 12 - 15 hours. The flowering will be inhibited in case of shorter or longer duration of light.

Phytochrome, a chemical found in the leaves, is responsible to measure the dark period length. It occurs in two forms - P_r and P_fr . When P_r absorbs red light in the daytime, it converts to P_fr . P_fr absorbs far-red light during the night time and gets converted to P_r . Since P_fr inhibits flowering, at night period when its concentration falls below a critical amount, the flowering occurs after the floral organs receive chemical messages.

a)

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b) For the first spinach plant, the light duration is 8 hours and dark duration is 16 hours. For the second spinach, the dark period is 16 hours, followed by light for 8 hours. For both cases, flowering won't occur since, for the long day plant, shorter dark period is necessary to flower.

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c) When spinach plant is given 8 hours light and 8 hours dark period in a growth chamber, it still will not flower. For long day plants, a shorter dark period is a critical factor. Hence, if the light duration is increased, it will flower. But in this condition, it will not flower.

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