Question

One set of plants exposed to 12 hr broad daylight and 12 hr night flowered. The other set with similar exposure to day-night period but with dark period interrupted by flash of light did not come to flower. The plant is
A. Long day
B. Day neutral
C. Indeterminate
D. Short day

Answer 1

It has been noted that some plants require a periodic exposure to light to induce flowering and they are able to measure the duration of exposure to light. For example, some plants require the exposure to light for a period exceeding a well-defined critical duration, while others must be exposed to light for a period less than this critical duration before the flowering is initiated in them.

Complete step by step answer: Response of a plant to relative length of sunshine and darkness within 24 hours is understood as photoperiodism.
Garner and Allard established the concept of photoperiodism.
Photoperiodism was first studied in relevancy flowering. Plants are described in relevance to their photo-period responses as short-day, long-day, day-neutral plants.
Short day plants (SDP):
These plants flower briefly days, i.e. with a protracted unbroken night.
However, day length is critical and varies from species to species. If these plants are kept at day length in way over crossroads, they still remain in vegetative state.
These plants normally flower in winter or early spring.
Examples: tobacco, strawberry, Oryza sativa, chenopodium, cocklebur and Chrysanthemum.
-Long day plants (LDP):
These plants flower in long days, i.e. with a brief unbroken night.
However, day length is critical and varies from species to species. If these plants are kept a day length below of crossroads, they still remain in vegetative state.
They normally flower in late spring or early summer.
Examples: Poppies, lettuce, spinach and potatoes, Tomato
-Day neutral plants (DNP):
These plants don't have any specific requirement of day length to flower, i.e. they're not tormented by day lengths and flowers round the year.
Examples: love apple (tomato), corn, cucumber, rice and a few strawberries.
Critical photoperiod:
The length of the daily period of sunshine to which a plant is exposed is termed photoperiod and therefore the number of hours of sunshine that determines its flowering response is alleged to be a critical photoperiod.
For short day plants, the critical photoperiod is that photoperiod at or below which it flowers.
For long day plants, the critical photoperiod is that the photoperiod at or above which it flowers.
Importance of dark period:
In 1938, K. C. Hammer and J. Bonner experimented with artificial lengths of dark and lightweight periods.
Cocklebur, a short-day plant, flowers as long because the dark period lasts over 8.5 hours.
If a dark period is interrupted by a flash, it doesn't flower; darkness amidst the day cycle has no effect.
Long-day plants require a dark period shorter than a critical length no matter the length of sunshine period.
Thus, length of the dark period controls flowering, not length of the sunshine period
Therefore, the correct option is D, short day.

Note: Phytochrome and Flowering: It's been indicated that the pigment phytochrome is involved in photoperiodic perception and flowering. Phytochrome- a blue-green leaf pigment that exists in two forms:
Pr (phytochrome red) absorbs red light (wavelength of 660 ;nm); it's converted to Pfr.
Pfr is a phytochrome far-red and absorbs far-red light (wavelength of 730 ;nm); it's converted to Pr.