A flash of red light followed by a flash of far-red light given during the middle of the night to a short-day plant will likely 
1. cause increased flower production.
2. have no effect upon flowering.
3. inhibit flowering.
4. stimulate flowering.

Answer: 2
Phytochromes are characterised by a red/far-red photochromicity. Photochromic pigments change their "colour" (spectral absorbance properties) upon light absorption. In the case of phytochrome the ground state is Pr, the r indicating that it absorbs red light particularly strongly. The absorbance maximum is a sharp peak 650-670 nm. But once a red photon has been absorbed, the pigment undergoes a rapid conformational change to form the pfr state. Here fr indicates that now not red but far-red (also called "near infra-red";705-740 nm) is preferentially absorbed. when pfr absorbs far-red light it is converted back to pr. Hence, red light makes pfr, far-red light makes pr. In plants at least pfr is the physiologically active or "signalling" state.

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