Q.Discuss photoperiodism in detail: its definition, the three categories of plants recognised by their photoperiodic response (short-day, long-day and day-neutral) with named examples of each, the concept of the critical photoperiod, and the site within the plant where the day-length stimulus is actually perceived.

[!TLDR]

Photoperiodism = flowering response to day/night length; SDPs (e.g. chrysanthemum) flower below a critical day length, LDPs (e.g. spinach) above it, DNPs (e.g. tomato) regardless; the leaf perceives the true trigger, night length. …

Photoperiodism is the phenomenon by which a plant's physiological responses -- most conspicuously, the induction of flowering -- are regulated by the relative duration of light and darkness (the photoperiod) it experiences in each daily cycle. The term, and much of the founding experimental work, is credited to W. W. Garner and H. A. Allard, who investigated why the tobacco variety Maryland Mammoth grew vigorously through the summer in open field conditions without ever flowering, yet flowered readily when grown over winter in a greenhouse; by systematically testing different day lengths, they established that the shorter day length (equivalently, the longer uninterrupted night) of winter was the actual trigger.

On the basis of their flowering response to day length, plants fall into three categories. Short-day plants (SDPs) -- chrysanthemum, tobacco variety Maryland Mammoth, cocklebur, rice and soybean among them -- flower only when day length falls below a species-specific critical photoperiod. Long-day plants (LDPs) -- spinach, radish, henbane and wheat among them -- flower only when day length exceeds their own critical photoperiod. Day-neutral plants (DNPs) -- tomato, cotton, sunflower, cucumber and maize among them -- flower on a schedule essentially unaffected by day length, relying instead chiefly on developmental age or size. The critical photoperiod itself is the specific day-length (or, equivalently, night-length) threshold value, genuinely different from species to species, that separates flowering from non-flowering conditions for a given photoperiodically responsive species.

A crucial refinement to this picture, established by experiments that interrupted an otherwise long dark period with a brief flash of light, is that what a photoperiodically responsive plant is actually measuring is the length of the continuous, uninterrupted dark period, not the length of daylight as such. A short-day plant given a long night interrupted partway through by even a brief light flash fails to flower, exactly as though the night had been short throughout; a long-day plant given the identical brief interruption of an otherwise long night is, conversely, induced to flower, exactly as though the night had been short. This is why night length, rather than day length, is now understood to be the physiologically decisive variable, even though 'photoperiodism' remains the conventional name for the overall phenomenon. …

Structure the answer in four parts: (1) define photoperiodism and its discovery (Garner/Allard/tobacco), (2) the three plant categories with named examples and the critical-photoperiod concept, (3) the night-interruption experiments showing night length is the true trigger, and (4) the l …

Presenting florigen as a confirmed, fully characterised hormone rather than honestly flagging that its precise chemical identity remains a subject of ongoing research -- …