Plants need to know when to produce flowers so that fruit and seed development can be accomplished before the next winter arrives...at least at high latitudes that have a winter. Of course the arrival of winter varies with location on our planet. To help understand this is a global map.
Plants living near the equator do not have to worry about preparing for winter or timing around winter. The sun goes directly overhead only between the two tropic lines on either side of the equator. These lines define the tropical zone. Here the variations between summer and winter are minimal. But you will notice that, except for Hawaii, the US is north of the Tropic of Cancer. So in most of the US, the plants must respond to the seasons. Do you know what defines the artic and antarctic circles?
Focusing on the 48 contiguous states, you can see that here in Connecticut, we are at a fairly high latitude: 41.7° N. We have considerable winter; the plants must prepare for this...but how?
Plants need to be able to detect the difference between one season and another in order to flower at the correct time of year. One environmental variable that reliably changes during the year...at least at high latitudes...is photoperiod. The length of day is longer in summer and shorter in winter as shown in this diagram. Are the summer and winter solstices shown on the correct dates? Are the vernal (spring) and autumnal equinoxes shown on the correct dates? Does this figure apply to all of the earth or only just one part of it? Which part?
You will notice that plants in our state, Connecticut, is at a latitude that gives plants a considerable swing in daylength between summer and winter.
To determine whether it is the correct season for flowering or not, the plant has evolved a way to determine the length of day and to respond correctly. There are at least two different response systems found in flowering plants. Some plants flower when the daylength exceeds a specific critical photoperiod...these are called long-day plants (LDP). Other species flower when the daylength is shorter than their specific critical photoperiod...these are called short-day plants (SDP). The LDP flowers in summer and the SDP flowers either in spring or fall.
As you can clearly see, the SDP example shown above is flowering whenever the photoperiod is shorter than a critical value of about 15 hours (black line). The LDP example shown above is flowering whenever the photoperiod is longer than its critical value of about 11 hours. Notice how the critical photoperiods between these two species are different...in fact the LDP has a shorter critical period than the SDP! The difference between LDP and SDP is not found in these values...rather it is found in which way the plant responds above and below its critical value. Thus the difference is not the position of the inflection, but rather in the shape of the curve!
Just to be sure we are not glossing anything over...some plants are day-neutral and do not use photoperiod as their signal! We call such plant DNPs and they generally just need to reach a certain size or maturity and then they flower regardless of photoperiod.
This question is not trivial...while clearly the graph above shows the responses as a function of daylength, note that the graph also includes the corresponding night length. So a SDP could actually be a LNP (long night plant), and a LDP could really be a SNP (short night plant). To distinguish these possibilities one would try some interruptions of the photoperiod (day) or of the noctoperiod (night). First we examine the results with Short-day plants...
Here you can see that SDPs flower when the day is shorter than the critical length and when the night is longer than the critical length. When the night of correct duration is interrupted by a brief flash of light, the plant fails to flower! Interrupting the night blocks flowering...so the plant is actually measuring the noctoperiod. So why don't we call this noctoperiodism? Tradition! An interruption of the day with a brief night, does not change the plant's response.
Now we turn our attention to a long-day plant for the same test...
You can see above, the LDP flowers when the day exceeds the critical value and fails when it does not, but does it need the long day or the short night? When you interrupt the night with a flash of light, indeed this plant can flower with short days if you cut the long night into short nights. So again, the LDP is, in fact a short-night plant. Again, this should be called noctoperiodism! Interrupting the day with a brief night does not change the plant's response.
As you can easily see, the night interruption works fine, but the day interruption does NOT change the responses. Furthermore if you go to non-standard cycles (those not totalling 24 in a day), again the critical value is the NIGHT length not the DAY length!
Does is matter whether the brief flash of light that interrupts a long night comes early in that night or later in that night? The answer is, yes it does. You can see below that the plant is maximally sensitive to a night interruption some 8 hours into the dark period for both an example SDP and an example LDP.
Since the plants are sensitive during the night, how does this sensitivity change during one long uninterrupted dark period? Well, you put many plants into a dark period, and then at intervals you expose a few to light and then put them back into darkness...then you see which ones responded. Here are the results for a SDP.
As you can see above, the night interruption that blocks flowering is maximized at about 14, 38, and 62 hours...with valleys of response at those times. Interruptions at other times do not reduce flowering as much. So the plant has some way of keeping time and is only sensitive to the interruptions at approximately 24-hour intervals...even though the plants are in continuous darkness other than the brief flash. Such a sensitivity response is showing a circadian rhythm.
Of course you might want to know if a LDP would show the same rhythm... here is the response seen in Arabidopsis, a LDP. Of course the interruption of the long night is what stimulates flowering rather than inhibiting it!
In the early 1900s the interruptions were tried with light of different wavelengths (colors). It was observed that red light (660 nm) was most effective in interrupting the long night...and far-red light (730 nm) was ineffective. Are you thinking what I'm thinking?
Would FR light undo a R interruption? Would R light undo a FR block of interruption? Well sequences of flashes given in the middle of the night ending in R acted as if an interruption had been effective...sequences ending in FR light acted as if no interruption had occurred at all.
Here are the corresponding action spectra:
If you are thinking...this is a phytochrome response!...you would be correct. Obviously the sensitivity of the plant in the middle of the night to light uses the pigment, phytochrome, to determine whether there is a light available or not. The plant then responds accordingly.
Once the phytochrome situation is correct for the plant in question, hormones or other signals must give the actual signal to flower. Years ago Anton Lang showed that the flowering signal could be transmitted through grafts from an flower-induced plant to an uninduced plant. Here are the results for Xanthium.
The induced cocklebur plant can be grafted to an uninduced one and make it flower under non-inducing conditions. That induced plant can then induce others through grafts.
When this work was repeated with Perilla, it was shown that it only took a graft of a single induced leaf to induce another plant to flower, and while this leaf could be sequentially grafted to other plants, inducing each one in turn, portions of the induced plants could not be grafted to other uninduced plants to produce flowering.
The conclusion of this work is that once a plant is induced to flower it can produce chemical signals that can cause other plants to flower. The transmissible substance was named florigen in mid-1900s, but this "hormone" has never been identified.