Today, in the Northern hemisphere* the winter solstice occurs with the shortest day and longest night of the year. For any photosynthesizers that haven’t completely shut down for the winter, this is their least (primary) productive day. Before you pity the plants, keep in mind that they knew this was coming. I’ve mentioned before than plants have a variety of ways of telling time because it benefits them to prepare for predictable changes in light cycles. In this way, plants can make sure they do things like flower and produce seeds when environmental conditions are most favorable to do so. But we’re not just talking about the time of day. Plants can also tell the time of year.
Plants have a series of biological photosensors that transmit information about light to their machinery for controlling their development. This ‘photoperiod’ response has been extensively studied in plants especially with respect to when a plant will produce flowers. Of course, different plant species have different photoperiod responses when it comes to flowering. Some species require a certain day length before they will flower. Plants like spinach or sugar beets require long days before they will flower. Other species are just the opposite, flowering when days are short. Rice and poinsettias are examples of short day plants. Short and long are merely relative to some internal critical photoperiod threshold for a given plant species. So, even though a plant may be ‘long day’ or ‘short day’, these are not absolute terms. One plant’s short day may be another’s long day. There are also other examples of plants that will flower no matter what the photoperiod and still others that integrate info about day length with seasonal temperatures in a more complex equation before making the transition to flowering.
Plants sense photoperiod and transmit that information to the shoot apical meristem (SAM), where developmental changes for flowering must occur. This means there is a transmissible signal within the plant that travels to the SAM to program those cells to start flower development. This is an active area of plant research. The current model involves a couple of transcription factors whose accumulation is controlled by photoperiod and the circadian clock. Once the critical photoperiod length is sensed, these proteins move to the SAM and turn on and off the plant genes necessary for making flowers. Check out the references below if you want the nitty gritty details and gene names (caution, some have paywalls).
Here’s the real twist on photoperiodism that you didn’t see coming. Plants don’t measure the length of the day. They measure the length of the night. They do this using a pigment-containing protein called a phytochrome that senses red and far red light. When the phytochrome absorbs light, it changes its form. So you can have a red form (PR) and a far-red form (PFR). The default state is PR that prefers to absorb red light, and once it does it changes to the PFR form which absorbs far-red light. The PFR form is converted back to the PR form when it absorbs far-red light. The PFR form is the active form for signaling. During the day, there is a continuous interconversion of the two forms as they are always absorbing red and far-red light. The PFR form can also be converted back to the PR form in the dark. Any PFR form that accumulated during the day will revert back to PR overnight. The longer the night is, the more PR form there is by the morning, but shorter nights yield more PFR form. The relative amounts of the PFR and PR form are the signal the plants use to estimate day length and what time of year it must be.
References and Links:
* The opposite is happening in the Southern hemisphere. They are experiencing the summer solstice- the longest day and shortest night of the year. For plants in the Southern hemisphere, this is their longest day for photosynthesis.