We think of plants as unsensing and incapable of reacting to their environment. It’s true that they don’t have a nervous system in the same way higher animals do but they do have mechanisms to cope with challenges and avoid problems. And while they don’t possess glands that produce hormones like many animals, they do produce hormones that regulate many functions. Rather than go into them all let’s tackle a few ways that plants sense and adapt to their environment. Today’s focus is on flowering and how to avoid conditions that would prevent the production of viable seed.
Plants from temperate climates need to avoid weather extremes when flowering, particularly freezing temperatures that would damage reproductive structures or abort developing seed. So they either must finish producing viable seed before killing frosts/winter or wait to flower until the threat of winter/freezing temperatures should be over. Plants from areas near the equator, without winter, don’t need a mechanism to avoid this challenge. So how do they do it?
They have a “hormonal” system and pigments termed phytochromes. There are red and far red phytochromes which switch back and forth with each other using light. When exposed to light phytochrome red rapidly converts to phytochrome far red. In darkness, the far red turns back to the red very slowly. If in the middle of the night there is a flash of light the red will rapidly turn back to far red and the process back to red starts over. The logical question is how does this help plants avoid cold?
Plants need to initiate flowering and produce mature seed far in advance of cold temperatures or after them so cold alone won’t work. Take a moment a think about an environmental change that starts to happen way before freezing temperatures in the fall arrive. For winter plants there is an additional factor but for them what starts to change before warmer temperatures arrive. The answer is sunlight.
Remember how red and far red change back and forth. The change to far red is almost instantaneous and the change back in the dark quite slow. So summer annuals need to avoid freezing fall temperatures. They are sensing a decrease in day length (increasing night length) so they are keyed by accumulation for phytochrome red. When nights reach a certain length the ration of red/far red increases to where it keys floral initiation. Soybeans are a good example. Winter wheat wants that red/far red ratio to decrease to a certain amount and it will allow flowering – increasing day length. The amount needed for a plant depends on the species and the hybrid/variety of that species. Summer annuals are long night/short day plants while winter annuals are long day/short night plants. For winter wheat there is one other safeguard.
Even if the daylight is increasing, winter wheat must vernalize before it can flower. In plain English, it must accumulate a certain amount of cold before changes in plant physiology allow for floral initiation. If you plant winter wheat in April, it will germinate, tiller, grow, and sense long days but it will never flower due to a lack of cold accumulation. There is a bit more involved here but this is a general explanation of how plants sense the environment and determine when to flower.