How do plants detect seasons?

Asked July 13, 2017, 9:44 AM EDT

How do plants detect seasons (e.g. what are the names of organs they use) and what are the underlying reasons why plants only fruit during a certain season? e.g. Why does a tomato need warm weather? Why does an orange need cold weather?

Minnesota

1 Response

I contacted a colleague of mine, Dr. Karl Foord, who forwarded the following answer:

Plants detect photoperiod or length of the night. As the length of night decreases, or the length of day increases, in the spring plants will perceive this change and react according to previously determined genetic systems arrived at through natural selection. There are plants that flower in reaction to lengthening days (examples: Pea (Pisum sativum), Cabbage (Brassica), Spinach (Spinacea), coneflowers, California poppies) and plants that flower in reaction to shortening days (chrysanthemums, poinsettias, coffee, cosmos). There are plants that flower but such flowering is not governed by photoperiod e.g. day neutral strawberries.

Trees and flowering
Most temperate fruit trees have a chilling requirement. I believe this is the cold you are referring to relative to oranges. The chilling requirement of a fruit is the minimum period of cold weather after which a fruit-bearing tree will blossom. It is often expressed in chill hours, which can be calculated in different ways, all of which essentially involve adding up the total amount of time in a winter spent at certain temperatures. Because oranges are a semi-tropical crop chilling has different effects.

Chilling of orange trees has two effects. First, it increases production of carotenoids and decreases chlorophyll content of the fruit, improving their appearance and, ultimately, their market value. Second, the "quasi-dormancy" experienced by orange trees triggers concentrated flowering in spring, as opposed to more or less uniform round-the-year flowering and fruiting in warmer climates.

We humans have moved plants from their native environments to our gardens. Tomatoes originated in warmer regions and we have planted them in regions to which they are not naturally adapted. They can survive if we provide optimum growing conditions. Tomatoes basically suffer under cool temperatures as do most plants in the solanaceae e.g. eggplant, and peppers. Other cool season plants like cabbage and broccoli tolerate and thrive under cooler conditions.