Apparently they use temperature. We know this because as the climate warms, plants' sense of seasons is changing, which it would not be if they were using light instead of temperature.
That said, I suspect that desert plants sense the seasons by the availability of water, rather than temperature, since water is what limits their growth.
There is a field of science called "plant gnosophysiology" which studies the sensing ability of plants to different stimuli. From this site I read that plants have sensors that respond to changes in Carbon dioxide, light, chemicals, temperature, etc.
I recommend you to read the book "Lab Girl" from Hope Jahren. You can download the audiobook from the Goleta Public Library. It is a good book about plants and the way they sense the environment. You will enjoy it for sure
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