Why do leaves turn colour in the autumn? Is it the cooler nights, the date on the calendar or what?
While some may think it is the cooler weather or just the changing seasons, it's really the "photoperiod."
The photoperiod is considered to be the environmental trigger that causes fall leaf colouring, senescence and leaf drop. The pigment phytochrome senses the photoperiod. Photoperiod is often thought to be the length of the daylight period.
In actuality it is not the length of the daylight time, but the length of the night.
Plants that respond to photoperiodism, do not measure the length of the daylight but rather the length of the dark period. For these maple leaves, their night has been shortened by the rays of the street lamp. I'm sure at some point they will respond as the rest of the tree has done. My speculation is that the other leaves will create enough of the hormones that trigger the autumn responses to eventually cause all the leaves to colour and fall.
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