How do trees measure the changing light?

Plant chemistry underlies the singular event we most associate with New England: the annual autumn foliage display!

Northern trees prepare quietly for their winter. Conifers form waxy buds and close needle pores, ceasing photosynthesis abruptly. Hardwood leaves fade as green chlorophyll production slows.

Unstable sugar molecules produced in leaves are moved via sap to roots to be stored as more stable starch.

How do seemingly inanimate trees measure the changing light?

Relative concentrations of leaf chemicals called “phytochromes” produced inside leaves during longer dark periods begin to exceed those produced during ever-shorter, sunlit hours. In response to changes in relative phytochrome concentrations, hormones signal leaves to form a corky layer to seal off leaf stems called petioles. You can hear the audible snap of leaf petioles on frosty, October mornings as trees discard the spent solar panels.

When chlorophyll production ceases entirely, residual green pigments breaks down and the underlying yellow and orange secondary pigments are revealed. Other hormones trigger production of red and purple secondary pigments where leaves remain exposed to weaker autumn sunlight.

Red “Anthocyanin” pigments are responsible for the red, purple, and blue colors of fruits, vegetables, cereal grains, and flowers. Anthocyanin pigments are brilliant scarlet on the sun-lit sugar maples, quintessential icons of the autumn foliage! Warm, sunny days following cool, frosty nights are prime ingredients for spectacular fall foliage.