Different plant parts and growth facets operate on differently timed cycles. Plants have several types of biological clocks, related to circadian cycles and rhythmical processes. This lets them anticipate environmental and biological events that occur at precise times of the day. Photosynthesis, fragrance emission, and blooming are all time regulated. Plants in a community often bloom at the same time to optimize the benefits of interbreeding.
A fungal pathogen attacks a rockcress plant, giving it a downy mildew disease: weakening the plant, and giving it a fuzzy mildew coating. The pathogen forms spores at night, releasing them at dawn.
Rockcress fights the pathogen by immunizing itself from the evening onward, with peak resistance at dawn. During the day, the genes that express protection are inactive.
Numerous plant processes are seasonally symphonic. Tree leaves fall at the same time as buds go dormant, cambial activity (cell growth) decreases, while contemporaneously preparation for cold tolerance steps up.
Yet the bioclocks involved in these seemingly synchronous activities may be quite different, or responses staggered. Bud dormancy is triggered by shortening days, as perceived earlier by the phytochrome in leaves, and pattern recognized.
Leaves fall after considerable preparation: retrieving recyclable nutrients and loading them with secondary metabolites. Cambial activity is regulated by a complex confluence of resource availability and perceived environmental conditions, which are synchronized with and summarized by bioclock indications.
Chemical reactions are highly temperature dependent. Chemical reaction rate doubles with a 10 °C increase in temperature. Plants compensate accordingly: planning their activities and metabolite productions based upon anticipated weather patterns.
Temperatures just above freezing (0–5 °C) typically retard plants’ biological clocks for the duration of cold exposure. After prolonged chill, warming resets clocks.