abscisic acid (ABA) A plant hormone (C15H20O4)
and weak acid that generally acts to inhibit growth,
induces dormancy, and helps the plant tolerate stressful
conditions by closing stomata. Abscisic acid was
named based on a belief that the hormone caused the
abscission (shedding) of leaves from deciduous trees
during the fall.
At times when a plant needs to slow down growth
and assume a resting (dormant) stage, abscisic acid is
produced in the terminal bud, which slows down
growth and directs the leaf primordia to develop scales
that protect the dormant bud during winter. Because
the hormone also inhibits cell division in the vascular
cambium, both primary and secondary growth are put
on hold during winter.
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