Why do most annual plants die
in the autumn? Larry D. Nooden and Susan L. Schreyer at the University of
Michigan are studying a chemical "death signal" possibly a hormone which
they have traced to plant seeds. The possibility is being considered that
seeds inside mature fruits such as soybean pods send out hormones, which
cause plants to yellow and die even before nights cold enough for freezing
cut them down.
Gardeners for years have
known that if faded flowers are picked before they form seeds the plants
will continue to produce more flowers. Pansies are a good example. Among
the vegetables okra will continue from early spring to frost if the pods are
kept picked before they harden.
Nooden says that this idea
was tested on soybeans. Growing pods were plucked from one side of the plant
only and allowed to remain on the other. The side with the mature pods and
seeds turned yellow and died, the other remained healthy.