Plant hormones are molecules within a plant that send signals to various cells throughout the plant, determining the growth of seeds, stems, branches, leaves, plus flowering and fruiting. Abscistic acid is believed to be the hormone primarily responsible for the dormancy of seeds. Aided by another hormone, Ethylene, abscistic acid also causes leaves to fall and other changes associated with plant dormancy.
Abscistic acid is a growth inhibitor produced by the chloroplasts of mature leaves. Chloroplasts contain chlorophyll, the substance that uses light to produce photosynthesis. Abscistic acid counters gibberellin and auxin, hormones that encourage seeds to germinate and plants to grow. After the leaves produce abscistic acid, the hormone moves short distances through the phloem vessels to other parts of the plant and is found stems, the caps of roots and in green fruits.
Abscistic acid encourages fruit to drop and leaves to fall. When the plant is low on water abscistic acid closes the stomata, pores in the leaves. It encourages both seeds and plants to go dormant.The hormone induces seeds to produce proteins to store when the seeds are dormant. Abscistic acid inhibits the growth of shoots, but does not have much affect on roots; sometimes it may even promote root growth.
Dormancy is a drop in a plant's metabolic rate. Dormant plants have a reduced need for nutrients and are more tolerant to drought and cold temperatures. Short days, falling temperatures, nutrients and rainfall and nutrients can all cause a plant to produce abscistic acid that induces it to go dormant.
The ability of gibberellins to break seed dormancy induced by abscistic acid can take hundreds of hours at low temperatures. Some seeds will not germinate until the abscistic acid is leached away. For these reasons seeds are sometimes refrigerated or soaked to induce germination.
When the plant growth hormones auxin and gibberellin begin to decrease, stem nodes, aging leaves and ripening fruits produce the hormone ethylene. Ethylene, secreted as a gas, works with abscistic acid to promote the aging and falling of fruits and shedding of leaves that precedes dormancy. Ethylene moves through the spaces between plant cells. When ripening fruits produce ethylene, nearby fruits ripen faster.