1 - Rooting hormones and rooting plant growth regulators help you create new plants from stems taken from some older plants.

Q.

Why are hormones recommended when I am trying to grow new plants from stems cut from old plants? What do the hormones do?

A.

Rooting hormones, or more accurately, plant growth regulators increase the chances that roots will form on the stems you cut from the original plants.

The process of starting new plants requires a plant organ (stem in your case) with its specific tissues to begin to produce new plant organs (roots) with tissues associated with roots. All cells within a plant have exactly the same genes in their nuclei (there are some exceptions to this, but that is another story). So, stem cells have the same genes as do root cells, but the genes expressed (producing appropriate proteins) and directing growth in the stem are stem genes; in the roots, root genes are expressed. Hormones are natural chemicals that regulate the expression of the genes. Plant growth regulators are synthetic variants of the hormones that perform the same function as the natural hormones.

When you purchase rooting hormones/plant growth regulators to stimulate rooting in the plants, you are purchasing chemicals in the hormone class called "auxins". Auxins stimulate production of roots by encouraging genes to produce proteins related to root growth.

Auxins perform other functions within a plant. Auxins promote cell enlargement. Auxins also promote "apical dominance" by inhibiting lateral bud growth and encouraging growth of the apical bud (the bud at the end of a stem). Auxins are produced in buds at the top of a plant and translocate (move) to the roots to stimulate root growth. On the way down the stem auxins serve to inhibit the growth of buds along the side of the stem. As the auxins move further down the stem they become diluted and lateral buds are able to grow. When you prune away the apical bud, the lateral buds are released to grow because the supply of auxin is reduced. When the auxins finally reach the root zone of the plant, they encourage growth of roots. That is the characteristic we are exploiting when we apply rooting hormones.

Growing new plants from stem cuttings is much more complicated than just applying hormones to a cut stem. Some plants will not regenerate roots even when rooting hormones are applied. There are other chemicals called rooting cofactors that work with the auxin in stimulating root development. Some plants that are difficult to propagate from stem cuttings, even when treated with rooting growth regulators, may be induced to more readily form roots by soaking the base of the cutting in a solution of rooting cofactors before applying the auxin. Such a solution of cofactors may be produced by extracting cofactors from willow twigs. Willows have high levels of rooting cofactors which are easily extracted by soaking pieces of willow twigs in water. Some plants will not even produce roots even when treated with rooting cofactors and auxins. These plants are usually propagated by grafting or other propagation techniques.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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