Adventitious Roots

Adventitious roots are different from other roots.

Roots are generally classified as primary or lateral. The primary, or main, root supports a number of side, or lateral roots. Root systems feature either a taproot or fibrous roots. Carrots are an example of a taproot. Fibrous root systems are more “all over the place”. In both cases, the roots are attached to the aboveground portion of the plant at the crown, or to other roots. This is not the case, when it comes to adventitious roots.

Banyan tree (Wknight94) CC BY-SA 3.0

What are adventitious roots?
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Adventitious roots emerge from a variety of non-root locations, and for different reasons. Unlike the classic, “stem goes up, roots go down” type of growth, adventitious roots appear at leaf and stem nodes, and at wound sites. What starts out as a normal bud or shoot can change its purpose and become an adventitious root. This often occurs as a result of low oxygen levels (flooding, burial) or high ethylene levels (pollution). Several different plant hormones can trigger this response, including auxins, cytokinens, gibberellins, abscisic acid, and ethylene. These roots are the ones that appear after you break a Jade plant leaf off of a parent plant. The wound dries and new cells form just underneath the callus. Some of those cells turn into adventitious root cells that allow the leaf to become an individual plant.

Adventitious stem and brace roots on corn (Jamain) CC BY-SA 3.0

Where are adventitious roots found?

Adventitious roots resulting from normal growth are classified according to where they occur on the plant:

• Nodal roots grow from leaf nodes. (Strawberries, grass)
• Stem roots grow along stems between nodes. (Ivy)
• Junction roots form where normal roots and shoots join. (Beans)
• Hypocotyl roots form on the embryonic stem found above the root and below the seed leaves, or cotyledons. (Beans, tomatoes)
• Crown roots are nodal roots that form below soil level. (Wheat, corn)
• Brace roots are nodal roots that emerge above soil level. (Corn)
• Prop roots grow from tree trunks to provide stability and to improve aeration. (Mangroves)

Adventitious stem roots (Nzfauna) CC BY-SA 3.0

Why do plants create adventitious roots?

As with everything else plants do, it’s all about genetic survival. Under normal conditions, many plants have evolved to use adventitious roots to access more food and water, provide greater stability, or to procreate. Bulbs use adventitious roots to send new bulblets out over a greater area. When plants are faced with conditions such as flooding, stem burial, grazing, and nutrient deficiencies/excesses, adventitious roots are used to help the plant recover. These roots may form at any of the above mentioned locations, depending on the stressor and the plant species.

How do we use adventitious roots?
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Adventitious roots are why we can propagate many plants from cuttings. Grape, apple, succulent, and stone fruit species are commonly cloned in this way. Tomato plants are often planted with the first set of stems buried below the soil level. This stimulates adventitious root growth, providing the plant with more water and nutrients, ergo better tomatoes. Tomato and pepper plants are often propagated with cuttings, to extend the growing season. Plants grown from cuttings tend to mature more quickly than those started from seeds. Many herbs, such as basil, rosemary, thyme, and sage can be grown from cuttings, thanks to their ability to generate adventitious roots.