Last Updated: August 1, 2018

Definition - What does Phototropism mean?

Phototropism is the movement of plants in relation to light. It is considered just one way plants move. This is probably the most notable in seedlings. Light triggers the plant's hormone known as auxins to move to the shaded part of the plant that is not exposed to the light.

In response to the hormone, the plant's cells start to elongate but the cells on the lighted side remain their normal size. This makes the plant bend towards the light. If the plant bends and stretches towards the light too much, it will grow tall and lanky, which is considered leggy marijuana, or leggy seedlings. It's a signal the plants are starving for more light.

MaximumYield explains Phototropism

Phototropism is undesirable in cannabis plants because it will cause the plant to grow tall, lanky, and thin as it stretches and bends to find an adequate light source. Marijuana plants always want to grow so they are pointing towards the brightest light source. It is imperative that growers provide ample and adequate light in the grow room so that the cannabis plants do not undergo phototropism.

A cannabis plant will bend and move in search of the plant’s coveted blue light. Most growers choose grow lights that offer abundant blue light such as metal halides. LED lights provide full spectrum lighting, which is also acceptable for the plant’s entire life cycle.

Other light-related plant movements include photonasty. This process is controlled by the plant, in response to light or its absence. An excellent example of this is a plant opening and closing leaves or flowers at dusk, when light levels drop, or in the morning, when light levels outdoors increase.

Vines do not produce the hormone auxin in response to the light. Phototropism in vines occurs because the light causes the plant to produce a growth inhibitor instead of auxin.

Phototropism is just one way plants move, but it is the one cannabis growers should be more concerned with. Other ways plants move include:

  • Gravitropism and geotropism (movement relative to a gravitational field, or toward the center of the Earth)
  • Thigmotropism (plant growth in response to physical contact)
  • Chemotropism (movement in response to a chemical in the environment)
  • Hydrotropism (growth or developmental response to water)
  • Thermotropism (response dependent upon temperature)
This definition was written in the context of Cannabis
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