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Why Do Plants Bend Towards Light? Phototropism Made Them Do It

Plants reach for light, the giver of life. Plants reach for light, the giver of life.


Plants will bend themselves outside of their natural growth habit to reach into light. A plant on a windowsill turns to face the sun, letting you know it’s not receiving enough of it. This phenomenon is known as phototropism. It demonstrates how perfectly necessary it is for plants to receive light and in the right amount. Although plants may vary in their light requirements, all need to consume light to survive.

A plant does not have a brain, nor does it have a nervous system. But it does have hormones and these hormones control the intricate workings of plant life. In this case, the hormone auxin is responsible for helping plants lean towards light. In the tip of every shoot and root, as well as under woody fiber and bark, is the meristem tissue. This is the only place plant cells divide and thus the only place from which a plant truly grows. It is also where auxin is produced.

Auxin flows from these centers to other parts of the plant, reacting to the presence of sunlight and moving away from it. If you see a plant growing directly upwards, its meristematic areas are receiving a balanced amount of light and its auxins are moving proportionately on all sides downward. In the absence of balanced light, auxins will soon distribute unevenly, not only away from the light but accumulating on the shaded side of a plant.

Interestingly enough, certain parts of a plant will grow depending on the wavelength of light it receives.

Where the auxin accumulates, a plant’s cells will drop in pH and acidify. It is only in this acidic environment that the affected cell’s hydrogen bonds are disrupted and the enzyme expansin can begin to break down the cell walls within the stem. As the cells weaken, they swell and elongate, causing the stem to bend. When the tip of the stem and its meristematic tissue begin to receive balanced light once again, the stem will cease to bend and resume growing straight, remaining forever in its contorted position.

How a plant moves towards the sun is certainly curious. In one sense, it seems almost a conscious reaction that a plant would move shaded parts of itself into light so that it can continue to photosynthesize to its full potential. At the same time, however, it remains an entirely unconscious gesture as it is natural for auxins to move towards where there is the least amount of light regardless.

Phototropism is very pronounced in winter, when days are shorter and plants struggle to get as much sun as they need. Filtered lights will always keep a plant growing straight. Interestingly, certain parts of a plant will grow depending on the wavelength of light it receives. While natural sunlight contains a complete spectrum of light, plants are stimulated most by red and blue light. Blue light corresponds with bushy, vegetative growth while red light can promote flowering. Plant tips respond especially to blue light, the same provided by fluorescent lights and blue LED lights. A blue LED light or cool white fluorescent placed on the other side of a room will balance natural light from a window during daylight hours or can be suspended above plants that receive no natural light at all.

Next time you see your plants leaning one way or the other, you can blame phototropism. Help your plants along with a properly placed light or a nice place to receive sun. Both will make for happy, upward growing green.

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Last modified on Wednesday, 10 July 2013 23:22

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