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I can't find info on NASA site, but saw a picture of romaine on ISS and am wondering how it maintains its shape in zero-gravity- why doesn't it spread out, what makes it grow up...?

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    $\begingroup$ I suppose plants grow towards light rather than "up". $\endgroup$ – Roman Reiner Aug 11 '15 at 15:49
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The structure of lettuce is determined by a combination of genetics and environmental effects. What the growth of the lettuce on the ISS indicates is that gravity is negligible in terms of its effect on lettuce growth. Atmospheric pressure would have a more noticeable impact.

The direction in which a plant grows (shoots up, roots down) is determined by light (for leaves) and water source (for roots). The process by which the plant shoot moves toward light is called phototropism, and gravity has negligible impact here. With the roots gravity does have some impact (geotropism), but the ISS project was able to overcome this by using stakes to guide the roots to the water. This water-seeking quality of the roots is called hydrotropism.

Where gravity would have an impact is in the ultimate height of a plant. Trees are ultimately limited in height by gravity, but if somehow they evolved in a microgravity environment they could potentially grow a lot taller.

Further Reference:

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