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I just read a question on the gardening stack site about why plants grow up, to which one of the answers has mentioned experiments on the ISS, and which stated:

experiments on the ISS have shown that plants will grow oriented in a manner such that “upward” (i.e. the stem, leaves, etc.) is toward a light source, even in the absence of gravity. Of course, this depends on the plant being able to determine which way is toward the light source.

But I am wondering now how a plant would grow if it had no way to determine some kind of "up" by either gravity, thermal, or EM (light).

Have there ever been any space experiments to determine how a plant grows if without any external indicators as to which direction it "should" grow?


As suggested by commenters, yes I was indeed assuming a test of this nature would likely involve either no light or a uniform distribution of light from different directions.

For the case of no light, plants do indeed begin to grow in the absence of light, but they appear sick, grow quickly (in an attempt to try and reach light, I think), and die soon.

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    $\begingroup$ Interesting question, but it will be difficult to grow plants without light. $\endgroup$ – Organic Marble Nov 2 '18 at 0:35
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    $\begingroup$ @OrganicMarble: Seeds can and do germinate in darkness. Also, one could illuminate a plant uniformly to allow it to grow without any phototrophic clues. $\endgroup$ – DrSheldon Nov 2 '18 at 3:20
  • $\begingroup$ @DrSheldon good point, the question is worded very carefully. $\endgroup$ – uhoh Nov 2 '18 at 16:09
  • $\begingroup$ Impossible to grow plants with out light. Impossible to grow plants without water. Impossible to grow plants without the chemistry photosynthesis requires. Impossible to grow plants outside the bounds of 50 to 100 degrees F. The complexity is not to be ignored. $\endgroup$ – stormy Nov 6 '18 at 23:01
  • $\begingroup$ @stormy Actually, plants will start to grow without light. I am not an expert, but I have done an experiment where a plant in near complete dark grew, and it even grew faster than its counterpart in the light, which I assume was an attempt by the plant to reach an area where it could get some light (which, of course, failed). The plant looked sick and died early, but it did grow several inches. However, such an experiment need only to have a uniform distribution of light. Darkness is only 1 such uniform distribution. An equally spaced array of grow lamps in all directions could be another. $\endgroup$ – Aaron Nov 6 '18 at 23:47
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Seeds include a plant embryo with a root and a shoot already developed. When the seed germinates, the root and the shoot each elongate through tissue growth at the tip (meristem). Without environmental cues, the root and shoot will each continue to grow in the same general direction they had inside the seed. I emphasize "general" because the root or shoot can branch, and the roots have also been shown to form a helix; in both cases, the overall growth is still in the same general direction.

To prevent plants from growing the "wrong way" into the ground, they have responses called tropisms. These cause the root or shoot to bend as it grows, as much as 180 degrees! The three major tropisms are gravity (geotropism or gravitropism), light (phototropism), and water (hydrotropism). As described below, each has been studied on the ISS, and sometimes in combination.

Although tropism experiments have been hosted in several different facilities aboard the ISS, the EMCS was specifically built for these types of experiments:

The European Modular Cultivation System (EMCS) is an ESA experiment facility that is dedicated to studying plant biology in a reduced gravity environment. It supports the cultivation, stimulation, and crew-assisted operation of biological experiments under controlled conditions (e.g. temperature, atmospheric composition, water supply, illumination, observation, and gravity). The facility has performed multi-generation (seed-to-seed) experiments and studies the effects of gravity and light on early development and growth, signal perception and transduction in plant tropisms.

https://www.nasa.gov/mission_pages/station/research/experiments/explorer/Facility.html?#id=336

An experiment in microgravity and darkness:

Biological Research In Canisters - 16: Investigations of the plant cytoskeleton in microgravity with gene profiling and cytochemistry (BRIC-16-Cytoskeleton) studies the effects of microgravity on the structure and organization of the actin cytoskeleton in plants using the model plant Arabidopsis.

[...]

Seedlings grown in total darkness in microgravity showed more skewing at the roots and more roots forming from shoot tissues than ground control plants.

https://www.nasa.gov/mission_pages/station/research/experiments/explorer/Investigation.html?#id=772

The results of the BRIC-16 study showed fewer seeds germinating, roots growing out of "above-ground" parts of the plant, and the roots corkscrewing as they grew:

The first major physical difference was observed at the root apex and proximal root where extreme screwing occurred in the flight seedlings compared to slightly skewed roots with the ground controls. Another major difference was the greater amount of adventitious roots (roots formed from the stem or leaves) found on the flight samples. Other effects included the percentage of plants arose from seeds were significantly lower in samples grown in flight hardware (FL, GC) compared to the growing seedlings in just the petri dishes. In addition, the endodermal cells (the deepest cells in the outer layer) were significantly smaller in seedlings grown in the BRIC-PDFU system compared to those in the HC. This change in the shape of endodermal cells indicates alterations in the cell wall and appears to be a true microgravity effect.

Another experiment in microgravity, with and without light:

The Characterizing Arabidopsis Root Attractions (CARA) experiment looks at mechanisms at the molecular and genetic level that influence the growth of a plant’s roots in the absence of gravity, and how those change with or without light. Researchers expose one set of seedlings to light, keep another set in the dark, and then examine how each environment influences the patterns of root growth. Some of the plants are also imaged with the Light Microscopy Module on orbit, and at the end of the experiment, all plants are harvested by the astronaut, and preserved for their return to Earth in order to evaluate genes associated with plant responses on orbit.

https://www.nasa.gov/mission_pages/station/research/experiments/explorer/Investigation.html?#id=1020

Gravity, water, and nutrients each examined separately:

Multiple-Tropism: Gravity, Nutrient and Water Interaction of Stimuli for Root Orientation in Microgravity (MULTI-TROP) separately evaluates the role of three stimuli – gravity, water and nutrients – on plant growth. Tropism refers to an organism directional response to an external stimulus, such as plant roots growing downward into soil in response to gravity on Earth. Previous research shows that plant roots grow randomly without specific direction in microgravity, presenting a challenge when developing facilities to cultivate plants in space.

https://www.nasa.gov/mission_pages/station/research/experiments/explorer/Investigation.html?#id=7473

The interaction of gravity and water:

The Hydrotropism and Auxin‐Inducible Gene expression in Roots Grown Under Microgravity Conditions (HydroTropi) experiment has three specific aims:

  • First, it demonstrates that gravitropism (a plant's ability to change its direction of growth in response to gravity) interferes with hydrotropism (a directional growth response in which the direction is determined by a stimuli in water concentration).

https://www.nasa.gov/mission_pages/station/research/experiments/explorer/Investigation.html?#id=753

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  • $\begingroup$ +1 Nice answer! $\endgroup$ – Organic Marble Nov 2 '18 at 11:27
  • $\begingroup$ Wow! I was not expecting so much. Thank you. One issue: you write "root and shoot will each continue to grow in the same direction they had inside the seed," but the quotes you provide seem to suggest otherwise with the corkscrewing and then in your second-to-last quote "grow randomly without specific direction in microgravity". Still, excellent answer, thank you - I have +1'ed, and I hesitate to accept the answer only because it is SE custom to give it a bit more time so as not to discourage other answerers, but I'll return to accept it later. $\endgroup$ – Aaron Nov 2 '18 at 17:50
  • $\begingroup$ @Aaron: I've improved my answer to help with your concern. $\endgroup$ – DrSheldon Nov 4 '18 at 1:37

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