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Great Plant Debate

Comments on the Hardware Design
Part I of the Great Plant Debate

by A. Gerard Heyenga, Ph.D.

Senior Research Associate
NASA Ames Research Center

For Gerard's Biography and picture see:

Over the last few weeks I have had the pleasure to read through the designs submitted for the SMORE project. While the shortage of time has limited my response to brief comments, I feel that each design has contributed towards at least some aspect of plant growth in space. It is indeed most impressive and a credit to you all that collectively you have addressed the primary issues involved in the design of a space plant growth chamber.

Here are some additional suggestions based on my own flight studies conducted on the space shuttle:

One of the initial design criteria to consider is the type of experiment to be conducted in a space flight environment!

(1) A typical investigation may involve looking at what effect the absence of gravity may have on plant growth and development. This in turn helps us understand the importance of gravity on our planet.

This type of research goes under the wonderful title of space plant physiology.

This type of investigation needs special care and a plant growth chamber which comes close to providing all the growth conditions the experimental plant is accustomed to on Earth (called a physiological environment). The ultimate goal is to compare plants grown in precisely the same environment with the exception that the Earth-grown plants experience a normal gravity force while the plants in space receive none. This study becomes very difficult to evaluate if the chamber conditions are variable.

(2) Another type of investigation jumps the gun a bit! It presumes that plants will grow well in a space flight environment and therefore examines the extent to which a crop can contribute towards keeping people alive by providing oxygen and food and absorbing carbon dioxide (known as regenerative life support).

In this case the investigation may concentrate on achieving maximum efficiency from a crop (and so worries less about the plant's happiness). This may include using higher light levels, temperature and carbon dioxide. It may even include breeding a new special crop species which is better suited to the highly artificial and enclosed environmental conditions of a space craft.

For this crop idea to be useful you would need a lot of plants, a relatively large growing area, a lot of electrical power and effort. The big question is, does it make sense for either the Shuttle or a space station!

For example, which option would you take if you had the choice of bringing your lunch to school OR converting most of your classroom into a farm or garden and growing everything you would need for your lunch!

At this stage it seems to make a lot more sense to bring your lunch and everything else with you into space for any period less than two years.

However, this approach would make sense for a large planetary base as suggested for the Moon or Mars (and maybe even on a futuristic Earth!). Here you would have the opportunity to build the support facilities required.

With respect to the design of a growth chamber:

As mentioned, the primary requirements necessary to keep a plant alive include the supply of light, a suitable gas environment (e.g., oxygen, carbon dioxide, etc.), a suitable temperature, water and nutrients. Supplying these in just the right amount in an enclosed environment raises a lot of difficulties.

For example,

(a) supplying sufficient light in a chamber (even with the coolest lights) causes a build-up of heat,

(b) the introduction of water into a chamber by plant transpiration causes an increase in humidity and

(c) the loss of carbon dioxide and production of oxygen during photosynthesis are all factors which need to be corrected if the experiment is to remain stable and happy.

This all adds up to a lot of technological support. The supply of water and nutrients in microgravity is another challenging problem. My approach has been to make things as simple as possible.

For example, in preparation for a ten-day plant experiment on the Shuttle using the standard Plant Growth Unit (PGU) (see my pictures under http://quest.arc.nasa.gov/smore/events/designs/heyenga.html ) the entire water and nutrients supply was made into a semi-solid gel which was enclosed in a special plastic 'pack' which allows gas diffusion. Plants were grown directly in packs which were placed inside the 'Plant Growth Chambers' housed in the PGU.

This approach has proved successful for the cultivation of plants for periods up to a few weeks depending on the rate of water loss. One of the advantages of using gel is that you can examine the interesting distribution of roots (see images ) not otherwise possible in a soil matrix.

A further development of this approach includes the passive (wicked) uptake of water into a pack which now allows the long-term support of plants with the great advantage that no pumps, valves or tubes are required.

Since there has been a strong "hydroponic" statement in the discussions, which indicated that "Soil is never used," it may be useful to give an additional view.

First of all, I think it is fair to say that we all tend to use the term "soil" in a very loose manner which in fact includes such broad properties from clay, silt and sand, to properties of high fertility to low fertility.

One of the interesting efforts involved in growing plants in microgravity has been to find a soil "type" which is rough enough (porous) to allow adequate aeration of gases while still allowing the wicked distribution of water, and also contain a good level of nutrients (macro and micro elements).

The advantage of this approach is that such a soil can support plants with minimal effort, i.e., NO power, pumps or sensor, etc., which is obviously a useful thing since we mentioned the scarceness of resources on a space mission. To date a number of such soils or aggregates have been used to support plants in space. As yet no hydroponic system has been used.

Best wishes


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