Aeroponic Science: Root Zone Droplets
Aeroponic systems such as the Tower Gardens, pictured above, offer creative ways to optimize floor space usage. Photo credit: Benjamin Wolf
Aeroponics is a soilless plant culture method with origins in research studies. Given the observation that plants in tropical, humid climates grow and thrive with their roots dangling in the air, scientists were eager to apply these conditions in the laboratory in order to gain visual access to the roots of a plant as it grows and matures. In aeroponic systems, a nutrient is misted in the root zone of a growing crop, providing good aeration, water, and nutrients. One of the things that I found interesting about the early days of aeroponics in the 20th century was its application to trees. It seems so ambitious! Why bother with small, well behaved model organisms like Arabidopsis thaliana when you can go whole hog and grow a bunch of aeroponic apple trees? Interestingly, W.A. Roach’s rationale for aeroponically growing trees was because hydroponic (or “water culture” as it was called in his paper) apple tree roots became “covered with a slime of fungi and bacteria and have then died”. No reference was given for this statement, so my guess is that Dr. Roach was probably venting personal frustration with prior apple tree experiments. “Water culture” must have been all the craze back in the ‘50s. Hoagland and Arnon revised their nutrient formulation in 1950, and this formulation became the basis of all commercial hydroponic nutrient mixes. I’m sure researchers were scrambling to apply this new Hoagland and Arnon solution to their pet crops. I guess in some instances, like Dr. Roach's apple trees, there were still many operational challenges ahead.
Following the proofs of concepts in the 1950s, the ‘60s and ‘70s saw experimentation in commercial applications of aeroponics in an effort to maximize the use of floor space in a growing area. Operational complexity aside, the benefits of aeroponics are several: disease propagation is minimized in the root zone, water use efficiency is maximized, the roots are amenable to colonization by beneficial microbes, and the grower is granted control beyond that of soil and hydroponic methods. Want to feed your plants one nutrient mix during the day, and a different one at night? Can’t do that with hydroponics! Want to grow plants in low gravity? Think aeroponics!
The variables and the mysteries
Speaking of low gravity, one of the claims to fame for aeroponics was NASA’s interest in the technique. I would say that this is one of the more interesting, and yet confounding aspects of aeroponics knowledge. You see, NASA realized that it might not be very practical to haul all the food for a space voyage right from the get-go. During a long voyage, say a manned mission to mars, it would make more sense to grow food in space than to spend the rocket fuel and space to haul it along. Given all the considerations, aeroponic crops seemed like the best bet. In comes the protagonist of the crops-in-space story—Richard J. Stoner. Together with researchers from University of Colorado, Stoner was funded by NASA to develop methods of growing aeroponic crops in space as part of a Phase I SBIR grant in the 1990s.
And here’s where things get complicated. I wanted to read through Stoner’s work as a starting point for understanding aeroponics, but quickly started to hit some roadblocks. First of all, I see a lot of references in aeroponics articles and literature to Stoner’s 1998 SBIR report, but I can’t seem to find it on the web. It’s one of those things that is either confidential or is just not published on the web. There’s no DOI associated with it, and so this research is a lot harder to access than your typical academic publication. This, along with the lack of peer review in SBIR reports, makes it very hard to perform something important with Stoner’s findings in science—replicate them. It’s a shame, because a lot of the claims about the interaction of plant roots with water droplets seem to come from this report, and it would be a benefit to the scientific community to use Stoner’s methods and findings as a starting point. What is accessible, however, is a technical paper published by Stoner, and the U of Colorado group for the Society of Automotive Engineers. What I found therein is a lot of discussion on how droplets interact with roots under simulated microgravity conditions. This is important because most of us Earthlings perform aeroponics under normal gravity conditions. Microgravity can induce profound effects on plant root development, which can be a confounding variable for understanding aeroponic root development in Earth’s gravity. For example, auxin, a plant hormone that plays an important role in regulating root development, is itself reliant on gravity to properly distribute itself within a plant. Another example: plant cell cycles seem to be affected by microgravity, resulting in a divergence between cell growth and cell proliferation, something that is normally correlated under normal gravity conditions. Things like this can have deep implications for growth in microgravity vs. 1 g aeroponic systems.
Microgravity is a unique experience for plants and humans alike. Photo credit: NASA
Research findings on droplet sizes
OK, so what do we know about droplets and microgravity? It comes down to the momentum of the droplet. Remember in physics class that momentum is the product of mass and velocity. A 1-micron (1/1000 of a millimeter) droplet under most conditions carries insufficient momentum to break from the air stream that goes around the roots. You can increase the velocity of a 1-micron droplet to increase its momentum, however this takes an impractical amount of energy. You’re better off increasing the mass of the droplet by generating larger droplets. Great, but now we have to factor in gravity. Big droplets will eventually fall out of the air and pool at the bottom of your aeroponics system under 1 g. I. A. Lakhiar et al. suggest that this happens when a droplet reaches about 100 microns, or about 0.1 mm. But aren’t small droplets equally affected by gravity? Yes, however because of their smaller mass, they exhibit a lesser terminal velocity. If the terminal velocity gets small enough, then the droplets can get “pushed around” via other environmental factors such as convection currents. I wouldn’t take these droplet size guidelines as hard rules, however, because environmental factors such as temperature and ambient humidity can also play roles in how the droplets interact with their environments.
It appears that Stoner also discovered differences in root morphology as it relates to droplet size, stating that too small a droplet causes excessive root hair formation at the expense of lateral roots. Again, I can only take that claim as-is from a second-hand reference because I don’t have access to Mr. Stoner’s original research. Also, I have reservations about lateral root development claims from microgravity studies (see paragraph above for why). That being said, let’s look at some peer-reviewed research experiments to see exactly how much of a role the droplet sizes play.
First off is a pretty recent study by I. A. Lakhiar et al., which looked at the effects of various aeroponic droplet sizes on the growth of lettuce. The specific cultivar wasn’t mentioned. They used three different atomizers: an air-based atomizer (droplet size: ~23 micron), an airless atomizer (droplet size: ~46 micron), and an ultrasonic fogger (droplet size: ~3 micron). The grew lettuce for 45 days in an aeroponic system prior to harvesting, using a tweaked Hoagland’s solution (called South China Agriculture University leafy vegetable solution B). Misting was performed in a 20-min ON, 3-hr OFF interval. Overall, the growth the lettuce was best with the airless atomizer, however the other two atomizers produced growth within 10%. Polyphenols and antioxidants were also the highest with the coarsest droplets, being about 20% better than the poorest performer, the ultrasonic fogger. Yes, the bigger droplets made significant differences, however the difference wasn’t really night and day.
In another lettuce study by Hikosaka and colleagues, the researchers set up trials with two different kinds of droplets: a fine “dry fog” with a particle size of < 10 microns, and a “semi-dry fog” with particle size of 20 microns. The misting system was continuous. Lettuce fresh weights were higher with the semi-dry fog, however the root mass primarily accounted for this difference. Interestingly, both of these treatments performed better than a deep flow hydroponics control. Hikosaka et al. also investigated the effect of air speed in the nutrient zone, comparing a 1 m/s vs. a 0.1 m/s airflow, and revealed that the slower air speed resulted in a 20-30% increase in yield in the fresh weight of the leaves. Something that I find interesting is that the nozzle for making the “dry fog” is air-assisted (the authors don’t mention if the “semi-dry fog” nozzle is also aerated). In both this and the Lakhiar et al. study, air-assisted droplet generation seems to be a less optimal solution. Two knobs are being tweaked in these studies: air assistance in droplet generation and physical droplet size. What I can tell is that bigger droplet + less air assistance = more fresh crop mass, however I don’t see enough evidence from either of these studies to find out which is the culprit. Additionally, the extra airflow appears to negatively impact the growth of the crop.
This is giving me some Kratky non-circulating hydroponics vibes. In Dr. Hideo Imai’s paper on non-circulating hydroponics (Imai and Kratky worked together on developing e non-circulating method), roots differentiated into oxygen, or “O” roots, and nutrient-water, or “NW” roots. Not only are the O-roots specialized for aeration, but they are not well-suited for nutrient uptake anymore. O-roots are damaged if they get submersed in a nutrient solution, which is why you don’t want to top off a non-circulating, Kratky system. On the other end of the spectrum, Tibbits et al. reported that potatoes growing under continuous misting in aeroponics systems were sensitive to system failures of the misting system, suggesting that these roots are more NW-like and are damaged by exposure to air. This is in contrast to Stoner's and Lakhiar’s reports, in which plant roots are perfectly fine with pause periods between misting. Perhaps something similar is happening in a highly aerated aeroponics system, whereby the roots are becoming more O-like and losing NW character. It’s hard to say, really. What I would like to see is 1) a good microscopic and biochemical characterization of the root differentiation in a Kratky non-circulating system, and 2) use this as a benchmark to examine roots of aeroponically-grown crops.
Thoughts and conclusions
Aeroponics is a technique with great potential in water-limited situations, as well as in more exotic scenarios, such as space missions. Indeed, the focus on aeroponics in space has generated a pool of interesting research findings, but we have to be very careful in the application of those findings to Earth-based systems. At the very least, parallel experiments need to be conducted under gravity in order to validate the microgravity experiments. Add in the fact that much of the aeroponics interest lies in growing root vegetables like potatoes and yams, and I can understand now why some hobbyists can get frustrated in finding information on this technique. There’s a lot of scattered information with little overlap with which one can find emerging trends. What I can say from my secondary research here is that droplets in 20-40 micron size range seem to work well for growing leafy greens like lettuce, and perhaps this is also the case for potatoes. Here’s the deal, though. Beyond a certain particle size, say 10-20 microns, growth optimization becomes more of an issue of managing root morphology, oxygen uptake, and nutrient/water uptake. Yes, you can tweak particle sizes, but you can also alter spraying intervals, as well as air flow. All of these variables are going to have an effect on root morphology, which in turn will affect the nutritional uptake rates of the plant.
But of course other variables are at play here. Changing the length or shape of a root system is going to alter the microenvironment. The roots on the perimeter of a root bundle are not experiencing the same environment as those in the inner core. Thick roots, wiry roots, hairy roots, or giant yams with roots are all going to create different local environments, and each system will have a different optimum. The good news is that, at least, from the lettuce studies, plants seem to acclimate to their environments and do the best they can. In doing so, plant roots can assume different shapes and sizes. This is a phenomenon called phenotypic plasticity. Despite the different droplet sizes, all of the plants in the studies performed well in terms of growth—differences were within ~20% of each other. For the hobbyist, I would imagine that this small performance difference isn't enough to warrant a deviation from the simplest solution available to them, whether it be ultrasonic misters or pumped spray nozzles.
An example of root phenotypic plasticity in romaine lettuce growing in a non-circulating (Kratky) hydroponics system. The roots on the top are exposed to a moist air headspace, and develop numerous root hairs and short, wiry horizontal roots. The glassier, hairless roots below are submerged in the nutrient solution. Each group of roots creates and experiences a different microenvironment.
As a future direction (and please, let me know if this is already going on in aeroponics) I think the field of aeroponics can benefit from 3D image analysis and modelling research of plant roots. Good databases of root image data can pave the way for simulations and machine learning, using adjustable parameters such as air speed, humidity, droplet size, etc. Plant root phenotyping is already an active research area (see here and here for examples), and I think simulations are the logical next step. These can be used to predict how roots, and perhaps even a whole plant, can grow in a given environment. Such models can make great starting points for the deployment of commercial systems in different geographies. Connecting the dots between root morphologies and root biochemistry will also prove beneficial. That being said, I think this field, given the immense variable space associated with it, can benefit from some standardization of experimental conditions. As it stands right now, it’s hard to draw comparisons between the results of experiments conducted by different research groups. Hopefully a clearer picture will be presented as research in the field moves forward.