If you’d asked me about soybeans a year ago, I would probably think up images of the tofu and soymilk in my fridge. Little did I know then that I would be working on a project over the summer and would learn much more about soybeans than I ever thought I could know.
This past summer, I planted almost 600 soybean seeds in an experimental garden at the Smithsonian Environmental Research Center (SERC).
The soybean experimental garden in raised beds at SERC.
In addition to learning that soybeans are very resilient, surviving harsh rains and dry sunny days without water, I also learned how soybeans impact our daily lives – even if you don’t drink soymilk. Soybeans are much more than meat and milk replacers; they are actually the largest source of animal feed and the second largest source of vegetable oil in the world. And as I drove home from SERC at the end of that summer, I found that I recognized the small green leafy plants in almost all the fields I drove by – they were all soybeans! To my surprise, I discovered that the United States is the leading producer of soybeans and the second largest soybean exporter in the world, making it an important crop for the US economy.
Because soybeans are such an important crop for the US, many researchers are looking at ways to increase the productivity of soybeans. Although they are the same species, there are many varieties of soybeans made by different seed companies, bred and genetically modified for certain traits. Some soybeans can survive in very little water, some are resistant to fungi, and some are less palatable to insects. Farmers can potentially use this diversity of soybeans to increase their soybean yield.
Soybeans are often planted in monocultures of one variety, but if something like an insect or fungal disease comes through, and the variety is not resistant to this, a farmer will lose their entire soybean crop. Planting in polycultures, with more than one variety, could remedy this issue. In addition, studies have shown that increasing species diversity increases yield while preserving the same nutrient quality in agricultural hay fields and increases ecosystem productivity in grasslands. In our soybean garden, we planted four different varieties to see how they interacted in a polyculture.
We planted the varieties in a 6-plant arrangement so that all the plants could interact. We planted monocultures as 6 plants of the same variety, 2-variety polycultures with 3 plants of each variety, and 3-variety polycultures with 2 plants of each variety. We used all possible combinations, and with treatment replicates there were 588 plants total.
A group of 6 plants comprises one treatment combination. These combinations were either all 1 soy variety, 2 different soy varieties, or 3 different soy varieties.
After planting the soybeans, we measured several factors, including growth stage, height, percent herbivory, aspects of photosynthesis, and other traits. We took measurements on a tool called the Photosynq Multispeq, which measures a variety of photosynthesis variables, including relative chlorophyll, leaf thickness, and the efficiency of the leaf’s photosynthesis. For the traits, we harvested a sample of the leaves from the garden and measured their area, wet and dry mass, toughness, thickness, and used a pressure chamber on them to see if they were water-stressed. We also counted trichomes, which are tiny hairs on the leaf that you can see under a microscope. Even though we have to look at them under a microscope, these hairs are quite large compared to a small insect, so they are likely important in protecting leaves from insect herbivory.
Taking photosynthesis measurements on the soybean plants using the Photosynq tool.
In the fall, we harvested the soybeans and counted and weighed their beans. This was one of the most important measurements, because soybeans are annual plants, and what we consider “beans” are actually seeds which plants pass on to the next generation. Seeds are one of the best measures of a plant’s fitness. We then analyzed the data to answer the question as to how diversity affects productivity of the soybean varieties. Shown below are some preliminary results, and more results and statistical analyses are pending.
First, we found that the number of healthy beans produced per plant likely varies more between varieties than between different levels of diversity.
Healthy Beans
Varieties 31, 32, 67, and 83 with their average healthy bean number per individual plant in monoculture, 2-variety combinations and 3-variety combinations. Varieties 31 and 32 seemed to do better overall, with no clear trend between bean count and diversity.
The bean mass per plant includes all beans, healthy or unhealthy, and also showed more between-variety variation than variation within diversity treatments.
Bean Weight
The average bean mass per plant for each variety (31, 32, 67, 83) in monoculture, 2-variety, and 3-variety polycultures. Variety 67 once again did worse while the other plants produced a greater mass of beans.
In analyzing one of the measured traits, we found that the number of trichomes on a small circle taken from a leaf increased as diversity increased, although this may not be a significant difference.
Trichome Count
This graph again shows the Varieties 31, 32, 67, and 83. Only the monoculture and 3-variety polycultures were measured for trichomes.
Given that trichomes may act as herbivore repellant for plants, soybean plants in polyculture might be more resistant to herbivores. Results show that percent damage from herbivory in Varieties 31, 67, and 83 decreased from monoculture to 3-variety polyculture, although damage increased in the 2-variety polyculture in Variety 83.
Insect Damage
Average percent damage on the 2nd highest leaf on the plant, for all varieties in all diversity treatments.
Although we did not see soybean yield increase strongly with diversity, other beneficial traits such as protective trichomes may increase with diversity. This could protect plants from a potential pest outbreak when planted in polycultures. By continuing to analyze these factors, we will be able to see whether planting soybeans with greater varietal diversity may be beneficial. If diversity increases growth and yield, we may even be able to produce more soybeans with less land and increase our food productivity.
Written by Zoe Read, (recently graduated!) undergraduate student