Entomology professor Esther Ngumbi studies how two varieties of tomato plants and tobacco hornworm larvae respond to flooding. The hornworm caterpillars are enclosed in plastic bags affixed to the tomato plants.
Photo by Fred Zwicky
CHAMPAIGN, Ill. - I can barely hear Esther Ngumbi over the roar of greenhouse fans as she shows me around her rooftop laboratory in Morrill Hall. The benches are full of tomato plants, and the tomatoes don't look good. Half of the plants are submerged in bins of water. Their leaves are yellow and withering. Some of the dying tomatoes have flowered. I see one or two baby tomatoes on a couple of spindly plants.
Ngumbi and her colleagues look for differences between plants grown in normal conditions and those that are flooded.
Photo by Fred Zwicky
This isn't the only torture inflicted on the tomatoes. Someone has tied little baggies to their stems. Inside the bags, fat green caterpillars are chowing down on the tomato leaves.
Ngumbi studies how the combined stressors of flooding and herbivory affect crop plants. In these experiments, the flooded and unflooded tomato plants are subjected to herbivory by the larval form of the tobacco hornworm.
Photo by Fred Zwicky
Entomology professor Ngumbi has questions - lots of them - and this is how she's set out to answer some of them. She is purposely flooding the tomatoes to see how they might respond to flooded conditions in farmers' fields - a scenario that is becoming more common as a result of climate change.
Each variety of tomato studied emitted a unique mixture of chemicals before and after flooding, the researchers found. These chemical emissions are essential to plant communication and defense.
Photo by Fred Zwicky
"In nature, there are many stressors on plants during flooding," Ngumbi says. "Once the tomatoes get flooded, they're already weak, so most likely they will be attracting insects, which like to eat weaker plants. We're investigating how the plants deal with the combined stress of flooding and herbivory."
Seeking oxygen, the flooded plants send out new adventitious roots.
Photo by Fred Zwicky
This explains the caterpillars. They are the larval form of Manduca sexta, the tobacco hornworm. They are feasting on one of the two heirloom tomato varieties Ngumbi is using in the experiment: Cherokee purple and striped German.
Half of the tomato plants in the greenhouse are not flooded, allowing the team to compare the stressed plants with those grown in more common conditions. But there are more investigations going on here.
"Also, within this experiment, we're looking at the microbes," Ngumbi says. "We want to understand how the microbial community changes in flooded conditions."
Microbial life in the soil is another factor that affects plant responses to flooding and herbivory, Ngumbi said. And flooding changes the composition of soil microbes.
Photo by Fred Zwicky
One of Ngumbi's key focuses is how soil microbes influence plant health and productivity. She's fascinated by mycorrhizal fungi, which form intimate associations with plant roots, offering essential elements like nitrogen to the plants in exchange for glucose supplied by the roots.
The tomato plants are all growing in soil from an Illinois farm, but half were also inoculated with mulch from a local farmer who has developed his own recipe for nurturing mycorrhizal fungi in the soil. Ngumbi wants to see if this inoculation makes any difference to the plants' ability to defend themselves from the fat caterpillars.
To measure plant defenses, Ngumbi's team collects samples of gases emitted by the plants and screens them for volatile organic compounds, the chemicals plants use to ward off bugs that would eat them.
