Robert Raguso/Cornell University

The hawkmoth Manduca sexta moments before entering a Datura wrightii flower to feed on its nectar

February 12, 2023

New Findings on Humidity Gradients Suggests a Potential Role in Signaling for Pollinators

Print More

The flower Datura wrightii actively maintains humidity gradients, discovered in a Dec. 2022 study by the Raguso Lab, led by Prof. Robert Raguso, neurobiology and behavior. This has potential implications for signaling to pollinators. 

The study was based off of findings from a 2012 paper from the Proceedings of the National Academy of Sciences titled “Floral Humidity as a Reliable Sensory Cue for Profitability Assessment by Nectar-Foraging Hawkmoths,” describing the establishment of these humidity gradients in the flower Oenothera cepitosa through the evaporation of nectar. 

A humidity gradient refers to the difference in water content. In this case, it indicates a difference between the flower’s levels of moisture and ambient levels.

This process resulted in a humidity gradient approximately four percent higher than the surrounding environment, which attracted more pollinators than the less humid counterparts, leading scientists to believe that these gradients revealed information about the availability of nectar to pollinators. 

Ajinkya Dahake grad worked with fellow lab members Caleb Vogt grad and William Kandalaft ’21 to executive the study. They also collaborated with Piyush Jain grad from the Stroock Lab. 

In their study, they used findings from the PNAS study to investigate the changes to the humidity gradient when the morphology, or shape, of the flower was changed. 

Unlike the O. cepitosa flowers which have a flatter surface, D. wrightii is a large funnel shaped flower.

“You can imagine that if it’s a flat surface, the humidity is going to easily escape versus in the funnel shaped flower,” Dahake said.

Measuring the humidity of the D. wrightii revealed that these flowers are around 30 percent more humid than the ambient humidity, a significant difference between the O. cepitosa flowers. 

Following this finding, Dahake and his fellow researchers were met with yet another surprise. It was previously shown that removing nectar decreases the humidity gradient for O. cepitosa flowers. The team then expected similar results when nectar was removed from the D. wrightii flowers. However, they found no difference in humidity between undisturbed versus nectar extracted flowers. 

This disproved the idea that nectar was the main contributing factor for floral humidity, leading them to question the cause of such humidity.

Dahake considered the idea that the pollinators of D. wrightii, the hawkmoth Manduca sexta, displace the trapped humid air through the flapping of their wings as they land and interact with the flower. In doing so, they predicted that this air displacement would also lead to a rapid decrease in humidity. 

“But actually, we did not see that. The humidity always stayed at least 15-20 percent above ambient [levels]. And when the moth exits the flower, it reconstitutes its original levels within about 30 seconds,” Dahake said. 

The team then discovered the existence of stomata on D. wrightii flowers in a higher number than observed in others like O. cespitosa. Stomata, which are often associated with leaves, are small openings that regulate gas exchange and the release of water vapor which is important for humidity.

Testing this hypothesis, they blocked the stomata using Vaseline, which blocked gas exchange, and discovered that the humidity decreased by 50 percent, suggesting that active processes using the stomata are responsible for the humidity gradient. 

In light of these findings, it raises the question of what exactly the humidity is signaling for pollinators. The lack of direct evidence on the purpose of this humidity gradient in relation to pollinators has prompted further research by Dahake and his fellow researchers on what the signal indicates and how applicable their findings may be to other pollinators and plants. 

Interestingly, their study found that hawkmoths showed a preference for more humid flowers, even when the team removed the nectar reward. When they blocked the moths’ ability to sense the humidity gradient, however, there was no difference in preference. 

Moths possess special organs on their antennae that provide them with information about how dry, moist and cool the air is. The observed lack of differences following the inhibition of this hygrosensation, or sensation of humidity, ability hints at an innate preference for it which may have evolutionary implications. 

In determining the relationship between humidity and signaling for pollinators, it is especially important to consider what type of signal it is defined as. This is significant in beginning to understand the relationship between the signal’s meaning for pollinators and the implications for the flowers, as well. 

The study suggests that humidity could be an icon signal, a signal where the form of the signal is similar to its content, according to Dahake. 

“For example, in an eyespot of a butterfly, the form of the signal is the spot, the eyespot,” Dahake said. “But the content that it’s trying to convey is an eye. So it matches the content very well.”

It is especially important to define the signal in order to understand communication between animals and the symbiotic relationships between species such as mutualism where two species work together to reap benefits, or parasitism, where only one species benefits as the other is harmed. 

“In that respect, here’s an animal and a plant, two different kingdoms, that are in a mutualistic relationship with each other. Broadly speaking, the question of how plants and pollinators interact with each other such as shaking hands or taking advantage of each other is an evolutionary puzzle to think about,” Dahake said.