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Engineering study examines sunflower stem growth

microscope images of vascular tissue at 6, 8 and 10 weeks growth
These microscope images show the cylindrical vascular tissue in a sunflower stem at six weeks, from left, eight weeks and 10 weeks of growth. With growth, the cross-section of vascular tissue becomes a more uniform circle, with an increase in cell diameter and cell wall thickness.

Examining the structure of a sunflower stem as it matures can help both the plant scientist and biomaterials engineer. That’s the premise that Anamika Prasad, an assistant professor in South Dakota State University’s Department of Mechanical Engineering, is putting into practice.

“This is the first study to quantify structural and compositional changes in the sunflower stem at multiple stages of crop development,” said Prasad, noting most of the literature from the engineering side on plants is on wood. Results of the study, which was supported by the SDSU Research and Scholarship Fund and the Department of Mechanical Engineering, are published in the August 2020 issue of Materialia.

Prasad, whose expertise is in materials science and biomechanics, has done research on the structure and mechanics of bone and cardiovascular tissue in collaboration with medical doctors for more than 10 years. However, this is her first experience applying those techniques to plant tissues.

Roy holding a sunflower stem specimen, left, and Prasad
Doctoral student Mukesh Roy and assistant professor Anamika Prasad discuss the vascular structures of a sunflower stem.

Doctors use CT scans of healthy and diseased human tissues “to identify what is going wrong,” she explained. However, a literature search revealed “spectroscopy-based techniques have been studied to diagnose plant diseases, but they are not common.”

This opportunity allows Prasad to “work with plant scientists to bring an engineering perspective to plant diseases and to focus on bio-inspired (composite materials) design.” Her research group is developing the infrastructure to incorporate cellulose nanofibers into structural engineering materials and biomaterials for medical applications.

Implications for plant diseases

To complete the study, Prasad collaborated with SDSU field crops pathologist Febina Mathew, whose research focuses on diseases of soybeans, corn, sunflowers and other broadleaf crops. The plants were cultivated under Mathew’s supervision in a greenhouse to protect them from biotic stressors, such as diseases, weeds and insects.

“This study gives us a different perspective on what is happening within a healthy plant and can be applied to study diseased plants,” said Mathew, who is an associate professor in the Department of Agronomy, Horticulture and Plant Science. In particular, Raman spectroscopy has helped identify specific metabolites, such as crocetin, that may be important for plant stress signaling and host defense during the early stages of sunflower growth.

Inspiration for composite materials

To see how the vascular tissues within the plant stem change as they grow, the researchers examined a non-oilseed sunflower variety, collecting samples at four, six, eight and 10 weeks, which is when the plant begins flowering.

Unlike trees that grow radially outward, annual plants, such as sunflowers, grow longitudinally during their short life cycle. That makes annual plants are a good template for designing flexible polymer composites, Prasad explained. “The stem is a fiber-reinforced structure and cellulose is the building block of that fiber.”

At the four-week growth stage, Prasad and doctoral student Mukesh Roy could only measure the girth because the stem was too soft for sectioning. Because this is a new area of research, they also had to figure out how to analyze the tissues.

Surprisingly, the number of vascular tissue cells do not increase, but the shape and thickness of the cell walls change considerably to accommodate mechanical and biological demands, Prasad explained.

At first, the cells are nonuniform cylinders, but as the plant grows, they take on a uniform circular cross section with their diameter and wall thickness also increasing. Correspondingly, the internal soft food storage cells in the stem pith decrease and the vascular tissues widen to accommodate the flow of water and nutrients.

“All of these internal modifications influence the load-carrying capacity and flow conduction properties,” said Prasad. “If I can figure out how plants accommodate that large strain, those mechanics can be used to design flexible composites.”

Furthermore, different layers within the cell wall grow in unison, while touching one another without breaking apart, Prasad pointed out. Understanding the underlying structural basis of this adhesive contact can provide further inspiration for the design of composite materials.