When contaminated with heavy metals, the color of leafy greens turns purple but this cannot be seen by humans. Researchers have succeeded in detecting this change with the aid of sophisticated hyperspectral imaging technology.
Researchers from Purdue University reported this observation in the
journal Environmental Pollution. They found that kale and basil changed in color to purple when contaminated by cadmium, a toxic heavy metal.
This new method is better than traditional testing techniques for detecting contamination in that it does not require plant damage for analysis. It also produces faster results.
The Purdue hyperspectral system is being used to guide the development of a soil amendment that can help to control contamination.
“It is very difficult to see heavy metal stress in plants,” said Professor Lori Hoagland, who led the study. “We need new tools for it. If we can quickly see it and accurately measure it as the plants grow, we will be better able to develop soil amendments that sequester the harmful metals, as well as identify contamination before it reaches our plates.”
Less evident threat
Commonly found in batteries, cadmium is a typical constituent of phosphate used in fertilizers. It also finds its way, along with other heavy metals, into agricultural soils from waste and pollution leaks.
Cadmium poses a real threat to health in both humans and animals when crops contaminated with it are consumed. Plants feel the effect of taint with this metal but will usually not show clearly visible signs of that.
Hoagland said cadmium contamination is regarded as a “silent killer” in that it cannot be seen in plants, except when the extent is very great. No testing is also done for it in most cases.
Detecting contaminated plants
Hoagland and her colleagues made use of a system that features a “dual pair of hyperspectral cameras.” These cameras are capable of full-spectrum optical sensing from the top as well as the sides of plants. With the hyperspectral system, plants can be scanned from seedlings up to a 15-foot stalk.
According to Purdue’s digital phenomics director Yang Yang, hyperspectral imaging captures much more color bands than the RGB bands that humans can see. This study was the first in which researchers used the technology to probe heavy metal contamination in plants.
Researchers carried plants to an imaging station at specific intervals with the aid of a conveyor belt in a controlled phenotyping facility to examine them. A phenotype is a set of observable traits of an organism that stems from the interaction between its genotype and the environment.
Findings showed that kale amassed more cadmium compared to basil under a similar soil condition. However, basil showed more serious stress symptoms from contamination. Cadmium stress was noticeable only during the early phases of growth.
Thinking that cadmium toxicity stress effect on chlorophyll production would be a good indicator, researchers first probed the green light spectrum. The resulting change in color was, however, very faint.
Hoagland said changes in metabolites as a stress response were found to instead provide a more visible hyperspectral stress signal.
The team made use of a machine-learning algorithm to arrange and categorize the data generated.
An evaluation of “vegetation indices” showed that the Anthrocyanin Reflective Index (ARI) was the most effective for spotting cadmium stress. The researchers went on to work out a detailed vegetation index ratio equation that can be used for it.
Soil amendments to combat contamination
This research, as well as the technology, will aid the development of soil amendments. These materials will be added to the soil to bind heavy metals and keep them from being easily absorbed by plants.
Hoagland and her team have created a soil amendment to help reduce the amount of cadmium that plants absorb. When tested, the formulation slightly reduced the amounts of cadmium in plants. Farmers may need to use more of it to more effectively reduce the heavy metal while researchers continue to work to improve it.
“These amendments are different mixes of bio char, which includes organic material waste and specially treated wood chips burnt at high temperatures,” Hoagland said.
The professor of Horticulture and Landscape Architecture revealed plans to use hyperspectral imaging technology to detect lead and arsenic contaminations.