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Plant Leaves Reveal Clues About Crop Health

Photonics Spectra
Apr 2008
Researchers are seeking to develop early-warning systems.

Michael A. Greenwood

For many farmers, the possibility of crop failure from diseases such as root rot is an ever-present worry.

The anxiety is compounded by the fact that, by the time corn, potatoes, cotton and other mainstay crops start to exhibit the tell-tale symptoms indicating that something is wrong, it may be too late to do anything about it.

Investigators at Texas AgriLife Research and Extension Center in Lubbock, part of the Texas A&M University system, are looking for a way to provide farmers with a timely and relatively inexpensive detection method that could allow crops to be saved before full-scale devastation sets in.

Researcher Christian Nansen said that the way a crop’s leaves reflect light can provide the necessary notice, even if the hallmarks of an impending disease still are not visible to the eye or if the infestation is occurring underground in the plant’s root system.

Because even subtle changes in a plant’s metabolism as a result of the early stages of stress — whether from a disease, insect damage or lack of water or nutrients — can change the way that light is reflected, Nansen is using a hyperspectral camera to examine the reflected light from plant leaves. Hyperspectral technology already is used from the air to determine the condition of large swaths of plants, and at ground level, it is used to collect profiles with handheld devices that are clipped onto plant leaves. However, Nansen is seeking to develop something in between: a ground-based system that can be attached to a vehicle or irrigation system for high-resolution, high-volume analysis. The technique has the added benefit of being nondestructive to the crop and of delivering real-time results.

TWPlant_Fig1_Corn.jpg

Figure 1. Members of the research team collect hyperspectral data from corn plants grown under various water regimes and with or without infestation from spider mites. Images courtesy of Christian Nansen.


So far, Nansen has experimented with biotic stress associated with four common crop maladies: zebra chip in potatoes, spider mite stress on corn (Figure 1), root rot in cotton and salt stress in cotton. Each crop was scanned with a hyperspectral imaging spectrometer from Resonon Inc. of Bozeman, Mont.

The camera’s objective lens is optimized for the visible and NIR spectra, and the camera collects 160 bands in the range of 435 to 769 nm. Spectral image cubes can be collected under controlled light conditions or can be used with sunlight as a light source.

TWPlant_Fig2_Cotton.jpg

Figure 2. To the naked eye, cotton leaves look similar; however, in response to salt stress, the hyperspectral profiles of four leaves show considerable variation in reflectance in the near-infrared range.


Nansen said the technique revealed that crops suffering from some malady exhibited a striking increase in spectral reflectance that was particularly prominent at around 700 nm (Figure 2). Plants that are under stress from disease exhibit an increased metabolic rate, while their photosynthetic rate goes down. The researchers still are collecting spectral profiles of healthy and unhealthy plants and are developing classification algorithms. Nansen discussed the technology at the High Plains Vegetable Conference in Canyon, Texas, in January.

In the future, the group intends to expand its testing to other crops and to other diseases. Nansen said that the research potentially could last for years and that he eventually would like to extend it to determining the biofuel potential of certain plants.


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