Remote Sensing Researchers Find Unexpected Mechanism Making Stressed Leaves Brighter

The magnified image of a maize leaf on the right appears brighter than the image on the left due to chloroplast avoidance movement, a protective response to water stress.
The magnified image of a maize leaf on the right appears brighter than the image on the left due to chloroplast avoidance movement, a protective response to water stress.

It has been known for decades that the amount of sunlight reflected from leaves increases when the leaf loses water. Researchers at UNL are investigating whether this phenomenon can be used to detect and monitor the initial stages of crop water stress – before the leaves curl, change color, or otherwise look stressed. The goal is to provide farmers, natural resource managers, and scientists with a way to detect plant stress as early as possible. Early detection of stress allows actions to be taken to avoid damage to plants or losses in productivity.

When light strikes an object, it is reflected, transmitted through the object or absorbed within the object. When we “see” a leaf, we see the portion of light that is not absorbed. Visible light is absorbed by plant pigments, such as chlorophyll, and the resulting energy is consumed in photosynthesis – the chemical engine that feeds virtually all life on Earth. If the amount of pigment decreases, then the leaf looks brighter to us. Infrared light is absorbed by water and the resulting energy “heats up” the leaf. If the amount of water in the leaf decreases, then the leaf looks brighter in the infrared light spectrum – invisible to the human eye but visible to scientific instruments.

Experiments conducted on maize in a UNL greenhouse used a hyperspectral photoradiometer (a device which measures light brightness at thousands of wavelengths across the optical spectrum) to confirm the increase in both visible and infrared reflectance when plants were stressed. The researchers, Art Zygielbaum (SNR), Betty Walter-Shea (SNR) and Tim Arkebauer (Agronomy & Horticulture), were surprised to discover that although the amount of water decreased significantly during the early stages of stress, the amount of chlorophyll did not. The increase in visible light reflectance was not, therefore, caused by a change in chlorophyll. This counters the long-held view that changes in visible light reflectance are due only to changes in the amount of leaf pigment.

Looking more closely at the plants, literally, and obtaining microscopy images like those presented here, demonstrates that increasing visible reflectance in the initial stages of water stress results, primarily, from a process called chloroplast avoidance movement. Chloroplasts are components in leaf cells that contain pigments such as chlorophyll. Chloroplast movement is a plant protective response to stress which reduces visible light absorption by minimizing chlorophyll exposure to light. The protective response is needed to prevent detrimental compounds, which are produced in stressed plants, from damaging the plant. Under normal conditions, chloroplasts are positioned to maximize light absorption. Under stressed conditions, the chloroplasts move and pile up against the cell walls so that they shadow each other and reduce light absorption, allowing for more light to be transmitted through or reflected from the leaf. Hence light absorption is decreased even though there is little change in the amount of chlorophyll. The research has demonstrated that changes in visible light reflectance from leaves of stressed plants results from at least two processes, changes in the amount of pigment and chloroplast movement.

Thus far experiments have been conducted at the leaf level (leaf reflectance and transmittance, leaf-level photosynthesis, and leaf water relations). Research is being conducted in a “bootstrap process.” Initial experiments were internally funded to establish the validity of the research and research methods. Results like those reported will be the basis for solicitation of outside research funding. The initial research was conducted in greenhouses at UNL’s East Campus where growth conditions could be controlled. The next step will be to conduct this research in the field to see whether or not the leaf reflectance change is observable at the canopy level. If so, observations will move to higher platforms – airborne and satellite based sensors. In the future, experiments will be extended to measure plant canopy properties which can be sensed remotely, i.e., canopy reflectance and gross primary production. CALMIT’s “Hercules” research platform and remote-sensing aircraft will facilitate these analyses.