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Instrument Measures Fluorescence From Chlorophyll in Plants

Effective discrimination against solar background radiation is achieved without critical optical and mechanical parts.

An optoelectronic instrument, known as a plant fluorescence sensor, is being developed for use as a working tool in agricultural settings. This instrument is a remote, passive monitor that provides a means of discerning plant stress at very early stages. With sufficient warning, the user could provide timely applications of fertilizer, water, and/or pesticide to achieve maximum crop yield at minimum cost. Figure 1 presents two views of the plant fluorescence sensor. The instrument is the subject of U. S. Patent 5,567,947.

Measurement of steady-state plant fluorescence offers the possibility of determining the physiological status of a green plant. The magnitude of plant fluorescence and its spectral (color) distribution is sensitive to a number of factors which are related to the ability of a plant to perform photosynthesis (the process by which green plants convert atmospheric water vapor and carbon dioxide into sugars and oxygen, using sunlight as fuel). For instance, the light capture efficiency of the plant is dependent on the type and amount of pigment molecules (such as chlorophyll) which in turn is dependent on adequate fertilization. Plants stressed from a lack of fertilizer will limit chlorophyll production and exhibit both lower overall level of fluorescence and shift in spectral distribution compared to healthy plants. Another factor, such as lack of adequate water, can serve to limit the rate of photosynthesis by causing the plant to close its stomata (the openings which allow the leaf to draw in carbon dioxide and water vapor); when this happens, the level of plant fluorescence will generally increase. Thus, measurements taken with this sensor can guide growers in the allocation of resources such as irrigation water, fertilizer, and pesticides.

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Figure 1. The Plant Fluorescence Sensor in the left view is lying, with its casing removed, on a utility cart. In the right view, it is installed on a high-clearance vehicle for monitoring a Nebraska corn field.
Under sunlight, the chlorophyll in plants fluoresces at wavelengths from about 660 to 800 nm. The major problem in measuring this fluorescence is to discriminate against scattered sunlight, which can contribute a spurious component to the measurement. The present sensor is of the class of apparatuses known as Fraunhofer-line or spectral-line discriminators, but this sensor differs from others of its class by virtue of a unique design that exploits the spectral absorption lines of oxygen in such a way as to obtain enhanced spectral discrimination at lower cost. The desired spectral resolution and discrimination are achieved without need for the highly precise, expensive optical components with critical mechanical adjustments (e.g., Fabry-Perot cavities) that are used in other spectral-line discriminators.