A plant with dry, yellow leaves is obviously unhealthy and usually beyond help. To minimize crop damage and loss, agricultural and horticultural producers are seeking a way to detect stress in plants before such visual clues emerge. A new sensor that measures chlorophyll promises the real-time monitoring of plant health. The Plant Fluorescence Sensor, which was developed by Aerodyne Research Inc. for NASA's John C. Stennis Space Center in Mississip pi, is a Fraunhofer line discriminator that detects the fluorescent response of chlorophyll to sunlight. Because it senses at atmospheric oxygen absorption bands, it is largely immune to noise from background radiation. The sensor collects and transmits light from the fluorescing plants through one of two selectable bandpass filters that allow the transmission of A-band (760 nm) or B-band (690 nm) radiation. A 6-inch-diameter quartz bulb, filled with an argon-oxygen mixture and used as the induced-fluorescence cell, is housed within an integrating sphere. The oxygen absorbs some of the light and re-emits it as photons that are detected by a photomultiplier tube. The oxygen in the cell absorbs light at exactly the same wavelengths as the oxygen in the atmosphere, so the residual response to sunlight is minimal. In the field, the instrument can collect two-band fluorescence information from as far as 200 meters from target plants. Initial field tests indicate that the plant fluorescence sensor can detect water and nitrogen deficiencies as well as other applied stresses. Let it Grow Aerodyne anticipates that users of the sensor will have to take intermittent measurements over the growing system to determine whether the plants are experiencing stress. The company is collaborating with growers, agricultural support services and researchers to develop a fluorescent signature database. Aerodyne envisions that, if this task can be successfully completed, the plant fluorescence sensor will have widespread use in precision farming and greenhouse applications, and in biotechnology research.