A passive two-band sensor of sunlight-excited plant fluorescence

We have designed and built a passive remote sensor of sunlight-excited chlo rophyll fluorescence (U.S. Patent No. 5,567,947, Oct. 22, 1996) which provi des for the real-time, in situ sensing of photosynthetic activity in plants . This sensor, which operates as a Fraunhofer line discriminator, detects l ight at the cores of the lines comprising the atmospheric oxygen A and B ba nds, centered at 762 and 688 nm, respectively. These bands also correspond to wavelengths in the far-red and red chlorophyll fluorescence bands. The s ensor is based on an induced fluorescence approach; as light collected from fluorescing plants is passed through a low-pressure cell containing oxygen , the oxygen absorbs the energy and subsequently reemits photons which are then detected by a photomultiplier tube. Since the oxygen in the cell absor bs light at the same wavelengths that have been strongly absorbed by the ox ygen in the atmosphere, the response to incident sunlight is minimal. This mode of measurement is limited to target plants sufficiently close in range that the plants' fluorescence is not itself appreciably absorbed by atmosp heric oxygen (similar to 200 m). In vivo measurements of fluorescence in th e 760 and 690 nm bands of vegetation in full sunlight are also presented. M easurements of plant fluorescence at the single-plant canopy level were obt ained from greenhouse-grown bean plants subjected to a range of nitrogen tr eatments. The ratio of the fluorescence obtained from the two measurement b ands showed statistically significant variation with respect to nitrogen tr eatments. (C) 1999 American Institute of Physics. [S0034-6748(99)01511-7].