Chlorophyll fluorescence of mature lemon trees was measured with a Fraunhofer line discriminator (FLD). An increase in fluorescence was correlated with plant water stress as measured by stomatal resistance and twig water potential.

Abstract. A field study was conducted to determine the relationship of solar‐excited chlorophyll a fluorescence to net CO2 assimilation rate in attached leaves. The Fraunhofer line‐depth principle was used to measure fluorescence at 656.3 nm wavelength while leaves remained exposed to full sunlight and normal atmospheric pressures of CO2 and O2. Fluorescence induction kinetics were observed when leaves were exposed to sunlight after 10 min in darkness. Subsequently, fluorescence varied inversely with assimilation rate. In the C4Zea mays, fluorescence decreased from 2.5 to 0.8 mW m‐2 nm‐1 as CO2 assimilation rate increased from 1 to 8 μmol m‐2 s‐1 (r2= 0.520). In the C3Liquidambar styraciflua and Pinus taeda, fluorescence decreased from 6 to 2 mW m‐2 nm‐1 as assimilation rate increased from 2 to 5 or 0 to 2 μmol m‐2 s‐1 (r2= 0.44 and 0.45. respectively). The Fraunhofer line‐depth principle enables the simultaneous measurement of solar‐excited fluorescence and CO2 assimilation rate in individual leaves, but also at larger scales. Thus, it may contribute significantly to field studies of the relationship of fluorescence to photosynthesis.

An infrared filter radiometer for atmospheric cluster-ion detection Rev. Sci. Instrum. 79, 106107 (2008); 10.1063/1.3002428 Laser-induced fluorescence imaging of plants using a liquid crystal tunable filter and charge coupled device imaging camera Rev. Photoacoustics as a tool for the diagnosis of radicular stress: Measurements in eucalyptus seedlings Rev. Sci. Instrum. 74, 709 (2003); 10.1063/1.1518567 Photorespiration and temperature dependence of oxygen evolution in tomato plants monitored by open photoacoustic cell technique Rev. Sci. Instrum. 74, 706 (2003);We have designed and built a passive remote sensor of sunlight-excited chlorophyll fluorescence ͑U. S. Patent No. 5,567,947, Oct. 22, 1996͒ which provides for the real-time, in situ sensing of photosynthetic activity in plants. This sensor, which operates as a Fraunhofer line discriminator, detects light at the cores of the lines comprising the atmospheric oxygen A and B bands, centered at 762 and 688 nm, respectively. These bands also correspond to wavelengths in the far-red and red chlorophyll fluorescence bands. The sensor 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 absorbs light at the same wavelengths that have been strongly absorbed by the oxygen 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 atmospheric oxygen ͑ϳ200 m͒. In vivo measurements of fluorescence in the 760 and 690 nm bands of vegetation in full sunlight are also presented. Measurements of plant fluorescence at the single-plant canopy level were obtained from greenhouse-grown bean plants subjected to a range of nitrogen treatments. The ratio of the fluorescence obtained from the two measurement bands showed statistically significant variation with respect to nitrogen treatments.