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Remote estimation of Sun‐induced chlorophyll fluorescence emitted by terrestrial vegetation can provide an unparalleled opportunity to track spatiotemporal variations of photosynthetic efficiency. Here we provide the first direct experimental evidence that the two peaks of the chlorophyll fluorescence spectrum can be accurately mapped from high‐resolution radiance spectra and that the signal is linked to variations in actual photosynthetic efficiency. Red and far red fluorescence measured using a novel airborne imaging spectrometer over a grass carpet treated with an herbicide known to inhibit photosynthesis was significantly higher than the corresponding signal from an equivalent untreated grass carpet. The reflectance signal of the two grass carpets was indistinguishable, confirming that the fast dynamic changes in fluorescence emission were related to variations in the functional status of actual photosynthesis induced by herbicide application. Our results from a controlled experiment at the local scale illustrate the potential for the global mapping of terrestrial photosynthesis through space‐borne measurements of chlorophyll fluorescence. A novel high‐resolution airborne sensor is flownBoth red and far red Sun‐induced fluorescence signals are accurately quantifiedRed and far red fluorescence tracks variations in photosynthetic efficiency

Transients of chlorophyll fluorescence in photosynthetic objects are often measured using short pulses of exciting radiation, which has recently been employed to capture kinetic images of fluorescence at the macroscopic level. Here we describe an instrument introducing this principle to recording of two dimensional fluorescence transients in microscopic objects. A modified fluorescence microscope is equipped with a CCD camera intensified by a micro-channel plate image amplifier. The microscopic field is irradiated simultaneously by three types of radiation: actinic radiation, saturating flashes, and pulsed measuring radiation. The measuring pulses are generated by a light-emitting diode and their duration is between 10 to 250 µs. The detection of fluorescence images (300×400 pixels, 8 bit) has a maximum time resolution of 40 ms and is gated in synchrony with the exciting pulses. This allows measuring on a background of a continuous actinic radiation up to irradiance that can elicit the maximal fluorescence yield (FM). On the other hand, the integral irradiance of the objects by the measuring radiation is very low, e.g., 0.08 µmol m−2s−1at 0.05 µm spatial resolution and 0.006 µmol m−2s−1at 4 µm spatial resolution. This allows a reliable recording of F0even in very short time intervals (e.g., 5×80 ms). The software yields fluorescence kinetic curves for objects in user-selected areas as well as complete false-colour maps of the essential fluorescence kinetics parameters (FM, FO, FV, FV/FM, etc.) showing a two-dimensional distribution of their values. Several examples demonstrate that records of fluorescence kinetics can be obtained with a reasonable signal-to-noise ratio with all standard microscope objectives and with object sizes reaching from segments of leaf tissue to individual algal cells or chloroplasts.

We demonstrate the feasibility of assaying and predicting post-harvest damage in lemons by monitoring chlorophyll (Chl) fluorescence. Fruit quality was assayed using a commercial instrument that determines photosynthetic performance by imaging Chl fluorescence parameters under different irradiances. Images of Chl fluorescence from individual lemons reveal that photosynthesis is active throughout the post-harvest ripening process. Because photosynthesis is highly sensitive to biotic and abiotic stress, variations in Chl fluorescence parameters over the surface of a lemon fruit can be used to predict areas that will eventually exhibit visible damage. The technique is able to distinguish between mould-infected areas that eventually spread over the surface of the fruit, and damaged areas that do not increase in size during ripening. This study demonstrates the potential for using rapid imaging of Chl fluorescence in post-harvest fruit to develop an automated device that can identify and remove poor quality fruit long before visible damage appears.

The sensitivity of marine algal biotest ISO 10253 to the photosystem 2 (PS2) herbicide diuron (DCMU) was determined. Using the diatom Phaeodactylum tricornutum, we found that the algal growth rate was reduced to 50 % of the control value (EC50) for ca. 200 nM DCMU. This value is too high to allow a practical application of the biotest for concentrations of the PS2 herbicides found in natural waters. The mechanisms causing the low sensitivity of the biotest to the PS2 herbicide were investigated by measuring parameters of photosynthetic apparatus in the diatom prior and during the biotest. The apparent dissociation constant for DCMU in P. tricornutumfound by measurements of inhibition of oxygen evolution and of variable fluorescence was in the range 60–90 nM. This should lead to a much higher sensitivity of the biotest than found in our experiments. The low biotest sensitivity is caused by an acclimation to sub-lethal DCMU concentrations. The acclimation is manifested by the chlorophyll content per cell that is increasing with the DCMU concentration. During a prolonged exposure to sub-lethal herbicide concentrations, we observed also a selection of DCMU resistant organisms indicating that also an adaptation may decrease the test sensitivity. The biotest sensitivity may increase when the acclimation and adaptation are limited by shortening of the experiment duration.

The earlier developed double-modulation chlorphyll (Chl) fluorometer was modified for measurements with intact leaves of higher plants. The Chl fluorometer is based on a non-periodic modulation of both actinic and measuring flashes. In addition, continuous orange actinic and far-red radiation were produced by separate arrays of light-emitting diodes (LEDs). Programmable timing of the flashes allows to cover a wide dynamic range from microseconds to minutes. We have demonstrated that the LEDs can produce single-turnover flashes that saturate QAreduction of intact leaves of Glyceria maximaand shoots of Picea abies.

A modification of the double-modulation fluorometer is described that allows measuring very dilute phytoplankton samples. The high sensitivity is achieved by increasing the sample volume and by collecting the fluorescence from the large volume by an integrating sphere. The sensitivity of the instrument increased approximately proportionally to the volume of the sample. A further improvement of the sensitivity was achieved by replacing the PIN photodiode of the earlier versions by a photomultiplier. The instrument was used to measure fluorescence induction, F0and Fmparameters, and QA-reoxidation kinetics at concentrations at and below 100 pM chlorophyll.

Photosystem II reaction centers in plants, algae, and cyanobacteria are susceptible to damage by excess light that irreversibly impairs activity and eventually results in the proteolytic degradation of at least one of the core proteins. The sequence of events and underlying molecular mechanisms that lead to photoinhibition are poorly understood. Here we present evidence for a one-electron redox component that exerts strong control over the rate of photosystem II photoinhibition in isolated thylakoid membranes. Monitoring the impact of various doses of visible light on the rate of water oxidation and on the variable chlorophyll fluorescence, we found that reduction of the redox component increased the rate of photoinhibition >15-fold. Anaerobic potentiometric titrations of the rate of photoinhibition revealed a redox component with a midpoint potential near 20 mV at pH 7.5. The titrations fit a Nernst equation for a one-electron reaction and were nearly pH independent. Although we have not yet identified the chemical species being titrated, a likely candidate is lowpotential cytochrome b-559. We believe this observation reveals an electron transfer pathway in photosystem II that functions to protect the reaction center against excess light energy.

In outdoor thin-layer sloping reactors algae are batch cultured and harvested at biomass concentrations of about 15 g (dw) I-1 whereafter a portion is used as inoculum for the next cycle. Light saturation curves of the oxygen evolution (PII curves) of the algae were measured using diluted aliquots of suspension taken from the reactors. The maximum specific photosynthetic rates (PBmax) and the light intensity at the onset of saturated photosynthesis (Ik) decreased whilst the maximum specific photosynthetic efficiency (aB) increased with an increase in the biomass concentration, during the production cycle. These differences reflect transition from light- to dark-acclimated state of the algae that occurs as a result of an increase of the suspension concentration during the production cycle. During these experiments the thin-layered smooth sloping cultures (TLSS, culture depth 5–7 mm) had higher photosynthetic rates per volume than the thin-layered baffled sloping cultures (TLBS, culture depth 5–15 mm). This was ascribed to the higherPBmax values of the algae grown in the TLSS cultures, allowing them to utilise high incident irradiancies more effectively. However, the areal productivity of the TLBS was higher than the TLSS indicating a higher photosynthetic efficiency of the TLBS reactors. The specific productivity decreased rapidly with an increase in the biomass concentration, but the yield remained linear during the batch production cycle, even at high areal densities.

High spectral selectivity of hole burning in fluorescence spectra was used to study excited energy transfer in the CP 43 chlorophyll-protein complex of core antenna of photosystem II ofSynechococcus. The observed energy transfer time is 7.6 ± 1.1 ps. The phenomenon of laser-induced hole filling was studied and explained in terms of energy transfer within the CP 43 complex.