(2007) investigated two different types of commercial portable UV fluorometers for in vivo screening of anthocyanins and carotenoids in leaves. The UV-A-PAM fluorometer (Walz, Germany) makes

use of a blue C188-9 mw reference beam, whereas the Dualex fluorometer (FORCE-A, France) makes use of a red reference beam. For measurements on green leaves, the two instruments gave similar results, whereas the anthocyanins common in fruits absorbed part of the blue light of the UV-A-PAM reference beam which led, for fruits, to higher estimates for epidermal UV transmittance compared to that by the Dualex fluorometer. Pfündel et al. (2007) also noted that the absence of Chl b (e.g., in the barley chlorina f2 mutant) affected the determination of the polyphenols. Ben Ghozlen et al. (2010) developed and described an improved instrument, which they called the Multiplex (FORCE-A, France). It contains four light-emitting diodes (LEDs): UV-A (370 nm), blue (460 nm), see more green (515 nm), and red (637 nm) and three diodes to detect Q VD Oph fluorescence emission at 590, 685, and 735 nm. The three diodes allow corrections for differences in the chlorophyll content of the sample. The red LED provides the

reference beam, because it corresponds to a wavelength not absorbed by anthocyanins or flavonols. The fluorescence induced at this wavelength is compared with the fluorescence intensity induced by the excitation wavelength specific for the polyphenol of interest (e.g., green 515 nm light for anthocyanins or 370 nm UV-A light for flavonols). Ben Ghozlen et al. (2010) derived formulas to correlate Dehydratase these ratios with

the actual polyphenol content of the sample. In summary, a fluorescence-based method and accompanying equipment have been developed to determine the anthocyanin and flavonol content of leaves and fruits. In the case of fruits, the choice of the color (blue or red) of the reference beam influences the results, something that does not affect leaf measurements. Question 32. Can Chl a fluorescence be used as an indicator for a specific stress in plants? To use Chl a fluorescence as a tool to identify a specific stress, the effects of that stress on the photosynthetic apparatus must be understood (Kalaji et al. 2012a, b). If heat stress destroys the donor side of part of the PSII RCs, it reduces the electron donation capacity of all PSII RCs together and, as a consequence, causes a slow down of the JI rise as measured by a PEA-type instrument (Srivastava et al. 1997 and see also Schreiber and Neubauer 1987). It also changes the recombination properties of the affected PSII RCs when measuring DF (Čajánek et al. 1998). In extreme cases, when all or nearly all PSII donor sides have been destroyed, the fluorescence rise levels off after ~300 μs of illumination (i.e., one charge separation) and then declines; this fluorescence pattern is called the K-peak (Guissé et al. 1995; Srivastava et al. 1997; Lazár et al. 1997). UV radiation may also destroy the donor side of PSII (e.g.