The ratio

between chlorophyll fluorescence at 735 nm and that at 700 nm (F735/F700) is linearly proportional to chlorophyll content (Gitelson et al. 1999). Conversely, as discussed in Question 24, the F M and F O values are not related to the chlorophyll content in leaves (Dinç et al. 2012). It may also be noted that there are simple chlorophyll meters on the market (CL-01, Hansatech Instruments, UK; SPAD meter, Minolta, Japan; CCM-200, Opti-Sciences, USA) that can be learn more used to follow changes in the leaf chlorophyll content (see e.g., Cassol et al. 2008; Dinç et al. 2012). These measurements can then be calibrated against measurements of the chlorophyll extracted from leaf areas measured before with the chlorophyll

meter (see e.g., Dinç et al. 2012). Chl measurements on dark-adapted leaves seem to give more reproducible results than measurements made on light-adapted leaves (Ceppi and Schansker, unpublished data, 2008). If the chlorophyll meter is used over the day on the same leaf, the readings change (Mishra, unpublished data, 2010), e.g., due to chloroplast movements, which change the absorbance properties of the leaf (see Wada 2013 for a review on chloroplast movements). Chloroplasts are known ABT-263 research buy to re-arrange themselves inside the cell in response to the ambient blue light intensity, adapting the absorbance properties of the leaf to the circumstances (Sakai et al. 2001; Kasahara et al. 2002). This does not only affect chlorophyll meter measurements, but also normal fluorescence measurements (Brugnoli and Björkman 1992). In practice, values measured using Quisqualic acid a Chl meter are often used as indicators for relative Chl changes. In that case, we assume that the measured values are a linear function of the leaf chlorophyll content between zero and the value measured on control leaves. However, in that case, it is important to test the validity of this assumption for each plant species and for each stress studied (Mishra, unpublished data, 2013). Question 26. Is it possible

to compare different leaves? It is easy to take randomly two leaves from two plants of the same species and to make a fluorescence measurement. But is it truly possible to compare these two measurements? It is likely that a difference in maximum fluorescence amplitude will be observed. Especially, when studying OJIP transients, the kinetics are often more interesting than the absolute amplitude, and in that case, the difference in the fluorescence amplitude is eliminated by double normalization between F O and F M. Arithmetically, this is done in the following way: (F t − FO)/(F M − F O). The effect of this calculation is to rescale each fluorescence value in a range going from 0 (corresponding to F O) to 1 (corresponding to F M). For a comparison of the kinetics of the individual rise phases of the OJIP transient, the same approach can be used.