e , (F t − F O)/(F J − F O)] In terms of nomenclature, double no

e., (F t − F O)/(F J − F O)]. In terms of nomenclature, double normalizations turn F GSK690693 mouse values into so-called V values, like V J, which is the double normalized F J value (see Strasser et al. 2004). An important source of variability between leaves is the development of stress symptoms. A common stress-related effect is chlorosis, and

it has been argued that a change in the chlorophyll content of the leaf has an impact on the fluorescence kinetics and thereby invalidates the analysis (Hsu and Leu 2003; Susila et al. 2004) but as discussed in Question 24, this is not the case as long as chloroplasts can adapt to their new light environment. In addition, if the development of the stress effects is followed over time, the gradually changing fluorescence properties will help the interpretation of the data. A comparison of leaf fluorescence measurements on stressed and unstressed plants in the field is hampered by the Protein Tyrosine Kinase inhibitor fact that such leaves are often acclimated to completely different light environments. It is important to realize that growth light intensity affects the stoichiometries and composition of many components of the photosynthetic membrane like the PSII to Selleck GS-9973 PSI ratio, the LHCII to PSII ratio, and

the amount of PSII-LHCII supercomplexes (e.g., Leong and Anderson 1984a, b; Walters and Horton 1994; Dietzel et al. 2008; Wientjes et al. 2013). Therefore, it is of fundamental importance that the light environment (full sunlight, shade, deep shade) of leaves/plants to be compared has been adequately analyzed before the effect of a certain stress is addressed by fluorimetric techniques. Several papers illustrate this, e.g., stressed and unstressed plants were compared by van Heerden et al. (2007), whereas Zubek et al. (2009) compared leaves of plants with and without mycorrhiza, both ascribing the observed difference in the initial slope of the measured OJIP transients Nintedanib (BIBF 1120) to an effect on the oxygen evolving complex of PSII. An alternative and more likely

explanation—a difference in the effective antenna size between the samples due to differences in the growth light conditions—was not considered. In summary, comparing leaves that develop under similar light conditions is relatively easy; however, comparing leaves that were growing under different light regimes is fraught with complications and should be avoided. Question 27. Can measurements made with different instruments during a large-scale field survey be compared in absolute terms? It is important to be aware that the use of different instruments, even from the same company and the same type, may yield different results in absolute terms. The light source used for saturating pulses of modulated instruments may age over time reducing its light intensity. The strength of the red LEDs of HandyPEAs often differs between instruments.

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