Early fluorescence measurements (Murata and Sugahara 1969; Wraight and Crofts 1970) detected the absolute fluorescence from

an illuminated sample and how it changed following different chemical treatments. Because the total fluorescence is proportional to the illumination intensity, comparing the amount of fluorescence across different illumination conditions requires measuring of the fluorescence quantum yield, \(\phi_\rm F.\) $$ \phi_\rm F = \frac\hboxnumber of photons emitted\hboxnumber of photons absorbed. $$ (1) PAM fluorimetry is a widely used tool for measuring changes in the chlorophyll fluorescence yield as plants acclimate to changing light conditions (Schreiber et al. 1986). PAM techniques are reviewed in Brooks and Niyogi (2011) and Schreiber (2004). While absolute fluorescence measurements use a single light source BMS-907351 ic50 to illuminate the sample and induce fluorescence, PAM fluorimeters only detect fluorescence resulting from a low intensity (<0.1 μmol photons m−2 s−1) modulated measuring light that minimally affects the photochemistry or NPQ in the plant.

Typical qE PAM fluorimeter measurements consist of a dark-acclimated sample exposed to actinic light (light that results in productive photosynthesis) until qE reaches a steady state (approximately 10 min), followed by a period of dark reacclimation until qE turns off. To distinguish the effects of photochemical quenching (irreversible charge Afatinib price separation in the RC) and NPQ, fluorescence yield measurements are compared when PSII RCs are open and closed. RCs are considered to be open when the primary plastoquinone electron acceptor in the RC, Q A, is oxidized and is considered closed when Q A is reduced (Baker 2008; Govindjee 2004). During the illumination and dark periods, short (<1 s) pulses of high intensity (up to 20,000 μmol photons m−2 s−1) actinic light are used to close PSII RCs. When RCs are open, excited chlorophyll can relax via photochemical

quenching, NPQ, fluorescence, or ISC. Adenosine When the saturating pulses close the RCs, the only available pathways are NPQ, fluorescence, or ISC. The rates of these processes affect the measured fluorescence quantum yield. To characterize the NPQ response of a plant, it is useful to compare the fluorescence yield when the PSII RCs are closed before and during light acclimation. F m is proportional to the maximum fluorescence yield measured during a saturating pulse of actinic light applied to dark-acclimated leaves. \(F_\rm m^\prime\) is the maximum fluorescence yield following exposure to light, also measured during saturating pulses. A parameter called NPQ can be calculated with these parameters (Schreiber et al. 1994). $$ \hboxNPQ = \fracF_\rm m-F_\rm m^\primeF_\rm m^\prime.