Imaging of Chlorophyll a Fluorescence: A Tool to Study Abiotic Stress in Plants

Chlorophyll (Chl) fluorescence is a tool which is widely used to examine photosynthetic performance in algae and plants. It is a non-invasive analysis that permits to assess photosynthetic performance in vivo (Baker, 2008; Baker & Rosenqvist, 2004; Chaerle & Van Der Straeten, 2001; Woo et al. 2008). Chl fluorescence analysis is widely used to estimate photosystem II (PSII) activity, which is an important target of abiotic stresses (Balachandran et al., 1994; Baker et al., 1983; Briantais et al., 1996; Calatayud et al., 2008; Chaerle & Van Der Straeten, 2000; Ehlert & Hincha, 2008; Gilmore & Govindjee, 1999; Guidi et al., 2007; Guidi & Degl’Innocenti, 2008; Hogewoning & Harbinson, 2007; Krause, 1988; Lichtenthaler et al., 2007; Massacci et al., 2008; Osmond et al., 1999; Scholes & Rolfe, 1996; Strand & Oquist, 1985). It is know as the energy absorbed by Chl molecules must be dissipated into three mechanisms, namely internal conversion, fluorescence and photochemistry (Butler, 1978). All of these downward processes competitively contribute to the decay of the Chl excited state and, consequently, an increase in the rate of one of these processes would increase its share of the decay process and lower the fluorescence yield. Typically, all processes that lower the Chl fluorescence yield are defined with the term quenching. Kaustky and co-workers (1960) were the first which observed changes in yield of Chl fluorescence. These researchers found that transferring a leaves from the dark into the light, an increase in Chl fluorescence yield occurred. This increase has been explained with the reduction of electron acceptors of the PSII and, in particular, plastoquinone QA: once PSII light harvesting system (LCHII) absorbs light and the charge separation occurs, QA accepts electron and it is not able to accept another electron until it has been passed the first one onto the subsequent carrier, namely plastoquinone QB. During this time the reaction centers are said to be closed. The presence of closed reaction centers determines a reduction in the efficiency of PSII photochemistry and, consequently, an increase in the Chl fluorescence yield. Transferring the leaf from the dark into light, PSII reaction centers are progressively closed, but, following this time, Chl fluorescence level typically decreases again and this phenomenon is due to two types of quenching mechanisms. The presence of light induced the activation of enzymes involved in CO2 assimilation and the stomatal aperture that determines that electrons are transferred away PSII. This induced the so-called photochemical quenching, qP. At the same time, there is an increase in the conversion of light energy into