Info

| Fig. 1. Different excited states of chlorophyll a caused by light absorption and energy conversion into heat, fluorescence, photochemical work, state 1-state 2 transfer and phosphorescence. (After Heldt 1999)

Ground state

Ground state

| Fig. 1. Different excited states of chlorophyll a caused by light absorption and energy conversion into heat, fluorescence, photochemical work, state 1-state 2 transfer and phosphorescence. (After Heldt 1999)

(continued)

hv hv hv hv hv

| Fig. 2. Use and dissipation of light energy in the photosynthetic apparatus. Light energy absorbed by the photosynthetic apparatus is mainly lost as heat. A small proportion (<10%) is emitted as fluorescence. A variable proportion can be used for photochemical work. With strong illumination the external antenna of PS II can dissociate and, at least in part, associate with PS I. Thereby PS II absorbs less and PS I more light energy. This type of balancing is called "state 1-state 2 transition" The rate constants (thickness of the arrows) can be changed by over-en-ergetisation. (After Schreiber et al. 1994)

Heat

Heat

| Fig. 2. Use and dissipation of light energy in the photosynthetic apparatus. Light energy absorbed by the photosynthetic apparatus is mainly lost as heat. A small proportion (<10%) is emitted as fluorescence. A variable proportion can be used for photochemical work. With strong illumination the external antenna of PS II can dissociate and, at least in part, associate with PS I. Thereby PS II absorbs less and PS I more light energy. This type of balancing is called "state 1-state 2 transition" The rate constants (thickness of the arrows) can be changed by over-en-ergetisation. (After Schreiber et al. 1994)

The photosystem is then closed. The more photosystems that are "open", the more oxidised acceptor is available, the smaller is the fluorescence. From the rate constants of fluorescence, i.e. its intensity, it is possible to determine the rate (efficiency) of the actual photosynthetic process. It is interesting that with a long period of excitation during which electrons are not transferred completely to NADP+ and further to CO? or another reducible substrate, a strong decrease in the maximum fluorescence occurs. This reduction of fluorescence with closed reaction centres shows the redistribution of the rate constants in favour of heat emission (thermal dissipation). This is called "non-photochemical quenching" (Figs. 2 and 3).

A number of parameters can be derived from the analysis of chlorophyll fluorescence (Fig. 3). The maximum fluorescence upon il lumination of a pre-darkened leaf: Fm. The reduced maximum fluorescence after a saturating light flash: Fm'. The minimal fluorescence of the antenna pigments of a pre-darkened leaf in which photosynthesis does not occur: F0; in an illuminated leaf, this is generally slightly reduced; it is then called F0'. The difference between Fm and F0 is the theoretical maximal useful energy available for photochemistry: Fv=Fm-F0. The actual fluorescence F, which is between F0' and Fm', shows the portion of energy which is not used for photochemistry, although in theory this could be used. The maximum quantum efficiency of PS II (i>n) is shown through the relation of Fv and Fm f/Jn=Fv/Fm. The effective quantum efficiency is i>' = (Fm'-F)/Fm'. The fluorescence quenching by photochemistry ("non-photochemical quenching"): qp=l-(Fm'-F0')/Fv is a measure of increased thermal dissipation.

(continued)

Measuring Actinic light + periodic saturating light flashes beam

F0: Fluorescence from the antenna

Fm: maximum fluorescence of a predarkened leaf

Fm': maximum fluorescence of an illuminated leaf (saturating light flash)

Fv: variable fluorescence during illumination with actinic light qQ: (new qp) photochemical quenching qE: (new qNP or qN) non-photochemical quenching at great ApH (increase in the rate constant for thermal dissipation)

q: total quenching under continuous light

Measuring Actinic light + periodic saturating light flashes beam j» 3 0

0 0

Post a comment