Offerman-Chapter 2

When a plant that has been in the dark is suddenly exposed to light, the chlorophyll first emits light as fluorescence. It rises quickly, then slowly decreases, and finally stabilizes.

• The initial rise happens beacuse the photosystem is full and cannot use all the light energy for photosynthesis, so some energy is released as light.

• The decrease happens when photosynthesiss starts using the light energy.

STF (Single turnover flash analysis) can determine very precisely what is happening within PSII.

OJIP determines very precisely what is happening between PSII and PSI.

• O = Origin

• J = electron accumulation in QA

• I = intermediate step

• P = peak fluorescence (maximum)

Quenching analysis: broader picture, less precise, but also gives information on Calvin-Cycle, Photorespiration, Stomates, Stress etc.

• Measure how the fluorescence decreases when light energy is used for photosynthesis or dissipated as heat.

• Can seperate photochemical quenching from non- photochemical quenching.

A widely used technique to measure chlorophyll fluorescence and assess photosystem II efficiency in plants.

While measuring fluorescence in plants, the light is needed. But at the same time, this light would also be used for photosynthesis and therefore interpretation of the data would be very difficult.

-> PAM fluorometer send a very short puls of light to the sample -> this will induce fluorescence but will not be sufficient to drive photosynthesis

• modulated measuring beam

-> this pulse is repeated a few times per second

-> the detection of the MMB is electronically locked to the frequency of excitation

1. Modulated (Measuring) Light : very weak light, not enough to drive photosynthesis but results in fluorescence emission. is used for example, to measure F0 in dark

2. Saturating light : extremely bright, but short pulse used to close all reaction centers and thereby increase fluorescence to max. Typically applied at the end of dark adaption phase in order to measure the max F. possible

3. Actinic light : light that is used to drive photosynthesis. This is continious light in normal physiological amounts -> used to see how F changes if photosynthesis is operating. Actinic light could be normal sunlight in a field measurement.

4. Far red light : Light that is used to selectively excite PSI -> thiw will drain all electrons out of the ETC and can therefore be used to measure F0 more quickly

simple PAM -> optics are attached to the leave -> spor measurement

imaging-PAM -> F is recorded as a picture of the whole plant/leaf

micro-PAM -> F can be measured in a microscope -> individual cells or even chloroplasts

integrated with gas-exchange measurements -> Licor device

In the darkness is the Electron transport chain oxidized. All the reaction centers are open.