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Box 4.6 Fluorescence analysis

A. Fluorescence induction kinetics (Kautsky effect)

In the first second after excitation fluorescence of photosystem II is induced and rises from the fluorescence (O) of the dark-adapted leaf obtained at very weak excitation energy to I, where all primary electron acceptors (Q in Box 4.2B) are reduced, and after a small shoulder further to P , where the plastoquinone pool (PQ in Box 4.2B) is also reduced.

Box 4.6 (Continued)

Box 4.6 (Continued)

The high fluorescence obtained at P after the first second is quenched as the electron-transport capacity at the acceptor side of photosystem I and other fluorescence-quenching processes are activated.

B. Fluorescence analysis

In a pulse modulated-fluorescence-analysis system the following kinetics are obtained during induction after a period of darkness (the example given was obtained from a leaf of Clusia multiflora):

Arrowheads pointing upwards and downwards respectively, indicate switching on and off of light, namely:

• m weak measuring light,

• p a single pulse of saturating actinic light,

• a actinic light with regular light-saturating pulses.

Box 4.6 (Continued)

Symbols in the graph have the following meaning:

• Fo minimal fluorescence yield of dark-adapted sample in weak measuring light,

• Fm maximum fluorescence yield of the dark-adapted sample,

• Fv maximum variable fluorescence,

• Fo minimal fluorescence yield of the light-adapted sample,

• Fm maximum fluorescence yield of the light-adapted sample.

Calculations which can be made include the following:

1. Fv/Fm as a measure of potential quantum yield of photosystem II after dark adaptation, which is lowered by photoinhibition,

2. A F/Fm as a measure of effective quantum yield, a F/Fm=( Fm- f )/Fm.

3. ETR = 0.86 x 0.5 x (AF/Fm) x PPFD as an empirical approximation of the relative electron transport rates, where PPFD is incident photosynthetic photon flux density, the factor 0.86 accounts for an average light absorption of leaves of 86% (unless measured specifically) and the factor 0.5 for equal distribution of absorbed photons to PSII and PSI.

4. The quenching coefficient for photochemical quenching of fluorescence, qP

5. The quenching coefficient for non-photochemical quenching of fluorescence, qN

6. The extent of non-photochemical quenching, NPQ,

(Refs.: Genty et al. 1989; van Koten and Snel 1990; Schreiber and Bilger 1993;

Bilger et al. 1995; Maxwell and Johnson 2000)

Fig. 4.9A, B Light dependence curves of net CO2 uptake (A) and photochemical quenching (B) of sun and shade leaves of Arbutus unedo. (Schreiber and Bilger 1987)

we benefit from instrument miniaturization for large scale operations in the field (Sect. 2.3.2).

Among the parameters explained in Box 4.6 potential quantum yield of PSII, Fv/Fm, is extremely useful in ecophysiology as it allows to distinguish between acute and chronic photoinhibition. A dark adapted sample where all elements of the electron transport chain are oxidized ("open") shows maximum Fv/ Fm upon a saturating light pulse. This is close to 0.83, because maximally 83% of the light are used (Bjorkman and Demmig 1987). If Fv/Fm is lower than that, the sample is under photoinhibition. Then one can study the time of darkening needed to restore it to close to 0.8 (Thiele et al. 1998). If the photoinhibition indicated by Fv/Fm was reversible within several tens of minutes one was observing acute photoinhibition due to built up of an electrochemical gradient at the thylakoid membranes and xanthophyll type energy dissipation. If it was not reversible before several hours, destruction of D1-protein must have been involved. If it was not reversible for an extended period, e.g. overnight, there has been irreversible photodamage and photoinhibition was chronic.

Effective quantum yield of PSII, A F / F m, and apparent electron transport rate, ETR, indicate the activity of photochemical work. A F/ F^ decreases with increasing light intensity as an increasingly less proportion of the incident irradiance is used for photosynthesis, and ETR increases up to light saturation. In this way for example from actual momentary measurements at varying irradiance, e.g. during the course of a day, light saturation curves can be obtained and non-photochemical fluorescence quenching can also be related to irradiance (Fig. 4.8). In Fig. 4.9 light-response curves of net CO2-uptake and photochemical quenching of a sun and a shade leaf are compared. The CO2-uptake curves show the typical characteristics of sun and shade types (see Fig. 4.1). Photochemical quenching is high in both cases at low PPFD and decreases much more rapidly with increasing light intensity in the shade leaf than in the sun leaf, suggesting increased over reduction of the photosynthetic electron transport chain.

4.2 Varying Irradiance on the Forest Floor and in Gap Dynamics 4.2.1 The Response to Light Flecks

Light flecks were already mentioned in relation to the vertical structure of forests (Sect. 3.4.1) describing the dynamics of light penetrating through the forest canopy. The importance of such dynamics is illustrated by modelling canopy photosynthesis with steady state and dynamic models, respectively, where the former overestimate carbon gain by 13.4% at open sites and even by 86.5% at low light environments of the understory (Stegemann et al. 1999; Timm et al. 2002, 2004). Clearly, light flecks must be important in any type of forest. However, in the very dark, moist tropical forests the dynamics of the responses of photosynthesis to light flecks play an essential role in fulfilling the energy demands of photosynthesis in lower canopy layers and particularly on the forest floor.

A prerequisite for photosynthetic utilization of the irradiance of rapidly formed and transient light flecks are swift and co-ordinated reactions of stomata as well as the biophysical and biochemical machineries of CO2-assimilation. Figure 4.10 shows that CO2-uptake, stomata-limited leaf conductance for water vapour (gH20) and intercellular CO2-concentration (pC02) in the leaves of Claoxylon sand-wicense and Euphorbia forbesii respond within minutes to a stepped increase of irradiance from low to high intensity. Both species occur in the understory of a Hawaiian forest (Pearcy 1983). In C. sandwicense, a tree with C3-photosynthesis, gH20

Fig. 4.10A, B Response of net C02 uptake, leaf conductance for water vapour (gh2o) and intercellular CO2 concentration (pCi o ) to a stepped increase of light intensity to a PPFD of 500^molm-2s-1 photons (arrow) after the leaves had been at 22^molm-2s-1 photons for 2h; A Claoxylon sandwicense; B Euphorbia forbesii. (Pearcy et al. 1985)

Fig. 4.10A, B Response of net C02 uptake, leaf conductance for water vapour (gh2o) and intercellular CO2 concentration (pCi o ) to a stepped increase of light intensity to a PPFD of 500^molm-2s-1 photons (arrow) after the leaves had been at 22^molm-2s-1 photons for 2h; A Claoxylon sandwicense; B Euphorbia forbesii. (Pearcy et al. 1985)

showed an immediate linear increase, which continued for more than 50 min; net CÜ2-uptake first increased very rapidly and then more gradually in correlation with gH2o; ^CÜ2 decreased initially in response to increased availability of light energy for photosynthesis and then increased again slightly as stomata opened more widely (increased gH2Ü) allowing CO2-uptake from the atmosphere. E. forbesii, performing C4-photosynthesis (see Box 10.2, Sect. 10.1.2), responded in a somewhat different fashion. There was a slight delay in stomatal opening, but then maximal stomatal conductance was attained within 15 min. After an initial decline, pCÜ2 was stabilised at an intermediate level while CO2-uptake reached a high, constant rate.

Are these responses sufficient to allow the plants to make efficient use of short light flecks? Interestingly there is an effect of accelerating CO2-uptake with time, when very short light flecks are imposed repeatedly over short periods. Figure 4.11 shows, for the two species discussed above, that CO2-uptake during artificial light flecks increases gradually when subsequent light flecks of a duration of 1 min are alternated with low background intensity for about 90 s. Conditioned or induced leaves have a considerably higher light use efficiency in light flecks than uninduced ones which is decreasing as light fleck length increases (Fig. 4.12; Valladares et al. 1997). The intensity of this background light is also important in maintaining the conditioning effect, which leads to increasing efficiency in the use of light flecks. As shown in Fig. 4.13, the efficient use of 5-s light flecks at 500 | mol photons m-2 s-1 increases considerably when the background light intensity following each light fleck is increased from 0 to 101 mol photons m-2 s-1. At very high intensities the irradiance of light flecks is used more effectively when there are interruptions by low intensity background irradiation. This is explained by after effects of the high

Fig. 4.11A, B Response of CO2-uptake of: A C. sandwicense; B E. forbesii during 1-min light flecks (PPFD = 510^mol photons m-2 s-1) on a background of 22 ^mol photons m-2 s-1, which was presented to the plants for 2 h prior to the first light fleck and which interrupted the individual light flecks. Arrows indicate increase (t) and decrease (|) of PPFD respectively. (Pearcy et al. 1985)

Fig. 4.11A, B Response of CO2-uptake of: A C. sandwicense; B E. forbesii during 1-min light flecks (PPFD = 510^mol photons m-2 s-1) on a background of 22 ^mol photons m-2 s-1, which was presented to the plants for 2 h prior to the first light fleck and which interrupted the individual light flecks. Arrows indicate increase (t) and decrease (|) of PPFD respectively. (Pearcy et al. 1985)

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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