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irradiance light flecks, namely post irradiance CO2 fixation (Kirschbaum and Pearcy 1988a; Leakey et al. 2005).

What is the nature of these conditioning processes, which include induction, the use of short-time high irradiance and the apparent after-effects during low background irradiation? The nature and mechanism of the important after effects are understood looking at a 20-s light fleck experiment with the tropical shade-plant Alocasia macrorrhiza where CO2 and O2 gas exchange were recorded simultaneously before, during and after the light fleck (Fig. 4.14). At the beginning of the light fleck, O2 evolution increased very rapidly, and during the first second it attained about twice the rate of steady state CO2-uptake. Subsequently it dropped again and matched the rate of CO2-uptake after 2.5 s. This suggests that light-dependent elec-

Fig. 4.14 Time course of photosynthetic O2 evolution and CO2 uptake of a fully induced leaf of Alocasia macrorrhiza during a 20-s light fleck (PPFD = 500 jmol photons m-2s-1). Arrows indicate duration of light fleck (Kirschbaum and Pearcy 1988a)

Fig. 4.14 Time course of photosynthetic O2 evolution and CO2 uptake of a fully induced leaf of Alocasia macrorrhiza during a 20-s light fleck (PPFD = 500 jmol photons m-2s-1). Arrows indicate duration of light fleck (Kirschbaum and Pearcy 1988a)

tron transport, indicated by photosynthetic O2 evolution, may proceed rapidly to fill up pools of reduced compounds in a very short initial period after stepped increase in irradiance, before it becomes limited by reactions of CO2-reduction. Biophysical light-reactions of photosynthesis (Box 4.2B) are extremely fast. Therefore, in light flecks the slower processes of induction are the biochemical reactions of CO2-fixation and assimilation as well as stomatal responses. Among them, the regeneration of ribulose-bis-phosphate the CO2-acceptor in photosynthesis, is comparatively fast. It was found to be 60- 120 s under transient light conditions as typical for light flecks, while light-activation of the activity of the carboxylase (RuBISCO) and stomatal reactions occurred in the range of 10-30min (Sassenrath-Cole and Pearcy 1992). On the other hand, the mechanisms which show slower induction are also subject to slower decay and may remain active during intermittent light flecks. At the end of a light fleck, when there is a stepped decrease of irradiance, O2-evolution drops immediately as photosynthetic electron transport stops. However, CO2-uptake declines only gradually showing an after effect of the high irra-diance during the light-fleck due to a surplus of reduced compounds formed during high PPFD. In this way the consequence of after effects is that high light of light flecks can be used more effectively when the high irradiance is not continuous because this allows a more efficient use of the intrinsic potential for reduction built up during absorption of light at high intensity. Thus, this phenomenon explains the particular efficiency of short duration light fleck utilisation as a co-ordination of more rapid responses (photosynthetic electron transport) and more sluggish processes (photosynthetic CO2 assimilation), with both adjusted to some extent in series so that each process may not be entirely simultaneous. Of course, the light flecks must be short, if this is to be relatively important quantitatively. The different time constants of the processes involved also explain the conditioning to intermittent light (Fig. 4.11). Thus, the specific dynamics of transients after light intensity is stepped up and down, make light flecks a quantitatively more important energy source for forest-floor photosynthetic carbon assimilation than one might expect from their intensity and duration alone. The dynamics of fluctuating light, stomatal conductance and biochemical activation and pools of key photosynthetic intermediates are also convincingly simulated by mathematical models (Kirschbaum et al. 1997). Another factor which is also involved is respiration the activity of which is regulated by ATP-demand of the plants during light fleck dynamics (Noguchi et al. 2001a).

One may also ask whether light fleck responses are different in shade and sun plants, since one might expect that the latter are less dependent on intermittent light. Indeed, such differences have been observed. They are largely based on stomatal dynamics. Stomatal relations are very important as the induction of photosynthesis during dynamic light flecks depends on stomatal opening and CO2 availability to the mesophyll. When stomata are already open at low background irradiance induction may be faster than when first a stomatal opening movement of guard cells is required (Valladares et al. 1997).

Responses of plants to light flecks are also species specific (Leakey et al. 2005). Species dependence of stomatal responses are also seen in Fig. 4.10. A comparison of two dipterocarp rain forest tree species showed that one species could use light flecks quickly and the other more slowly but instead performed photosynthesis continuously at low light (Zipperlen and Press 1997). In species of Piper, acclimation of stomatal responses to different light intensities was observed, which was important for the performance of the plants in varying light environments (Tinoco-Ojanguren and Pearcy 1992). A comparison of Piper auritum, a pioneer tree, and Piper aequale, a shade tolerant shrub of Mexican tropical forests, showed that differences in induction of photosynthesis could be accounted for by differences in stomatal behaviour. The shade tolerant shrub, P. aequale, had the larger and more rapid response of stomatal conductance (gH2O) to light flecks, which was shown to improve carbon gain during subsequent light flecks for shade adapted plants. Conversely, low-light acclimated plants of the pioneer tree, P. auritum, showed even slower and smaller conductance responses than sun-acclimated plants, and there was no significant improvement in use of subsequent light flecks (Tinoco-Ojanguren and Pearcy 1993a,b). Another comparison was provided by Poorter and Oberbauer (1993), who studied saplings of a climax tree species, Dipteryx panamensis, and a pioneer tree, Cecropia obtusifolia, in a rainforest of Costa Rica. The results of their comparative investigation are compiled in Table 4.4. Remembering the differences in general photosynthesis characteristics of these groups of plants (Sect. 4.1.1) it appears that the climax-tree saplings exploit temporal variation in light availability by refining the speed of the induction response. In contrast, the pioneer species adjust by realising higher rates of light-saturated photosynthesis under high irradiation.

Different light fleck responses have also been reported in relation to leaf-longevity (Kursar and Coley 1993). In shade-tolerant species with short lived leaves (1 year) induction to attain 90% of maximum photosynthetic rates took 3-6min, while 11 - 36min were needed in long-lived leaves (> 4 years). In this case, however, RuBISCO activation seemed to be the time-limiting factor.

Table 4.4 Comparison of the responses of saplings of two Costa Rican rainforest tree species to light flecks in situ (Poorter and Oberbauer 1993)

Character

Dipteryx panamensis Climax species in bright microsites

Cecropia obtusifolia Pioneer species

Induction time needed to reach 90% of light-saturated rate of photosynthesis in the morning Daily average induction time needed

Duration of maintenance of high levels of induction Behaviour when grown in shaded sites as compared to bright sites

16 min

Shorter than in C.o. Longer than in C.o.

Faster rates of induction

No difference in light-saturated rates of photosynthesis

10 min

No difference in rates of induction

Lower light-saturated rates of photosynthesis

On the forest floor plants heat up in light flecks and this much adds to sudden light stress (Leakey et al. 2003, 2005). Thus, light flecks may not only have beneficial effects and it can not be generalized that light fleck activity is directly associated with greater carbon gain (Leakey et al. 2005). It is an intriguing question if the plants adapted to life in the deep shade of forest floors do not get under severe problems of photoinhibition and photodamage when subjected to high irradiance in the light flecks. Shade leaves of some plants do not appear to be pho-toinhibited during light flecks, they have a limited xanthophyll-cycle strategy and use increased synthesis of D1-protein (Schiefthaler et al. 1999). However, mostly it is observed that there is photoinhibition due to NPQ but no photodamage. This is managed because of the short time constants of zeaxanthin functions (Sect. 4.1.4). With the rise and fall of the transthylakoid ApH in relation to incident irradiance xanthophyll-cycle dependent energy dissipation is engaged rapidly during light flecks preventing photooxidative damage and disengaged rapidly after light flecks pass (Demmig-Adams et al. 1996; Logan et al. 1997; Watling et al. 1997; Adams et al. 1999).

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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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