K00 1800

Fig. 5.3 Daily course of CO2 uptake (JCO2), intercellular CO2 concentration (pco ), leaf temperature (Tl), air temperature (TA) and solar irradiance (L) for a plant of Pitcairnia integrifolia in Trinidad and photograph of a plant with the head of the porometer attached, which was used for measurements. (Lüttge et al. 1986)

uptake and rehydration as well as possible repair mechanisms may restore photosynthetic capacity.

The phenomenon of reduced gas exchange during the hottest time of the day is called midday-depression. It is very frequent among trees, shrubs and herbs in hot

Fig. 5.3 (Continued)

and arid regions (Schulze et al. 1974, 1975a,b; Tenhunen et al. 1980, 1981, 1984; Pathre et al. 1998) and also frequently observed among trees in savannas and cerrados (Sect., Figs. 10.10-10.13). The midday-depression may be smaller or larger. Gas exchange may be totally absent during this time by full stomatal closure. Moreover, recovery in the afternoon, as shown for example in Fig. 5.3, may be expressed to different extents and with increasing drought it may not occur at all. Usually nocturnal rehydration may provide more effective recovery but this as well will be reduced as drought becomes increasingly severe.

In addition to mechanisms of stress tolerance, there are also means of stress avoidance. P. integrifolia for example may roll its leaves, exposing only the lower abaxial surface to the sun. This surface is densely covered by silvery trichomes. Bromeliad trichomes have evolved for absorption of water and nutrients (Sect. 6.4). The trichomes of P. integrifolia are non-absorbent and composed of dead cells which effectively reflect the light. Compared to white paper (100% reflectance) the reflectance of the abaxial leaf surface with scales was found to be 46.5% but only 19.8% when the scales were removed. As an alternative role of the non-absorbent bromeliad trichomes functioning as a water repellent has been discussed rather than reflection of excessive light and reduction of photoinhibition (Pierce et al. 2001). CAM: Escape from the Dilemma Desiccation or Starvation

Choosing between limiting the effects of any one stress represents a daily "damage limitation exercise" such that plants with C3-photosynthesis face the dilemma of desiccation or starvation, when under water stressed conditions. With the midday-depression, the strategy is to try to avoid desiccation by stomatal closure at the expense of CO2-supply for photosynthesis. Desiccation is always more rapid and is the more immediate danger than starvation. One escape from this dilemma is provided by the evolution of crassulacean acid metabolism (CAM) (Box 5.1), where CO2 is fixed during the night, when water-vapour pressure saturation deficit of the atmosphere is much lower than during the day, and hence stomatal opening has a smaller effect on the water budget of the plants. The CO2 fixed is stored in chemical form of organic acids mainly as malic acid, remobilized again during the day and made available for photosynthesis, so that the plants can utilize the light energy of solar irradiance for CO2-assimilation behind closed stomata.

Fig. 5.4 Phylogenetic tree of plant families with Crassulacean acid metabolism

This mode of photosynthesis was first discovered in plants of the genus Kalan-choe (see Sect. 2.5), which belong to the family of the Crassulaceae, and hence the name. However, it has evolved independently several times, i.e. polyphyletically, since there are CAM-performing taxa on almost all branches of the phylogenetic tree of vascular plants (Fig. 5.4). Among the plants of the thornbush-succulent forests many are CAM-plants, i.e. the cacti in the new world (Figs. 3.11B and 3.13), the succulent Euphorbiaceae in the old world, the Didieraceae (Fig. 3.12) and many of the rosette plants in the Bromeliaceae (which may cover the whole floor of neotropical dry forests like the rosettes of Bromelia humilis in Fig. 3.10A), Agavaceae and Liliaceae, to name the major ones.

However, CAM may not only operate in the simple day-night fashion described above. In fact it provides an enormous range of plasticity in form and function, allowing responses to environmental conditions to be optimised (see also Sect. 2.5). The best way of describing these options is by reference to the four phases of CAM according to the nomenclature introduced by Osmond (1978; see Box 5.1). Phase I represents nocturnal stomatal opening with CO2-uptake, fixation and storage as malic acid, whereas during phase III daytime stomatal closure with CO2-remobilization and assimilation occurs. Phases II and IV are transitional phases in the early morning and in the afternoon. Phase IV often plays an important role, because when CAM plants are well watered it may be quite extensive. Then CAM plants take up CO2 directly from the atmosphere and assimilate it directly by the C3-mode of photosynthesis via RuBISCO. This can make a major contribution to their productivity.

Conversely, water stress may become so severe that even CAM plants face the dilemma of desiccation or starvation. Then, stomata may be closed even during the night, and CAM represents an option for survival by recycling CO2 internally. The CO2 evolved nocturnally during respiratory metabolism is refixed and stored as malic acid; the day-time remobilization and reassimilation, using solar radiation, recycles carbohydrate reserves for the subsequent night (Box 5.1). Under severe drought stress cacti, for instance, keep stomata closed continuously for many months (see also Sect. By CO2-recycling they do not gain carbon, but very little is lost and solar energy can be used to maintain metabolism and remain competent until water is available again. At the same time, with totally closed stomata, the plants lose only a little water via cuticular transpiration. Water storage tissues in cacti and other succulents also provide reserves and help to overcome drought periods.

In a drought deciduous forest in western Mexico, Lerdau et al. (1992) studied the performance of the arborescent cactus Opuntia excelsea. In the dry season, when trees had shed their leaves, the cactus had a competitive advantage, as there was no light limitation. However, a factor associated with plant size, possibly water status, limited carbon gain during the dry season. Larger individuals were able to utilize water stored in their trunks and main branches (see also Sect. Light availability in the forest understorey constrained CO2-assimilation of the cactus in the wet season.

Daytime CO2-remobilization from nocturnally stored organic acids behind closed stomata also participates in controlling photoinhibition which would be amplified

Box 5.1 Crassulacean acid metabolism (CAM)

In CAM plants there are two ways of primary CO2 fixation, namely via the enzymes phosphoenolpruvate-carboxylase (PEPC) and ribulosebisphosphate-carboxylase oxygenase (RuBISCO). In its typical performance CAM has four phases (Osmond 1978):

Nocturnal dark fixation of CO2 via PEPC generating malic acid, which is translocated into the vacuole by proton pumps (H+-ATPase and H+-pyrophosphatase - PP; ase - transporting protons) and an inward rectifying malate anion channel (transporting malate2-) at the tonoplast.

A transition phase in the early morning, after light energy becomes available, with primary CO2 fixation partially via PEPC and RuBISCO, respectively.

Efflux of non-dissociated malic acid from the vacuole, malate decarboxyla-tion and refixation of the CO2 via RuBISCO behind closed stomata.

Opening of stomata in the afternoon, when nocturnally accumulated malic acid is consumed, and primary CO2 fixation via RuBISCO.

CAM may play a role as a water-conserving mechanism at different levels of drought stress.

• In the typical performance dominating nocturnal CO2 uptake reduces tran-spirational loss of water related to CO2 acquired and thus increases water-use efficiency, because the evaporative demand on leaves with open stomata is smaller in the dark than in the light.

• At increased drought stress first phase IV and then also phase II are eliminated, and stomata remain closed for the whole light period, further restricting transpirational loss of water.

• At still more severe drought, stomata may also be partially or totally closed during the dark period. In this situation the CO2 fixed nocturnally for the accumulation of malic acid partially or totally may come from internal sources, i.e. mainly respiration (CO2 recycling). This further reduces transpirational loss of water but also limits carbon acquisition.

The scheme of CAM (►) shows the key reactions in metabolism. With PYR pyruvate; PEP phosphoenolpyruvate; OAA oxaloacetate; MAL malate; P; inorganic phosphate; [CH2 O] carbohydrate and transport across the tonoplast: with MC malate transporter; the H+-ATPase and H+-PP;ase, and passive malic acid efflux.

Net CO2 exchange by the CAM-plant Kalanchoe daigremontiana (►►) with increasing drought stress: o—o well-watered; +---+ low and •—• high drought stress. Phases I to IV are indicated. Phase II and IV CO2 exchange is expressed only in the well-watered plant; onset of phase I CO2 exchange is delayed in the severely stressed plant (Smith and Luttge 1985).

Box 5.1 (Continued)

when internal CO2-levels at high irradiance were low as in the midday depression of C3-plants (Sect. (Osmond 1982; Adams and Osmond 1988; Griffiths 1989). In fact, internal CO2-levels (pCo2) behind closed stomata during the light period of CAM may be very high and reach up to a few percent (Cockburn et al. 1979; Kluge et al. 1981; see Lüttge 1987, 2002). However, in correlation with the internal CO2-concentrating mechanism of organic acid remobilization from the vacuoles and the

Fig. 5.5 Chlorophyll-fluorescence variables (see Box 4.6 for explanation) in Clusia minor in the C3

phase IV,-phase III).

(Haag-Kerwer 1994)

related high rates of CO2-reduction, high internal oxygen concentrations also build up, i.e. close to 40% or twice the atmospheric O2-concentration (Luttge 2002). Thus, other protective mechanisms of energy dissipation (Sects. 4.1.4 and 4.1.6) must also be active.

Experiments measuring chlorophyll fluorescence in the neotropical facultative CAM-tree Clusia minor, which can perform both CAM and C3-photosynthesis (Sect., have shown that photoinhibition, if it occurs, is most likely to be observed during phase IV of CAM, when stomata are open and plants fix CO2 via RuBISCO rather than in phase III. Light response characteristics of chlorophyll-fluorescence variables (see Box 4.6) in phases II and IV were similar to those observed with C. minor in the C3-state and very different to those of phase III of CAM (Haag-Kerwer 1994; Fig. 5.5).

When nocturnal accumulation of malic acid occurs from recycled CO2 alone (= 100% recycling), this is an extreme case. However, stomata may only be partially closed during the night and malic acid accumulation may be due to both recycled CO2 and CO2-uptake from the atmosphere. Since the stoichiometry of CO2-fixed to malic acid formed is unity, recycling can be calculated in absolute terms as malic acid accumulated minus CO2 taken up or in relative terms (% recycling) as malic acid accumulated minus CO2 taken up 100 malic acid accumulated

The degree of recycling may then depend on the severity of drought stress. This is illustrated in Fig. 5.6 by a study of Aechmea aquilega and its higher altitude coun-

500 1000 1500 2000 2500 Annual Precipitation (mm)

Fig. 5.6 Net nocturnal CO2 uptake from the atmosphere and internal CO2 recycling of Aechmea (A. aquilega at the three lower altitudes and A. fendleri at the highest altitude) in relation to altitude and precipitation in Trinidad. (After data of Griffiths et al. 1986)

500 1000 1500 2000 2500 Annual Precipitation (mm)

Fig. 5.6 Net nocturnal CO2 uptake from the atmosphere and internal CO2 recycling of Aechmea (A. aquilega at the three lower altitudes and A. fendleri at the highest altitude) in relation to altitude and precipitation in Trinidad. (After data of Griffiths et al. 1986)

terpart Aechmea fendleri along a gradient of altitude and precipitation in Trinidad. A. aquilega grows both terrestrially and epiphytically from very dry deciduous thornbush-forests to quite wet forests, and A. fendleri is epiphytic in wet forests. Figure 5.6 shows that with increasing altitude and precipitation total CO2-uptake by the Aechmeas increased and relative CO2-recycling decreased considerably.


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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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