V

where AP and AV are turgor pressure and volume changes, respectively, and V is cell volume, it then follows that a given change in volume (AV) leads to a lower change in pressure when s is larger or cell walls are stiffer, i.e. in the epiphytic Ficus plants a larger volume of water can be lost before turgor is lost than in the terrestrial trees. As a result leaves of the more succulent epiphytes and the conspecific less succulent trees of Ficus species lost turgor at approximately the same relative water content.

(see Box 6.1) at P = 0, or zero turgor, f = — n, and this is substantially higher (less negative) in the epiphytes due to the lower osmotic pressure n. These observations agree with the general trends for higher succulence, higher f and lower n in epiphytes (see above and Table 6.7).

In contrast to Ficus species, in Clusia differences in all of these water relation parameters between epiphytes and trees were very small. Thus, while Clusia has instantaneous plasticity of responding to changing water supply and evaporative demand by photosynthetic options (C3-CAM transitions) Ficus shows intrinsic developmental changes during the transformation from epiphyte to tree which is associated with improved acquisition of water.

In CAM plants water relation parameters ty, P and n also oscillate together with the day-night malic acid rhythm. Figure 6.25 describes an experiment with the atmo-

Fig. 6.25 Experiment showing the capacity of cells of the atmospheric CAM bromeliad Tillandsia usneoides for water uptake during a day-night cycle. Plants were weighed (initial FW) dipped for 10 min into water, dried superficially and then weighed at intervals to determine the point where rapid evaporation of surface water is completed and water is only lost from the living cells by transpiration, which allows to estimate water uptake by extrapolation (A). It is seen that osmotic pressure n (B) and malate levels (C) increase during the night, and increased water uptake (D) and turgor pressure (E) measured directly with an intracellular pressure probe are associated with this. (see Luttge 1987)

Fig. 6.25 Experiment showing the capacity of cells of the atmospheric CAM bromeliad Tillandsia usneoides for water uptake during a day-night cycle. Plants were weighed (initial FW) dipped for 10 min into water, dried superficially and then weighed at intervals to determine the point where rapid evaporation of surface water is completed and water is only lost from the living cells by transpiration, which allows to estimate water uptake by extrapolation (A). It is seen that osmotic pressure n (B) and malate levels (C) increase during the night, and increased water uptake (D) and turgor pressure (E) measured directly with an intracellular pressure probe are associated with this. (see Luttge 1987)

spheric CAM-bromeliad Tillandsia usneoides, showing that nocturnal accumulation of malic acid provides an osmoticum, which may drive cellular water uptake. It can be seen that cell-sap osmotic pressure (n) increases together with malic-acid levels. Water uptake, measured after dipping the plants for a short period into water as shown in Fig. 6.25A, clearly increased during the night together with n, and this also led to an increase in turgor pressure. It should be noted, that while atmospheric bromeliads could occur in rather dry habitats, they are often found at sites where fog forms during the later part of the night and in the early morning. Water from condensed fog and dew is then available at times when malic-acid concentration in the cells is high and can lead to osmotic uptake of water.

6.6.2.3 CAM and Flexibility: The Case Study of Clusia

A major advantage of CAM in habitats where there are large short term and seasonal variations in water availability is the inherent flexibility in this mode of carbon acquisition. The different expression of the four CAM phases (see Box 5.1) in constitutive CAM plants already allows highly variable responses. If water supply were to range from very severe to moderate and low drought stress, there may be, respectively total stomatal closure and CO2-recycling (also called CAM-idling), predominant nocturnal opening of stomata (Phase I) or increasing use of phase IV and phase II CO2-uptake during the daytime hours. There may even be continuous CO2 uptake day and night under well watered conditions. In addition there are species which are true C3-CAM intermediates. They can switch from C3 photosynthesis to CAM as drought stress increases, and back again when the stress is released. Among the epiphytic bromeliads Guzmania monostachia is such a C3-CAM intermediate and it is the only one in the Bromeliaceae family (Maxwell 2002; Maxwell et al. 1994, 1995, 1999). However, there are other taxa with C3-CAM-intermediate epiphytes. The epiphytic fern Pyrrosia confluens and the Crassulaceae Kalanchoe uniflora belong to this group (Griffiths 1989) as well as species of Peperomia (Sipes and Ting 1985; Ting et al. 1985; Holthe et al. 1987). The plants showing the most flexible response, however, are in the hemi-epiphyte and strangler genus Clusia. A separate book is monographically devoted to Clusia (Luttge 2007a) and it is only briefly used here as case study. Each of the photosynthetic modes mentioned above are expressed in Clusias:

• pure C3-photosynthesis,

• C3/CAM-intermediate behaviour with environmentally controlled reversible changes between the two modes of photosynthesis,

• night time stomatal closure with fixation of respiratory CO2 and vacuolar malate accumulation plus day-time stomatal opening and reduction of CO2 both from the atmosphere and the nocturnally stored malate (CAM-cycling),

• stomatal closure around the clock and only recycling of respiratory CO2 (CAM-idling),

Fig. 6.26A-I Modes of photosynthetic CO2 exchange in Clusia.

Left panel. Comparison of four species under identical conditions in a phytotron, Clusia venosa with C3 photosynthesis (A), Clusia minor with CO2 uptake day and night (B), Clusia major and Clusia alata both with CAM but differing in the development of phase IV (C,D). Center panel. Clusia minor in a growth chamber with C3 photosynthesis under well-watered conditions at high irradiance (1,700 ^mol photons m-2 s-1) and medium leaf/air water vapour pressure difference (AW = 6.6 mbarbar-1) (E); CAM with the well-expressed four phases (I-IV) under drought stress at low irradiance (400 ^mol photons m-2 s-1) and high AW (13.5 mbarbar-1) (F); and CO2 uptake day and night under well-watered conditions, low irradiance (400 ^mol photons m-2 s-1) and low AW (3.4 mbarbar-1) (G).

Right panel. Clusia rosea in the field with C3 photosynthesis (H) and CAM with an extended phase II in the first half of the day (I).

Black bars on the abscissa indicate the dark periods. (Luttge 1991)

Fig. 6.26A-I Modes of photosynthetic CO2 exchange in Clusia.

Left panel. Comparison of four species under identical conditions in a phytotron, Clusia venosa with C3 photosynthesis (A), Clusia minor with CO2 uptake day and night (B), Clusia major and Clusia alata both with CAM but differing in the development of phase IV (C,D). Center panel. Clusia minor in a growth chamber with C3 photosynthesis under well-watered conditions at high irradiance (1,700 ^mol photons m-2 s-1) and medium leaf/air water vapour pressure difference (AW = 6.6 mbarbar-1) (E); CAM with the well-expressed four phases (I-IV) under drought stress at low irradiance (400 ^mol photons m-2 s-1) and high AW (13.5 mbarbar-1) (F); and CO2 uptake day and night under well-watered conditions, low irradiance (400 ^mol photons m-2 s-1) and low AW (3.4 mbarbar-1) (G).

Right panel. Clusia rosea in the field with C3 photosynthesis (H) and CAM with an extended phase II in the first half of the day (I).

Black bars on the abscissa indicate the dark periods. (Luttge 1991)

Variability of ecophyiological response is observed:

• between different species under given environmental conditions (Fig. 6.26, left panel),

• for a given species under different environmental conditions (Fig. 6.26, center and right panels),

• even for the two different leaves of a given node in the same plant when they are kept under different conditions (Fig. 6.27).

The rapid changes between C3-photosynthesis and CAM, that may be performed by Clusia are determined by the external control parameters:

i) water relations, ii) day/night temperature regime, iii) light.

Fig. 6.27A, B CO2 gas exchange of two opposite leaves at the same node of Clusia minor in a growth chamber. After four days of drought stress the plant was rewatered on the fifth day (arrow head). The leaf kept in a cuvette at high VPD (AW = 13.1 mbarbar-1) continued to perform CAM (A) with all phases I-IV expressed. The other leaf (B) at low VPD (AW = 6.2 mbarbar-1) rapidly switched to daytime CO2 uptake and suppressed nocturnal CO2 uptake. The black bars on the abscissa indicate the dark period. (Luttge 1991)

Fig. 6.27A, B CO2 gas exchange of two opposite leaves at the same node of Clusia minor in a growth chamber. After four days of drought stress the plant was rewatered on the fifth day (arrow head). The leaf kept in a cuvette at high VPD (AW = 13.1 mbarbar-1) continued to perform CAM (A) with all phases I-IV expressed. The other leaf (B) at low VPD (AW = 6.2 mbarbar-1) rapidly switched to daytime CO2 uptake and suppressed nocturnal CO2 uptake. The black bars on the abscissa indicate the dark period. (Luttge 1991)

i) In an experiment with a plant of C. minor rewatered after a period of several days of drought, it was possible to get two opposite leaves at a given node to perform C3-photosynthesis and CAM respectively, at the same time. One leaf, when maintained in an atmosphere with a low leaf-air water vapour pressure difference (AW or VPD), i.e. kept under a low transpiratory demand, switched to C3-photosynthesis a few hours after watering with CO2 uptake

Fig. 6.28 Change of Clusia minor from C3 photosynthesis to CAM as night-time temperatures are lowered to give an increasing day/night temperature difference (AT). Jco2 net CO2 exchange, gH2o leaf conductance for water vapour. Day-time temperature was always 30 °C. (Haag-Kerwer et al. 1992)

Fig. 6.28 Change of Clusia minor from C3 photosynthesis to CAM as night-time temperatures are lowered to give an increasing day/night temperature difference (AT). Jco2 net CO2 exchange, gH2o leaf conductance for water vapour. Day-time temperature was always 30 °C. (Haag-Kerwer et al. 1992)

markedly reduced in the subsequent night. The other leaf, kept at high VPD, continued to perform CAM with the four phases clearly noticeable, as both leaves had done during the drought period before watering (Fig. 6.27).

ii) By varying the temperature regime, it was found that a certain day-night temperature difference was important for expression of CAM in C. minor (Fig. 6.28). The shift between CAM and C3-photosynthesis was fully reversible when the temperature regimes were changed between equal day/night temperature and day/night temperature differences (Fig. 6.29).

iii) In well-watered plants a drastic increase in light intensity led to an elimination of nocturnal dark-CO2-fixation and an increase in daytime C3-photosynthesis. Obviously, this represents an optimal use of high light energy provided that water is not limiting (Fig. 6.30).

Clusia spp. are also remarkable in several other ways:

• showing the highest nocturnal acid accumulation ever observed for CAM plants (Table 6.8),

• accumulating large amounts of citric acid during the dark period additionally or alternatively to malic acid (Table 6.8).

The latter observation also requires a comparative evaluation of the relative ecophysiological advantages of malic and citric acid accumulation during CAM (Table 6.9). Consideration of intermediary metabolism suggests that different com-partmentation and different contributions of mitochondrial and cytosolic reactions may both be involved. Citric acid accumulation, in contrast to malic acid accumulation, does not lead to a net gain of carbon, although it contributes to carbon recycling. However, carbon recycling via citric acid may be favourable because daytime breakdown of citric acid may possibly result in the liberation of more CO2 than

Fig. 6.29A-C Change of Clusia minor from C3 photosynthesis to CAM and back to C3 photosynthesis as a day/night temperature difference of 10 ° C is introduced and removed again. Gas exchange after seven days at 25/25 °C day/night (A) followed by five days at 30/20 °C (B) and by four days at 25/25 °C (C). JCO2 net CO2-exchange, gH2o leaf conductance for water vapour. (Haag-Kerwer et al. 1992)

Fig. 6.29A-C Change of Clusia minor from C3 photosynthesis to CAM and back to C3 photosynthesis as a day/night temperature difference of 10 ° C is introduced and removed again. Gas exchange after seven days at 25/25 °C day/night (A) followed by five days at 30/20 °C (B) and by four days at 25/25 °C (C). JCO2 net CO2-exchange, gH2o leaf conductance for water vapour. (Haag-Kerwer et al. 1992)

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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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