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and since

intrinsic water use efficiency is obtained as

1. 6WUEratio VPD

(The factor 1.6 accounts for the ratio of diffusivities of water vapour to CO2 in air.) The results show that WUEintrinsic increases with increasing salinity and VPD (Fig. 7.20) and suggest improved use of water as stomata partially close and VPD increases. This supports the conclusions reached from considering WUEratio (Fig. 7.19), because assuming constant pCO2 and V (denominator in (7.6)) WUEintrinsic for the values in Fig. 7.19 in A. marina at all salinities would increase from 6 Pa/kPa to 24 Pa/kPa VPD by a factor of 1.3 to 1.5 and in A. corniculatum at 1/10- and 1 /2-strength sea water by a factor of 1.25. At 1/1-strength sea water and VPD from 6 Pa/kPa to 12 Pa/kPa the increase in WUEintrinsic would be by a factor of 2.

Hence, in terms of both WUEratio and WUEintrinsic mangrove trees prove well equipped for economic water use in their habitats which are characterized by high salinity and solar radiation leading to high VPD. With respect to the role of VPD it is also necessary to mention leaf and air temperatures which in addition to atmospheric water vapour partial pressure are essential determinants of VPD. Reduced transpiration with increased WUE would reduce transpirational cooling (Sect. 5.2.2.1). Leaf angle position towards solar radiation and morphological characteristics of

strength sea water

Fig. 7.20 Intrinsic water-use-efficiencies (WUEintnns^) of averaged 19 different mangrove tree species studied in the field in Australia and Papua New Guinea in relation to approximate sea water strength salinity and leaf-to-air water vapour pressure differences (VPD). (After Clough and Sim 1989, from Luttge 2002)

strength sea water

Fig. 7.20 Intrinsic water-use-efficiencies (WUEintnns^) of averaged 19 different mangrove tree species studied in the field in Australia and Papua New Guinea in relation to approximate sea water strength salinity and leaf-to-air water vapour pressure differences (VPD). (After Clough and Sim 1989, from Luttge 2002)

leaves are additional attributes in optimization of these relations by mangrove trees (Ball 1996).

The relationships discussed above may be interpreted as illustrating the compromise of the desiccation-starvation dilemma (see Sect. 5.2.2). Since the flow of salt into the leaves is proportional to salinity and transpiration, control of transpiration also reduces the salt load and the danger of serious water deficits and salt toxicity in the leaves. The reduction of CO2-gain, which also follows partial stomatal closure, is partially offset by effective CO2-fixation at low , whilst maintaining high WUE (as noted by Ball 1986). Interestingly, it was also observed that of all the mangroves studied, the salt-excreting species (Avicennia marina) afford the highest rates of CO2-uptake and water-vapour conductances, although the salt load of leaves may be similar in excreting and non-excreting species (Fig. 7.14).

7.5.3 High Irradiance, Photoinhibition and Oxidative Stress

Kitao et al. (2003) observed a correlation of light saturated electron transport rates (ETRmax) of mangrove trees in a gradation from pioneer species, to intermediate species and shade tolerant climax species (Table 7.3). [This is also a good example for the potential of ETRmax-analyses for the assessment of intrinsic photo-synthetic capacities of plants adapted to various sites (Sect. 4.1.7).]

Photoinhibition has manifold protective functions as well as potentially being irreversibly destructive (Sect. 4.1.7). As shown by measurements of potential quantum yield of photosystem II (Fv/Fm of PSII, see Sect. 4.1.7) mangroves are often highly resistant against photoinhibition. Cheeseman (Cheeseman 1994; Cheese-man et al. 1991) did not observe photoinhibition in Rhizophora mangle under water stress in the greenhouse and in Bruguiera parviflora in the field. Under extremely challenging conditions Fv/Fm-values of ~ 0.8 were detected in Rhizophora stylosa (Cheeseman et al. 1997). Sobrado (1999) studied Avicennia germinans during the rainy and the dry season at a site with high salinity (30-50%c in the wet season,

60%c in the dry season) and at a site with low salinity (5 - 15%o and 40%c in the wet and dry season, respectively; where 30-35% correspond to 0.52-0.55 M NaCl, i.e. the salinity of sea water). Predawn values of Fv/Fm were ~ 0.75 under all conditions indicating only very mild chronic photoinhibition not reversible over night. Similar predawn values of Fv/Fm were measured with Avicennia marina under 1 /1-strength sea water salinity and hyper-salinity of 2/1-strength sea water (Sobrado and Ball 1999), and there was no evidence for pronounced chronic photoinhibition under severe salinity. The shade tolerant plants of Table 7.3 showed very slight chronic photoinhibition after darkening of 5 h or longer (Fv/Fm = 0.78). Thus, mangrove tree chloroplasts must be well protected against chronic photoinhibition and photodestruction.

On the other hand, during high insolation mangrove trees certainly can become subject to acute photoinhibition, which is not reversible after short periods of darkening. Bjorkman et al. (1988) reported a large decrease of Fv/Fm for various mangrove species at high solar radiation in the field. In the different seasons and sites where Sobrado (1999) did not observe chronic photoinhibition as mentioned above she detected Fv/Fm values as low as 0.45-0.55 at midday, i.e. severe acute photoinhibition, which then was largely reversible over night. This may be related to the protective functions of acute photoinhibition where excess photosynthetic excitation energy is dissipated in a harmless way, mainly in the form of heat (see Sect. 4.1.4). The involvement of xanthophylls in this protective process has been shown in mangroves (Christian 2005). The depression of Fv/Fm in mangrove-tree leaves at midday was found to be correlated with the concentration of zeaxanthin per unit leaf area (Lovelock and Clough 1992). This was not seen, however, in a study, where the performance of Avicennia marina was compared at 1/1- and 2/1-strength sea water salinity. The hyper saline condition reduced net photosynthetic CO2-uptake ( JCO2) from 7.6 to 4.3 |molm-2s-1 and stomatal conductance for water vapour from 123 to 53 molm-2s-1. Despite the much reduced CO2-assimilation under the hyper saline regime, xanthophyll pool sizes and epoxidation states as well as non-photochemical energy dissipation (i.e. not connected to CO2-assimilation)

Table 7.3 Photosynthetic electron transport rates (ETRmax) for pioneer, intermediate and shade-tolerant climax species of mangrove trees at an irradiance of 1,0001 mol m-2 s-1 (i.e. at saturation for the intermediate and shade tolerant species and near saturation for the pioneer species). (After data of Kitao et al. 2003)

Mangrove trees

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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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