Vascular Plants

11.4.3.1 Evolution and Diversity of Poikilohydrous Vascular Plants

Inselbergs are the diversity centres of poikilohydrous vascular plants (Meirelles et al 1997; Biedinger et al. 2000; Porembski and Barthlott 2000a,b; Proctor and Tuba 2002). Desiccation tolerant vascular plants, especially phanerogamic species, were initially known especially from inselbergs of Africa (Gaff 1977) and then also from South America, Australia and India (Gaff 1987; Meirelles et al. 1997). Although dormant dried developmental stages, such as spores, pollen grains and seeds, are known from the life cycles of all vascular plants, desiccation tolerance of the vegetative plant bodies is rare and there are only 200 (Kappen and Valladares 1999) to 350 (Proctor and Tuba 2002) desiccation tolerant species among vascular taxa. It has been proposed to consider desiccation tolerant cryptogams such as cyanobacteria, algae, lichens and bryophytes as constitutively poikilohydrous as they are exohy-dric and cannot actively control their water relations as compared to the vascular plants which are constitutively homoiohydrous with their complex regulation of water relations. In the former desiccation tolerance is a primary trait during evolution, in the latter it is a special late development in highly advanced taxa (Kappen and Valladares 1999; Proctor and Tuba 2002). Indeed, although among cryptogamic vascular plants desiccation tolerance occurs in a high proportion of taxa (e.g. in the class Lycopodiopsida, order Selaginellales; in the class Pteridopsida orders Schiza-les and Pteridales), tolerance appears to be less pronounced than observed in an-giosperms. Among the phanerogams desiccation tolerant species mostly are mono cotyledons and there are fewer desiccation tolerant dicotyledons (Gaff 1977), with the latter apparently absent from South America (Gaff 1987). Evolution of poikilo-hydry in the angiosperms was polyphyletic and occurred at least eight times (Proctor and Tuba 2002). The poikilohydrous vascular plants of the more basic taxa and of the dicotyledons generally are homoiochlorophyllous and retain their photosynthetic apparatus in a recoverable form. Poikilochlorophylly is only found among the desiccation tolerant monocotyledons (Kappen and Valladares 1999; Proctor and Tuba 2002) and appears as an advanced trait in evolution where plants lose all of their chlorophyll and 70-80% of their carotenoids (xanthophylls and j-carotene) and the internal structure of their chloroplasts (thylakoids) only retaining the outer envelope. While homoiochlorophylly has the advantage of rapid resumption of photo-synthetic metabolism during rehydration, poikilochlorophylly provides much better protection from oxidative stress in the dehydrated stage which appears to outweigh the disadvantage of much slower recovery.

11.4.3.2 Dynamics of the Performance in Dehydration/Rehydration Cycles

The dynamics of dehydration/rehydration cycles are best documented considering capacity of photosynthesis.

In homoiochlorophyllous species recovery upon rehydration is generally much faster than in poikilochlrophyllous plants. A unique example of a homoiochlorophyllous angiosperm is the aquatic species Chamaegigas intrepidus (Scrophulari-aceae) living in rock pools of granite outcrops in Namibia (Hartung et al. 1998). Effects of dehydration on photosynthetic quantum yield are maximal between 10 to 15 h and photosynthetic quantum yield rises again rapidly to high values within a few hours of rehydration and reaching maximum values after 10 h (Fig. 11.24).

For poikilochlorophyllous species desiccation and recovery has been studied in much detail in the Velloziaceae Xerophyta scabrida growing in the Uluguru Mountains in Tanzania at 650 m a.s.l. (about 05° 30' S, 35° 30' E), where they form a semi-desert like bush vegetation on cliffs.

Fig. 11.24 Quantum yield (circles) during a dehydration/rehydration cycle of a single plant of Chamaegigas intrepidus and development of abscisic acid levels during dehydration (Hartung et al. 1998)

Fig. 11.24 Quantum yield (circles) during a dehydration/rehydration cycle of a single plant of Chamaegigas intrepidus and development of abscisic acid levels during dehydration (Hartung et al. 1998)

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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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