Chapter 1: The Characeae Plant

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28 There is often a high degree of innate dormancy in charophyte oospores. Germination of fresh or field-collected oospores is typically less than 10%, and often less than 1% of the total oospore pool (Bonis and Lepart 1994; Skurzynski 2009). Rarely germination rates in excess of 40 % have been recorded (Casanova and Brock 1996; Dugdale et al. 2001). The role of such a uniformly high degree of dormancy is to ensure that populations remain viable, even if the habitat becomes unsuitable for characeae growth for long periods of time. A possible additional role is to allow transport of oospores long distances within soil, or the intestinal tracts of herbivores (Proctor 1962). The conditions that break dormancy vary for different species, so that species that germinate in Spring and Summer can require chilling, or stratification before germination will occur, and other require conditions that simulate winter inundation (Casanova and Brock 1996). The timing of germination in relation to dormancy breakage can vary as well. The high degree of dormancy and variability in the conditions required to break that dormancy make germination ‘on demand’ for laboratory experiments difficult to achieve. Characeae growth rates vary in relation to species (Blindow 1988; Casanova 1994; Casanova and Brock 1999a) and season (Blindow et al 2002) and an extension rate of 5 mm day -1 in the field is not exceptional. When water levels are constant some populations have been shown to maintain their position in the water through apical growth while the basal portions of the plant decay, initiating new rhizoids from sequentially higher nodes as the plant sinks through the water (Andrews et al 1994). Where water levels increase apical growth can maintain the upper photosynthetic parts of the thallus in the photic zone, and where water levels fall the thallus can lean over in the direction of water movement and initiate rhizoids at the nodes that are in contact with the substrate (Casanova 1994). In contrast, characeae can be found almost emergent at the edges of shallow and temporary water bodies, often dying back as the exposed shoots dry. In some exceptional circumstances populations of characeae can exist in a film of water in moist terrestrial habitats. Initiation of sexual reproduction in characeae populations can be a consequence of time since germination (e.g. for monocarpic annuals) or as a response to environmental conditions or resources (Casanova 1994). Polycarpic perennial species, such as Chara australis can produce oospores every year through the warmer months (Casanova 1994). Annual species can produce reproductive organs on the first whorl of branchlets before significant vegetative
29 growth has taken place (e.g. Chara muelleri). Other species establish a vegetative thallus before fertile shoots are produced (e.g. Nitella leonhardii). Although there are few studies on the interactions among characeae and between characeae and animals, the dense structure of characeae stands indicates that competition for light could affect growth, and the occurrence of characeae meadows that exhibit an abrupt, unexplained growth boundary (in plant height or depth distribution) suggest the presence of biotic interactions or controls. Characeae species vary with respect to their palatability to animal herbivores (Proctor 1999), so that the existence of perennial beds of characeae could be explained by their resistance to herbivory, and the absence of particular species to their susceptibility to herbivory (Proctor 1999). In general, interactions with herbivores are difficult to detect except through direct observation and experimentation. However characeae would be exceptional (i.e. unlike all other plants) if herbivory or creation of structures or behaviours to resist herbivory did not play a role in their ecology. 1.5.8 Application of ecological knowledge Knowing something of the ecology of characeae can have at least two positive consequences for physiological research: 1) the development of enhanced techniques and protocols for culturing and maintaining characeae for laboratory work, and 2) as a stimulus for physiological research. Observations of characeae in the natural environment can lead us to asking ‘why is it so?’, and physiologists and physicists will look for answers in relation to the physiology and biophysics of characeae plants and cells. Observations of the distribution of characeae in relation to environmental constraints can allow development of better germination and growth protocols, and a better understanding of the relationships among species, as photosynthesisers, as food for herbivores and as part of an integrated community of plants and animals. The rest of this book contains information about some of the studies that have been undertaken on the physiology of characeae plants, informed by a knowledge of charophyte morphology, cytology and ecology.
Page 1 and 2: 1 Chapter 1: The Characeae Plant 1
Page 3 and 4: 3 1.1 General Morphology The chara
Page 5 and 6: 5 largest cells, and for this reas
Page 7 and 8: 7 Lamprothamnium in 1916 to avoid
Page 9 and 10: 9 of inflated branchlets develop,
Page 11 and 12: 11 where the antheridia are termin
Page 13 and 14: 13 zygote and the maternal spiral
Page 15 and 16: 15 1.4.1 Systematics and the conce
Page 17 and 18: 17 definition of species (Proctor
Page 19 and 20: 19 bulbilifera. The natural range
Page 21 and 22: 21 do they occur?’ are central t
Page 23 and 24: 23 of animal and algal epiphytes (
Page 25 and 26: 25 1.5.4.4 Temperature Storage tem
Page 27: 27 annuals (i.e. reproducing once
Page 31 and 32: 31 Figure 1.15 The characeae oospo
Page 33 and 34: 33 antheridia clustered at the bas
Page 35 and 36: 35 Casanova, M.T. and Karol, K.G.
Page 37 and 38: 37 Proctor, V.W. 1960. Dormancy an

Chapter 1: The Characeae Plant

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