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Volume 1, Chapter 7-5: Water Relations: Physiological Adaptations Glime, J. M. 2017. Water Relations: Physiological Adaptations. Chapt. 7-5. In: Glime, J. M. Bryophyte Ecology. Volume 1. 7-5-1 Physiological Ecology. Ebook sponsored by Michigan Technological University and the International Association of Bryologists. Last updated 17 July 2020 and available at <http://digitalcommons.mtu.edu/bryophyte-ecology/>. CHAPTER 7-5 WATER RELATIONS: PHYSIOLOGICAL ADAPTATIONS TABLE OF CONTENTS Water Relations on Land ..................................................................................................................................... 7-5-2 Drought Tolerance vs Avoidance ........................................................................................................................ 7-5-3 Desiccation Tolerance .................................................................................................................................. 7-5-4 Desiccation Avoidance ................................................................................................................................ 7-5-9 Life Cycle and Life Strategy Adaptations ........................................................................................................... 7-5-9 Seasonal Changes .............................................................................................................................................. 7-5-11 Physiological Adaptations ................................................................................................................................. 7-5-14 Mode of Conduction .................................................................................................................................. 7-5-16 Osmotic Potential and Turgor .................................................................................................................... 7-5-21 Water Content ..................................................................................................................................... 7-5-24 Water-logging ..................................................................................................................................... 7-5-26 Inducible vs Constitutive Desiccation Tolerance ....................................................................................... 7-5-26 Hardening ........................................................................................................................................... 7-5-28 Desiccation-induced Changes .................................................................................................................... 7-5-28 Cell Contents....................................................................................................................................... 7-5-30 Chloroplast Responses ........................................................................................................................ 7-5-31 Photosynthesis .................................................................................................................................... 7-5-31 Mitochondria ....................................................................................................................................... 7-5-32 Nuclei .................................................................................................................................................. 7-5-32 Vacuoles and Vesicles ........................................................................................................................ 7-5-32 Membranes ......................................................................................................................................... 7-5-32 Plasmolysis ......................................................................................................................................... 7-5-33 Liverworts ........................................................................................................................................... 7-5-33 Summary ........................................................................................................................................................... 7-5-34 Acknowledgments ............................................................................................................................................. 7-5-34 Literature Cited ................................................................................................................................................. 7-5-34 7-5-2 Chapter 7-5: Water Relations: Physiological Adaptations CHAPTER 7-5 WATER RELATIONS: PHYSIOLOGICAL ADAPTATIONS Figure 1. Riccia cavernosa, a thallose liverwort that dries out during drought and recovers in the fall when rain returns. Photo by Jan-Peter Frahm, with permission. Water Relations on Land Proctor (2014) points out that one of the basic needs of relative ease with which genes can be moved into them or bryophytes is that of coping with the intermittent knocked out of them and their expressions be observed. availability of water. To this end, poikilohydry is efficient And both bryophyte and fern gametophytes exhibit at the small scale of a bryophyte, whereas endohydry is desiccation tolerance, whereas this ability is rare among more beneficial for the large tracheophytes. sporophytic seed plants (Watkins et al. 2007). Long live Physiological adaptations relate on one end to the the gametophytes! Even the lichens seem to have less morphology and on the other to the biochemistry. desiccation tolerance than the bryophytes (Green et al. Although we have recognized morphological characters for 2011). a very long time, few have actually been tested Oliver et al. (2000) hypothesized that for experimentally on a large scale for their adaptive value in photosynthetic plants to move onto land, desiccation altering physiology. The biochemical adaptations, on the tolerance was crucial. Using species of "resurrection other hand, constitute a new and emerging field of plants" from both bryophytes and tracheophytes, Fisher bryology, one that coincides closely with physiology of (2008) concluded that desiccation tolerance arose among tracheophytes. By using the more easily studied propagules as a means of survival. In bryophytes, nearly bryophytes, we have gained the possibility of better every part is a potential propagule in most species. For understanding of the physiology of tracheophytes. This example, Maheu (1902) found that the moss Tortula unusual interest in bryophytes is largely because of the muralis (Figure 2) would regenerate protonemata after Chapter 7-5: Water Relations: Physiological Adaptations 7-5-3 being stored dry for 14 years. Physiological adaptations water-stressed and the plant itself has become dry; it suffers may permit the bryophyte to retain water or to recover from dehydration of all its metabolic systems. Such vegetative loss of water, and to change its strategies with the seasons desiccation tolerance is rare among tracheophytes, with few or the climate. species withstanding vegetative desiccation: 60-70 species of fern and fern allies and 60 species of angiosperms (Oliver et al. 2000). Instead, most tracheophytes survive through reproductive structures. Bryophytes (and lichens), on the other hand, exhibit vegetative desiccation tolerance as well as through reproductive structures (Kappen & Valladares 1999; Proctor et al. 2007). Figure 2. Tortula muralis, a moss species that can survive drought as protonemata. Photo by Christophe Quintin, through Figure 3. Grimmia pulvinata, a drought tolerator growing Creative Commons. on concrete. Photo with permission from Botany Department Alpert (2000) presented two main puzzles from the website, University of British Columbia, Canada, with observed habitat patterns of desiccation-tolerant plants. permission. "What are the mechanisms by which plants tolerate For sake of clarity, let us consider drought to be a desiccation?" and "Why are desiccation-tolerant plants not condition of the environment and desiccation to be a more ecologically widespread?" There appear to be condition of the plant, in this case the bryophyte. For multiple mechanisms of tolerance, including protection tracheophytes, drought in the environment nearly always from oxidants and loss of normal configuration of causes desiccation in the plant, but for bryophytes, this may macromolecules during dehydration. Alpert suggests that not so often be the case. their inability to occupy a wide ecological range is due to Using that terminology, drought tolerance can be their inability to maintain a cumulative positive carbon accomplished in two ways: desiccation tolerance and balance during their repeated wet/dry cycles and the desiccation avoidance. Desiccation avoidance is the tradeoffs between desiccation tolerance and growth rate. ability to prevent desiccation from occurring within the plant or the ability to go into a dormant stage during Drought Tolerance vs Avoidance periods of low water availability; it is often characterized As clear as the two words tolerance and avoidance by plants that die and leave stress-tolerant diaspores (any may seem, they can lead to confusion because of structures that become detached from parent plant and differences in perspective. During (1979) tells us that gives rise to new individuals) that will grow the next drought tolerance is the ability to survive and maintain season. Note the use of the word stage here, not state. For activity despite a lack of water in the environment. Proctor bryophytes, spores and gemmae provide
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