Studies on the physiology of lichens with special reference to Peltigera praetextata (Flk.) Vain

Scott, George Dickson (1957) Studies on the physiology of lichens with special reference to Peltigera praetextata (Flk.) Vain. PhD thesis, University of Glasgow.

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Part 1 comprises tm investigation of several lichens for fixation of elemental nitrogen. Three lichens, Collema gronosum (Scop.) Sehaer., leptogjum lichenoides (L.) Zahlbr., and Poltigera praetextate (Flk.) Vain., each containing blue-green algae, were examined by a technique using the heavy isotope of nitrogen., 15N All were shown to fix atmospheric nitrogen. Evidence is provided to show that the fixation should be attributed to the algal symbiont (Nostoe) of these lichens. In the case of Leptogium and calculation of the total fixation of nitrogen over a period of five days, shows that the rate of fixation ( approx. 0.05 mg. nitrogen per gm, fresh weight of lichen) was too high to he attributed to any nitrogen-fixing agent (e.g. Azotobacter) other than the Nostoe. Confirmation of the fixation of nitrogen by Peltigera praetextata was obtained by a study of the growth, in nitrogen- free mineral solution, of dises of this lichen. It is also shown, by this method of culture, that nitrogen fixed by the algal symbiont is transferred to the associated fungal hyphao. Application of the isotopic test to two lichens containing green algae, Cladonia imnexa Harm. F. laxiuseula (Del.) Sendst, and Lobaria nulmonaria (L.) Hoffm., gave no evidence of fixation of nitrogen, it is considered, on the basis of these tests, that the triple symbiosis theory of Henckel and Yuzhakova (1936) can not he universally applied to lichens. In Part II is described an investigation of the growth, in culture, of discs of poltigera praetextata. Experiments are described which relate to the effect, on growth of discs, of the concentration of nutrients in the culture medium, and of the moisture content of the discs. Data for the rate of growth, under greenhouse conditions, of an entire plant of this species are given. This is shown to be higher than the rate of growth of discs of the lichen in culture. A physiological investigation of the formation of isidia on p. praetextate, and on other species of the genus, leads to the view that these structures are initiated by a light stimulus acting on the algal symbiont. An isidioid growth, somowhat similar to that commonly found on P.praetextata, has been induced to form on P.horizontalis, a species on which these structures are not found in Nature. Considerable evidence is produced, both from culture work and from examination of herbarium material, to support the view that the isidia of P. praetextata are potential reproductive organs. The isolation of the algal symbiont of P. praetextata, in bacteria-free culture, is described in Part III. Three isolation techniques were used. The isolated alga was irradiated with ultra violet light to produce a bacteria-free culture. Various stages in the development of the algal symbiont in culture are described and Illustrated. The alga is shown to be capable of heterotrophic growth, with the continued, formation of chlorophyll, using glucose as the source of energy. Part if comprises an investigation of the characteristics of dissemination and germination of ascospores of Peltligera praetetata. An aqueous extract of Nostoc, isolated from P. praetextata alone provided a suitable medium for spore germination. A detailed investigation of the causes of spore rupture in various synthetic media and in media containing agar, showed that this effect was due to an excess of vitamins la these media, The addition of thiamin (100 gamma/1.) or biotin (10 gamma/1.), to vitamin-free media produced a similar effect. The advantages of silica-gal as a substitute for agar are discussed. Tabulation of the data recorded for 83 apothecia of Peltigera praetextata,, used in experiments over a period of three years, shows that spores arc ejected, from the apothecia throughout the year, At no season arc they incapable of germinating. Consideration of the data obtained regarding spore rupture leads to the view that a large percentage of spores ejected from apothecia to the soil surrounding the plant, must succumb to this effect and hence be unable to germinate. The continued use of the term "gonidia" in the description of lichen-algae is deprecated. The term "phycobiont" is proposed, to replace "gonidium" and synonymous terms such as "algal symbiont", "algal host" and "lichen-alga". The term "mycobiont" is proposed, to replace the terms "fungal symbient" and "lichen-fungus".

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Advisers: John Walton; George Bond
Keywords: Botany, Physiology
Date of Award: 1957
Depositing User: Enlighten Team
Unique ID: glathesis:1957-72559
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 05 Jun 2019 14:27
Last Modified: 05 Jun 2019 14:27

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