STRUCTURAL RESISTANCES TO MASS TRANSFER IN THE LICHEN XANTHORIA PARIETINA

Abstract

SUMMARYQuantitative morphological data obtained from Xanthoria parietina by light and electron microscopy is related to measurements of photosynthesis rate and estimates of biotrophic ribitol transfer. It is concluded (a) that diffusive CO2 flux into the alga is greatly reduced by the resistance of the fungal cortex, (b) that diffusion can easily account for the mass transfer of ribitol from alga to fungus, and (c) that haustoria have no role in the biotrophic transfer of carbohydrate in lichens.

CARBON DIOXIDE EXCHANGE IN LICHENS: RESISTANCES TO CO2 UPTAKE AT DIFFERENT THALLUS WATER CONTENTS

“…It is probable that determination of these resistances in lichen thalli at different water contents could provide more useful information on NAR depression at high water contents. The only published value for the CO2 resistance of a lichen is that of Collins and Farrar (1978). The total resistance found (Ir = 138 x 10^ s m'^) was far higher than typical values for higher plants.…”

Tansley Review No. 117

“…Once taken up by the mycobiont, the carbohydrate is rapidly and irreversibly metabolized into mannitol via the pentose phosphate pathway (Lines et al, 1989), and is thereby made unavailable to the photobiont (Galun, 1988). The mechanisms behind the induction of carbohydrate export and mass transfer from photobiont to mycobiont are still a matter of debate ; so far no specific polyol or glucose transporter has been isolated from lichens, although such a carrier has been postulated (Collins & Farrar, 1978).…”

“…In this context, the distance between respiring tissue and photobiont cells is important, as also are the species-specific CO # acquisition characteristics of the particular photobiont (see section IV.2). Clearly, even though it has now been convincingly shown that the depression of photosynthesis at supraoptimal WC is caused primarily by increased thallus resistance for CO # diffusion (Collins & Farrar, 1978 ;Cowan et al, 1992), we need to consider how ultrastructural and biochemical characteristics of different lichens and their particular bionts influence the degree of this resistance. The quantitative effect of depressed CO # acquisition at high thallus WCs must also be considered in relation to the beneficial effect of being able to remain metabolically active for longer periods if this is coupled to a relatively large water-holding capacity.…”

“…Still, species that require high WC can still show depression of photosynthesis at supraoptimal hydration, whereas some species with low water-holding capacities and lower WC requirements for photosynthesis can be less inhibited by high WC (Lange et al, 1993 ;Sundberg et al, 1997b). The photosynthetic WC response of a particular lichen species is therefore apparently determined by a range of different factors, with morphological constraints being most important (Collins & Farrar, 1978 ;Lange et al, 1993Lange et al, , 1999. For instance, the structural composition of the lichen determines the relative distribution of water between intracellular, apoplastic and extracellular compartments within the thallus (Beckett, 1995(Beckett, , 1997.…”

Bioaccumulation of lead by the lichen Acarospora smaragdula from smelter emissions

“…We suggest that airborne particles are initially accumulated on the surface of lichens and, through gravity, trapped between areoles. Considerable intercellular space (18% in Xanthoria parietina ; Collins & Farrar, 1978) exists in lichens, especially the loosely interwoven pseudoplectenchymatous medulla. Our study is consistent with SEM studies on Caloplaca aurantia (Garty et al, 1979), Lecanora conizaeoides (Johnsen, 1981), and Stereocaulon vesuvianum (Jones et al, 1982), which all report that particulates are accumulated within the medulla.…”