INTRA-SPECIFIC VARIATION IN THE PHOTOSYNTHETIC RESPONSES OF CYANOBIONT LICHENS FROM CONTRASTING HABITATS

The photosynthetic properties of cyanobiont lichens from contrasting h abitats were measured to identify whether the increased assimilation r ates which characterized Peltigera membranacea (Ach.) Nyl. from an exp osed habitat were correlated with increased carbon-concentrating mecha nism (CCM) activity. The results were contrasted with data obtained fr om two populations of Peltigera praetextata (Florke ex Sommerf.) Zopf collected from dry and damp microhabitats within a shaded woodland and Peltigera leucophlebia (Nyl.) Gyelnik, which has been shown to lack a carbon-concentrating mechanism. The differences in assimilation rates between the cyanobiont lichens were not accounted for by differences in chlorophyll content. Peltigera membranacea from the exposed habitat which had the highest assimilation rates had the lowest Gamma and K-0 .5 values and accumulated the greatest C-i-pool indicating that increa sed C-i accumulation contributed towards the higher assimilation rates shown by these species. The convexity of the light response curve for the cyanobiont lichens decreased with increasing assimilation rates. This might have indicated a diversion of electron transport to energiz e the carbon-concentrating mechanism, The apparent quantum efficiency of CO2 assimilation (Phi CO2) was correlated with the genus of lichen photobiont. All cyanobiont lichens had comparable values for Phi CO2 w hich were greater than that of the tripartite Peltigera leucophlebia. Light compensation points reflected the exposure of the habitats with higher compensation points characterizing the cyanobiont population fr om the exposed crag and the tri-partite population from the open grass land. Carbon isotope discrimination values for organic matter and meas ured instantaneously were the same for all cyanobiont lichens and were comparable with values recorded for species with a carbon-concentrati ng mechanism. Carbon isotope measurements for P. leucophlebia were typ ical of those recorded for species without a carbon-concentrating mech anism. Variation in source isotope signature and refixation of respira tory CO2 were considered to be significant factors in determining orga nic matter and instantaneous carbon-isotope discrimination. These fact ors might have masked any subtle variation in carbon-isotope discrimin ation which resulted from variable CCM activity. The functional signif icance of increased carbon-concentrating mechanism activity in cyanobi ont lichens occupying exposed habitats is discussed.