INTRASPECIFIC VARIATION IN PHOTOSYNTHETIC RESPONSES OF TREBOUXIOID LICHENS WITH REFERENCE TO THE ACTIVITY OF A CARBON-CONCENTRATING MECHANISM

The photosynthetic responses of a range of trebouxioid lichens were in vestigated to determine whether variations in net assimilation rates s hown by populations of the same species collected from different habit ats could be correlated with adjustments in carbon-concentrating mecha nism (CCM) activity. The activity of a CCM was inferred from the high affinity for CO2 [i.e. low CO2 compensation point (Gamma); low externa l CO2 concentration at which half-maximal assimilation rates an reache d (K-0.5 CO2)], the release of a pool of accumulated dissolved inorgan ic carbon (C-i) during light/dark transient measurements of CO2 exchan ge and values for carbon isotope discrimination intermediate between t hose characteristic of C-3 and C-4 terrestrial plants. Higher net and gross assimilation rates were expressed by lichens collected from shad ed woodland habitats. The higher rates were not accounted for by varia tions in chlorophyll content. Lichens with high assimilation rates als o showed an increased affinity for CO2 as demonstrated by low CO2 comp ensation points and K-0.5 values and the magnitude of the C-i pool acc umulated upon illumination and released after darkening of the thalli. However, there was no correlation between assimilation rates and orga nic matter or instantaneous carbon isotope discrimination measursments , with the latter remaining roughly consistent whatever the provenance or species of the lichen material. The data are discussed with refere nce to significant environmental factors which are likely to control p hotosynthesis in the habitats studied.