Gas exchange and carbon isotope discrimination in lichens: Evidence for interactions between CO2-concentrating mechanisms and diffusion limitation

✍ Scribed by C. Máguas; H. Griffiths; M. S. J. Broadmeadow

• Book ID: 104658439
• Publisher: Springer-Verlag
• Year: 1995
• Tongue: English
• Weight: 912 KB
• Volume: 196
• Category: Article
• ISSN: 0032-0935
• DOI: 10.1007/bf00193222

✦ Synopsis

The characteristics of gas exchange and carbon isotope discrimination were determined for a number of lichen species, representing contrasting associations between fungal (mycobiont) and photosynthetic (photobiont) organism. These parameters were evaluated with regard to the occurrence of any CO2-concentrating mechanism (CCM) expressed specifically by the green algal (phycobiont) or cyanobacterial (cyanobiont) partner. Carbon isotope discrimination (A) fell into three categories. The highest A, found in lichens comprising a phycobiont plus cyanobacteria limited to pockets in the thallus (known as cephalodia), ranged from 24 to 28%0, equivalent to a carbon isotope ratio (~13C) of around -32 to -36%o vs. Pee Dee Belemnite (PDB) standard. Further evidence was consistent with CO 2 supply to the carboxylating system entirely mediated by diffusion rather than a CCM, in that thallus CO2 compensation point and online instantaneous A were also high, in the range normally associated with C 3 higher plants. For lichens consisting of phycobiont or cyanobiont alone, organic material A formed two distinct ranges around 15%0 (equivalent to a ~13C of --23%0). Thallus compensation point and instantaneous A were lower in the cyanobiont group, which also showed higher maximum rates of net photosynthesis, whether expressed on the basis of thallus dry weight, chlorophyll content or area. These data provide additional evidence for the activity of a CCM in cyanobiont lichens, which only show photosynthetic activity when reactivated with liquid water. Rates of net CO2 uptake were lower in both phycobiont associations, but were Abbreviations: ANOVA=analysis of variance; CCM = CO2-concentrating mechanism; cyanobiont = cyanobacterium; DIC = CO:+HCO3 +CO32-(dissolved inorganic carbon); photobiont = photosynthetic organism present in the association; phycobiont = green alga; phycobiont + cephalodia = green algae + cyanobacteria in cephalodia; Pmax=maximum photosynthetic rate; PPFD = photosynthetic photon flux density, 400-700 nm; Ru-bisco=ribulose-l,5-bisphosphate carboxylase/oxygenase; A=carbon isotope discrimination (%0); 613C = carbon isotope ratio (%)