PHOTOSYNTHETIC PERFORMANCE OF 2 CLOSELY-RELATED UMBILICARIA SPECIES IN CENTRAL SPAIN - TEMPERATURE AS A KEY FACTOR

Net photosynthesis (NP) and dark respiration (DR) of thalli of the lic hen species Umbilicaria grisea and U. freyi growing together in the sa me habitat in the Sierra de Guadarrama, central Spain, were measured u nder controlled conditions in the laboratory and under natural conditi ons in the field over a range of photosynthetic photon flux densities (PPFD), thallus temperatures and thallus water contents. Laboratory ex periments revealed that the photosynthetic response to PPFD at optimum thallus water content is very similar in both species. The light comp ensation points of NP increased from PPFD of c. 20 mu mol m(-2) s(-1) at 0 degrees C up to c. 100 mu mol m(-2) s(-1) PPFD at 25 degrees C. I n both species light saturation was not reached up to 700 mu mol m(-2) s(-1) PPFD except at 0 degrees C. By contrast, the temperature depend ence of CO2 gas exchange differed substantially between U. grisea and U. freyi. Both species gave significant rates at 0 degrees C. Optimal temperatures of NP were always higher in U. grisea at various PPFD lev els if the samples were kept at optimal thallus water content. NP show ed maximal rates at 95% dw in U. grisea and 110% dw in U. freyi respec tively. In U. grisea a much stronger depression of NP was observed wit h only 5% of maximal NP reached at 180% dw. At all PPFD and temperatur e combinations U. freyi showed higher rates of NP and more negative ra tes of DR if calculated on a dry weight basis. This was also true unde r natural conditions at the same site, when U. freyi was always more p roductive than U. grisea. The differences in the photosynthetic respon se to temperature between both species correlated well with the differ ent distribution patterns of both species. The possibility of genetic control of the physiological performance of these species and its infl uence on their distribution patterns and autecology is discussed. (C) 1997 The British Lichen Society