PREDICTING CO2 GAIN AND PHOTOSYNTHETIC LIGHT ACCLIMATION FROM FLUORESCENCE YIELD AND QUENCHING IN CYANO-LICHENS

Modulated chlorophyll a fluorescence is useful for eco-physiological s tudies of lichens as it is sensitive, non-invasive and specific to the photobiont. We assessed the validity of using fluorescence yield to p redict CO2 gain in cyano-lichens, by simultaneous measurements of CO2 gas exchange and chlorophyll a fluorescence in five species with Nosto c-photobionts. For comparison, O-2 evolution and fluorescence were mea sured in isolated cells of Nostoc, derived from Peltigera canina (Nost oc PC). At irradiances up to the growth light level, predictions from fluorescence yield underestimated true photosynthesis, to various exte nts depending on species, This reflected the combined effect of a stat e transition in darkness, which was not fully relaxed until the growth light level was reached, and a phycobilin contribution to the minimum fluorescence yield (F-o). Above the growth light level, the model pro gressively overestimated assimilation, reflecting increased electron f low to oxygen under excess irradiance. In cyanobacteria, this flow mai ntains photosystem II centres open even up to photoinhibitory light le vels without contributing to CO2 fixation. Despite this we show that g ross CO2 gain may be predicted from fluorescence yield also in cyanoli chens when the analysis is made near the acclimated growth light level . This level can be obtained even when measurements are performed in t he field, since it coincides with a minimum in non-photochemical fluor escence quenching (NPQ). However, the absolute relation between fluore scence yield and gross CO2 gain varies between species. It may therefo re be necessary to standardise the fluorescence prediction for each sp ecies with CO2 gas exchange.