Photosynthetic productivity of the epilithic lichen Lecanora muralis: Long-term field monitoring of CO2 exchange and its physiological interpretation - I. Dependence of photosynthesis on water content, light, temperature, and CO2 concentration from laboratory measurements

CO2 exchange of the epilithic, green algal lichen Lecanora muralis was monitored continuously for 15 months by means of an automatic cuvette in its natural habitat in the Botanical Garden, Wurzburg (Bavaria, Germany). As a first stage in the interpretation of the results, identically treated parallel samples were used, under controlled laboratory conditions, to determine the dependence of net photosynthesis (NP) and dark respiration (DR) on light, CO2 concentration, thallus hydration and temperature. Results of these measurements are communicated in the present, first part of a series of publications. The lichen showed an extremely large and extended suprasaturation depression of its CO2 assimilation. NP at high hydration was reduced almost to the compensation point. Maximal NP was only possible in a very small window of thallus water content (WC). Thallus water relations dominated the lichen's responses to the other environmental factors and the combination of 'optimal' moisture and light conditions was a rare event and lasted only for short times under natural conditions. Light response curves showed typical saturation kinetics with 'sun-plant' characteristics. Maximal NP, light saturation, maximal quantum use efficiency, and light compensation point were highly dependent on WC. Temperature optimum of NP was about 22 degreesC. But, at 2 degreesC, NP was still 60% of maximal NP. Under optimal WC, NP became saturated at c 1000 ppm external CO2 and, at natural, ambient CO2, NP was 62% of maximal NP with a CO2 compensation point of 16 ppm. Suprasaturation depression of NP was not completely removed by 5000 ppm external CO2, Seasonal changes in photosynthetic capacity and other photosynthetic parameters were extremely small. However, dark respiratory capacity showed a clear adaptation to low winter temperatures. This resulted in a higher light compensation point for CO2 exchange in winter than in summer. The daily and seasonal courses of CO2 exchange of L. muralis under natural conditions will be reported in further communications. The field performance of the lichen will be interpreted and analysed by means of the data presented here.