PHOTOSYNTHESIS OF TERRESTRIAL CYANOBACTERIA UNDER LIGHT AND DESICCATION STRESS AS EXPRESSED BY CHLOROPHYLL FLUORESCENCE AND GAS-EXCHANGE

Terrestrial mats of cyanobacteria with other associated microscopical cryptogams were obtained from various sites in the tropics, i.e. rocks of mountains and rock-outcrops and bare soil surfaces, a valley in th e Austrian Alps and a glasshouse. Species diversity of each sample was analysed qualitatively, The samples came from very different light cl imates, Responses to light and desiccation stress were studied using t he saturation pulse method for recording chlorophyll fluorescence vari ables as well as by measuring net CO2 and O-2 exchange in order to con firm results by independent methods. Under light stress, shade and hig h-light, samples showed a reduction of gas exchange and of the fluores cence variables photochemical fluorescence quenching coefficient (qp), potential quantum yield of photosystem II (F-v/F-m) and effective qua ntum yield (Delta F/F'(m)) and recovery to different degrees, Short-te rm desiccation led to suppression of variable fluorescence yield and i nhibition of gas exchange, but recovery occurred rapidly within 1 h af ter rewetting, It is concluded that together with the well-known abili ty for assimilation of atmospheric dinitrogen, desiccation tolerance o f terrestrial cyanobacteria mats is the major reason for their success as pioneers on bare substrata, In addition, the capacity to produce s un and shade forms adapted to a very wide range of light intensities a ppears to be important.