THE EFFECT OF LONG-TERM EXPOSURE OF LOLIUM-PERENNE L PLANTS TO ELEVATED CO2 AND OR ELEVATED AIR-TEMPERATURE ON QUANTUM YIELD OF PHOTOSYSTEM2 AND NET PHOTOSYNTHESIS/

The effects of long-term exposure of Lolium perenne L. plants to CO2 c oncentration elevated to 700 mu mol (CO2) mol(-1) (EC) and air tempera ture elevated by 4 degrees C (ET) on the quantum yield of electron tra nsport of photosystem 2, PS2 (phi(2)) and on the potential yield of ph otochemical reactions of PS2 (F-v/F-m) measured by the chlorophyll (Ch l) fluorescence method, were studied. The plants were exposed for 6 mo nths in opened field greenhouses to four treatments simulating global atmospheric changes: (1) ambient CO2 (AC) and ambient air temperature, AT (ACAT - control), (2) EC and AT (ECAT), (3) AC and ET (ACET), and (4) EC and ET (ECET). When the plants were exposed to stepwise increas ed irradiance, a decrease in phi(2) was found under both AC and EC mea suring concentrations. At high irradiances a significantly higher yiel d of PS2 was detected when measured under EC compared to AC regardless of long-term CO2 and temperature treatment (i.e., positive short-term . effect of EC). The short-term effect of EC on phi(2) as related to n et photosynthetic rate (P-N) Shift was detected from irradiance respon se curves. At high irradiances and AC, phi(2) was reduced in compariso n to control for the plants of EC and ET treatments (i.e., negative lo ng-term effect of treatment). The long-term effect of both EC and ET o n the yield of PS2 was attributed to a down-regulation of P-N caused b y the treatment. The phi(2) was related to the actual rate of photosyn thesis and the relationship between phi(2) and phi(CO2) was linear ove r a wide range of irradiances. No effect of long-term treatments on th e dark-adapted F-v/F-m ratio was found in plants cultivated under natu ral greenhouse irradiance.