THE SUSCEPTIBILITY OF PS-II OF LOLIUM-PERENNE TO A SUDDEN FALL IN AIR-TEMPERATURE - RESPONSE OF PLANTS GROWN IN ELEVATED CO2 AND OR INCREASED AIR-TEMPERATURE/

The effect of a sudden fall in air temperature from 20 to 5 degrees C on fast kinetics of chlorophyll fluorescence, maximum yield of the pho tosystem II photochemical reactions (Fv/Fm), quantum yield of the phot osystem II electron transport (Phi(II)) coefficients of photochemical (qP), non-photochemical quenching (qN) was studied in Lolium perenne u sing a modulated chlorophyll fluorescence technique. Before fluorescen ce measurement, the plants were cultivated in the treatments simulatin g the likely future climate characterized with elevated air temperatur e and CO2 concentration and combination of both. On fast kinetics curv es the risetimes of the I and D points characterizing the redox state of Q(A) were affected by lowering the air temperature. At 5 degrees C both the I and D points were reached later than at 20 degrees C. Also the I to D risetime was prolonged at 5 degrees C and it was found sign ificantly longer in plants cultivated in ambient + 4 degrees C tempera ture. While a significant difference was found in the area over the ri sing part of the fluorescence curve between 20 and 5 degrees C, no dif ference was found in area over the relaxation curve part. Lowering of air temperature to 5 degrees C had no effect on Fv/Fm values in contro l plants and in the plants cultivated in elevated CO2 but brought sign ificant decrease in plants cultivated in the ambient + 4 degrees C air temperature. Both Phi(II) and qP decreased with the temperature lower ed to 5 degrees C while the values of qN increased. The changes in flu orescence parameters indicated altered functioning of PS II at low tem perature. The changes in parameters are discussed as a consequense of decreased enzymatic activity, decreased turnover of plastoquinone pool and photoinhibition. (C) 1998 Elsevier Science B.V. All rights reserv ed.