CHLOROPHYLL FLUORESCENCE QUENCHING AND VIOLAXANTHIN DEEPOXIDATION OF FBPASE ANTISENSE PLANTS AT LOW-LIGHT INTENSITIES AND LOW-TEMPERATURES

Genetically modified potato (Solanum tuberosum L, cv. Desiree) and tob acco (Nicotiana tabacum cv. Samsun N.N.) plants were used to analyze t he effects exerted by the chloroplastic (cp) fructose-1,6-bisphosphata se (FBPase) on the regulation of light energy discrimination at the le vel of photosystem II. The cp-FBPase activity was progressively inhibi ted by an mRNA antisense to this FBPase. The chlorophyll fluorescence quenching parameters of these transgenic plants were compared to those of wild-type and transgenic plants that were acclimated to low temper atures. In particular various lines of the transgenic potato and tobac co plants were exposed to a temperature treatment of 10 and 20 degrees C for 10 days. Light intensities were kept low to reduce photoinhibit ion so that we could analyze exclusively the effects of a modification in the carbon fixation cycle on the chlorophyll fluorescence quenchin g parameters. The photon flux densities (PFDs) employed at the level o f the middle leaves of all plants were set to two different values of 10 pmol m(-2) s(-1) and 50 mu mol m(-2) s(-1). Subsequent to this 10-d ay acclimation the chlorophyll-fluorescence parameters of all plants w ere measured. Photoinhibition as expressed by the F-v/F-m ratio was mi nor in plants subjected to a PFD of 10 pmol m(-2) s(-1). Higher photon fluence rates of 50 pmol m(-2) s(-1) at temperatures of 10 degrees C gave rise to a significant reduction in the F-v/F-m, ratios obtained f rom the transgenic plants which were characterized by a restriction in cp-FBPase capacity to 20% of normal activity. Furthermore, a progress ive inhibition of the cp-FBPase activity induced an amplified nonphoto chemical quenching of chlorophyll fluorescence within the genetically manipulated species (except at 10 degrees C and 50 pmol m(-2) s(-1)). The increase in nonphotochemical quenching depended upon light and tem perature. Photochemical quenching of light quanta within the antisense plants declined relative to that in the wild type. To further charact erize the mechanisms producing higher levels of nonphotochemical fluor escence quenching, we analyzed several of the xanthophyll cycle pigmen ts. The deepoxidation state of the xanthophyll cycle pigments in potat o plants increased with attenuating FBPase activities under all condit ions. For tobacco plants, this elevation of the deepoxidation state wa s only observed at a PFD of 50 mu mol m(-2) s(-1).