LONG-TERM CHILLING OF YOUNG TOMATO PLANTS UNDER LOW LIGHT AND SUBSEQUENT RECOVERY .2. CHLOROPHYLL FLUORESCENCE, CARBON METABOLISM AND ACTIVITY OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE

To identify possible reasons for the persisting impairment of photosynthesis after long-term chilling, young tomato (Lycopersicon esculentum Mill.) plants were exposed to 6-10-degrees-C for two weeks under low illumination during the daily light period (60-100-mu-mol quanta . m-2 . s-1). The time courses of leaf carbohydrate contents, phosphorylated intermediates and chlorophyll-fluorescence parameters were followed. While starch formation was impaired during chilling at 6-degrees-C, soluble sugar contents increased from the first day onwards and reached up to eightfold the values found in unchilled plants within two weeks. At 8 and 10-degrees-C, a less drastic increase in soluble-carbohydrate contents was observed. During chilling, glucose-6-phosphate and fructose-6-phosphate accumulated up to 16 mM (assuming they are restricted to the cytoplasm). At the same time, non-photochemical quenching of chlorophyll fluorescence had increased and did not return to control values during the first week of recovery. The 3-phosphoglyceric acid/triose phosphate ratio remained nearly unaffected by the chilling treatment, indicating that the assimilatory power of the plants was still high even at the low temperatures. As a consequence of the chilling treatment, ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) activity in the chilled leaves was irreversibly decreased. It is suggested that, in addition to a possible (orthophosphate-mediated) feedback inhibition by internal sugar accumulation, the low activity of Rubisco can play a significant role in the strong decrease of photosynthetic capacity during long-term chilling in tomato.