Résumé
Batch suspension cultures of chicory cells (Cichorium intybus L. var. Witloof) possess a NADH-specific nitrate reductase activity that peaks on day 3 of a 10 d growth cycle. When both nitrate and ammonium are used as nitrogen sources, chicory cells absorb nitrate irst. Ammonium uptake becomes predominant at day 3, even though NO3− was still present in the medium. Although abscisic acid impairs growth as well as 15NO3− uptake and reduction, it promotes nitrate reductase activity as measured both in vivo and in vitro. Specific activity is 50% higher in ABA-treated cells than in controls. These conflicting data may be explained either in erms of nitrate reductase levels or by the availability of reducing power and energy. Since NRA is generally controlled by the availability of the reducing power, the energy status of the cell, the adenylate nucleotide pools, were measured simultaneously with the carbohydrate levels within the cell and the growth medium. The energy charge was not modified during the growth cycle, regardless of the rowth conditions. Yet ABA modified the intracellular carbohydrate metabolism and inhibited the acidic invertase, the sucrose synthase and the sucrose phosphate synthase activities. Modified assimilation rates of nitrate in chicory cells grown in the presence of ABA, were probably correlated to modified carbohydrate metabolism pathways leading to increased availability of reducing power, energy and C-skeletons.