CARBOHYDRATE ACCUMULATION AFFECTS THE REDOX STATE OF ASCORBATE IN DETACHED TOBACCO-LEAVES

A decreased utilization of NADPH for CO2 fixation as a result of carbo hydrate accumulation in chlorotic leaves is generally believed to be a ssociated with an increase in oxidative stress. Molecular oxygen may s erve as an alternative electron acceptor of photosynthesis under these conditions. In order to test this hypothesis mature leaves of tobacco plants (Nicotiana tabacum) were detached, fed with glucose (50 mM) vi a the petiole, and used to study the effect of carbohydrate accumulati on on the pigment content and the components of the superoxide dismuta se-ascorbate-glutathione cycle. During a period of five days the conce ntration of total soluble carbohydrates increased substantially in lea ves supplied with glucose in comparison with control leaves supplied o nly with water. This increase was accompanied by a twofold decrease in the chlorophyll content. In detached water-fed leaves the levels of m ost of the antioxidative components increased, whereas glucose feeding had no or only little additional effect on the activities of the prot ective enzymes, but caused a 1.6- and 4-fold increase in the contents of glutathione and ascorbate, respectively. In relation to the total f oliar ascorbate pool, the amount of reduced ascorbate increased from a bout 30% to 60% upon feeding with glucose. These results do not suppor t the idea that the accumulation carbohydrates per se cause an increas ed rate of superoxide production which necessitates increased activiti es of antioxidative enzymes. It rather appears that glucose-fed leaves have an increased reductive capacity that can be released via glutath ione into the ascorbate pool, thereby, regulating the redox state of a scorbate.