We also examined the changes in carbohydrate contents during
aging of sunflower primary leaves in order to identify their poten-
tial roles as metabolic signals for senescence. The plants grown at
elevated CO2 concentrations exhibited higher contents of starch
and soluble sugars (glucose and fructose) throughout development
than the control plants (Fig. 4A and B). In both CO2 treatments,
the concentrations of soluble sugars increased during leaf aging,
but the starch content strongly declined (Fig. 4D). The hexoses
(glucose + fructose) to sucrose ratio increased at the beginning of
senescence especially at elevated CO2 concentrations. This suggests
that accumulation of hexoses in leaves may play a role in regulat-
ing leaf senescence, mainly in the plants grown at elevated CO2
concentration (Fig. 4).