Storage of sugars and mannitol in petioles of celery leaves

Mannitol and sucrose are produced photosynthetically in the mesophyll and translocated in the phloem of celery leaves. The thick and fleshy petioles of these leaves act as reversible carbohydrate sinks, storing mainly mannitol, glucose and fructose but very little sucrose (less than 3.3 mg ml^-1 sap) in their parenchyma. The concentrations of soluble carbohydrates and the activities of enzymes of sucrose cleavage were asayed in the petiole parenchyma during leaf development. Mannitol (about 20 mg ml^-1 sap), glucose and fructose (about 10 to 15 mg ml^-1 sap each), changed little with development. Soluble acid invertase accounted for over 95 % of the total enzymic sucrose cleavage measured in vitro. Its activity was closely related to development with a sharp decrease during leaf maturation. With protoplasts from petiole parenchyma and vacuoles isolated from them it was demonstrated that 28% of the total soluble acid invertase of the tissue was in the apoplast and 72 % in the vacuole. Alkaline invertase and sucrose synthase were totally cytosolic. Glucose and fructose, the main storage sugars of these protoplasts, were vacuolar in location, whereas mannitol was both cytosolic (19%) and vacuolar (81%).