Distribution of Saccharides Between Cytoplasm and Vacuole in Protoplasts

Abstract

Glucose and fructose have a key function in metabolism. They can be utilized as a source of carbon in most synthetic pathways and can also satisfy the demand for metabolic energy. It is not surprising, therefore, that plant cells always contain a considerable pool of free hexoses and sucrose. It confers to the cell a certain independence on the supply of carbon and energy. Depending on species, type of organ, developmental state, and environmental conditions, pools of hexoses, sucrose, and other saccharides such as oligosaccharides and water soluble polysaccharides (fructans) may be exceedingly high, particularly in storage tissues. When, e.g., barley leaves are excised and illuminated strongly they are not able to export the enormous amounts of fixed carbon. They then store it in the form of fructan which may eventually account for as much as 70% of the dry weight (Wagner et al. 1983). It is obvious that such a large mass of solutes cannot be accommodated in the cytoplasm. Solutes imported or produced in excess are stored in the large internal extraplasmatic space of the plant cell, the central vacuole. This compartment offers to the cells an almost unlimited capacity for the accommodation of saccharides.