A detailed investigation of the activity and control of starch phosphorylase in barley endosperm was carried out. The existence of at least two isoenzymes was indicated by studies of pH dependence and phosphate inhibition, and was further supported by acrylamide gel electrophoresis and column chromatography using DEAE -cellulose. A change in the isoenzyme pattern during grain development was observed. Synthesis of starch by phosphorylase in an apparently primer - free system was demonstrated, and this activity integrated with starch synthetase.

The activities of enzymes concerned with starch meta- bolism in barley endosperm were investigated. It was established that mechanisms by which sucrose can be converted to nucleotide sugars and thus incorporated into starch exist in the endosperm by 5 days after anthesis. Possible variations in these pathways during grain development are discussed.

A pathway for the conversion of sucrose to glucan polymers via GIP in very young endosperm is proposed. It is suggested that at least one, and possibly both, isoenzymes of phosphorylase are glycoproteins. Spatial arrangements of phosphorylase, starch synthetase, and branching enzyme in relation to the amyloplast are discussed.

The partition of starch synthetase between soluble and amyloplast fractions of the endosperm was established at various stages of development, and changes in affinity to- wards UDPG and ADPG were demonstrated. The relevance of these results to branching enzyme activity and amylopect in content is considered.

Concentrations of major biochemical constituents of developing barley endosperm were measured and correlated with morphological changes during maturation.