ACTIVITIES OF SUCROSE METABOLISM ENZYMES IN GLYCEROL-GROWN SUSPENSION-CULTURES OF SWEET ORANGE (CITRUS-SINENSIS L OSBECK)

Activities of the enzymes involved in sucrose metabolism were investig ated in embryogenic nucellus-derived calli of 'Hamlin' sweet orange, C itrus sinensis L. Osbeck, cultured in suspension medium containing gly cerol or sucrose as the only carbon sources. Activities of sucrose pho sphate synthase (SPS) and sucrose synthase (SS), determined after 14 d ays of culture, were about 2.5-fold higher in the 2% glycerol-grown ti ssues, as compared to the 5% sucrose-grown tissues. Furthermore, activ ity of SPS was about two-fold and three-fold higher in the glycerol- a nd sucrose-grown tissues, respectively, when 40 mM glucose 6-phosphate (G6P) was added to the assay medium containing uridine 5'-diphosphogl ucose (UDPG) and fructose 6-phosphate. SPS activity increased 43% for the glycerol-grown tissues and 130% for the sucrose-grown tissues when only UDPG and G6P were present in the assay reaction. Gel electrophor etic analysis and catalytic determination confirmed the presence of ph osphoglucose isomerase in the tissues of both glycerol- and sucrose-co ntaining medium. Arbutin, an inhibitor of SS in plants, depressed most activity of SS in both glycerol- and sucrose-grown tissues when 5 mM was added to the assay medium. High activities of SPS, SS and invertas e and appreciable accumulation of sucrose, reducing sugars and starch in the glycerol-grown tissues indicate the ability of 'Hamlin' orange tissue cultures to efficiently use glycerol as a source of carbon for growth and differentiation.