Physiological Studies of Enzymes Catalyzing the Synthesis and Hydrolysis of Sucrose, Starch, and Phosphorylated Hexose in Sugarcane

Abstract

Four-month-old sugarcane plants were subjected to low levels of nitrate, potassium, and water in sand culture to induce variable sugar levels. Leaf samples were frozen for enzyme assay at intervals of 7 or 14 days following treatment. The objective was to study interrelationships cf enzymes responsible for the synthesis and hydrolysis of sucrose, starch, and glucose phosphate. Enzymes measured included sucrose phosphorylase, starch phosphorylase, invertase, amylase, hexokinase, peroxidase, and the phosphatases hydrolyzing ß-glycerophosphate, glucose-1-phosphate, and ATP. A slight but generally consistent increase of sucrose was obtained by each of the three treatments. Sucrose phosphorylase and invertase were unaffected by any of the treatments, which placed the causes of sucrose changes upon systems less closely related to sucrose level. Phosphatases and amylase were generally suppressed by every treatment. Both the high and low extremes of sucrose content were followed, after 7 or 14 days, by starch-phosphorylase changes which were inversely related to the sucrose levels. Preliminary evidence suggests that starch phosphorylase fluctuated as a result, rather than as the cause, of sucrose changes. Availability of glucose- 1-phosphate seems to have been the key factor for both the sucrose-and starch-forming systems. Periods of high amylase activity were accompanied, or followed closely by concurrently high starch-phosphorylase activity. No starch was detected in either leaf or sheath extracts. The role of amylase in cane is discussed in detail, particularly as a means of preventing the loss of glucose from pathways of active metabolism, or from sucrose formation. Hexokinase varied independently of sucrose level. The enzyme was particularly active among low-potassium plants at the periods when glucose was abnormally high.