Diurnal Variations in Some Enzymes of Carbohydrate Metabolism in Tapioca Leaves

Abstract

Diurnal activities of some enzymes of carbohydrate metabolism and values for protein, total nitrogen, non-protein nitrogen and dry solid contents have been studied in the leaves of the tapioca plant (Manihot utilissima Pohl). Leaf samples were collected at 4 hourly intervals for a 24 hour period starting from 8 a.m. on the months Feb.-March, when the nights were comparatively cool and days warm and the sky clear, and the duration of the day about 12 hours. The various assays were carried out on homogenates in water. Fructose-1:6-diphosphate aldolase, starch phosphorylase, and alkaline fructose di-phosphatase activities were considerably higher in the day time than in the night Phosphoglucomutase activitiy tended to be higher in the night There was no diurnal fluctuation in the activities of glumatic oxaloacetic transaminase, and acidic and neutral fructose diphosphatases. The results from assays on mixed homogenates discounted the possibility that the presence of activators or inhibitors at different times of the day was the cause of the observed alteration in enzyme activities. It was concluded that the rhythmicity was due to changes in net catalytic sites available. The fluctuation in phosphorylase activity was not accompanied by any change in its intracellular location. The content : of protein or total and non-protein nitrogen showed no marked fluctuation, as also the pH of the homogenates and dry weight of the tissue. It would appear that specific enzyme proteins were being formed and removed by transformation occurring among the various proteins, rather than by fresh synthesis from non-protein nitrogen. The significance of the observed diurnal activity of the enzymes studied is discussed from the point of view of carbohydrate synthesis in the day time and breakdown in the night Also discussed is the selective role of alkaline fructosediphosphatase in photosynthesis.