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Diurnal Variation in Nonstructural Carbohydrates of Sorghum sudanense (Stapf) as Influenced by Environment¹

Sudangrass (Sorghum sudanense (Stapf) herbage (leaves plus stems) and leaves, with 0 and 140 kg/ha of N fertilizer, were sampled four times daily on 8 days and eight times daily on 3 days to i) measure the diurnal variation in several nonstructural carbohydrates, ii) late carbohydrate trends to environmental conditions, and iii) determine the influence of nitrogen fertilization on diurnal carbohydrate changes.

Reducing sugars and sucrose were extracted from freeze-dried, ground tissue with alcohol. Starch in the residue was hydrolyzed with takadiastase. Ferricyanide, resorcinol, and phenol-sulfuric acid procedures were used to quantitatively assay sugars in the extracts.

Sucrose concentration in sudangrass herbage varied from 1.8% of the dry matter at 6 AM to 5.8% at 6 PM. Starch increased from 6.6 to 8.8% in the same period. Half of the daily increase in carbohydrate disappeared between 6 PM and 12 M. The diurnal trends in leaves were similar to those observed in herbage. Nitrogen fertilization (140 kg/ha) reduced the average total carbohydrate concentrations from 16.6 to 14.8% but did not change the magnitude of diurnal trends. Starch concentration declined from 9.9 to 5.9% of the dry herbage and sucrose increased from 1.2 to 6.6% during the 16-day sampling period. This change was more pronounced in N-deficient sudangrass. Rates of sucrose accumulation varied with radiation level and temperature, but the maximum concentrations reached each day were about 5.5%, indicating a physiological limit. A cold night caused the leaves to retain unusually high levels of sucrose overnight. Sucrose concentrations were as high as 2.5% compared to concentrations of less than .3% on warm nights. Sudangrass temporarily stores most excess photosynthate as sucrose in contrast to alfalfa (Medicago sativa L), which accumulates much more starch.

Key Words: Sucrose • Starch • Nitrogen

² Assistant Professors, Purdue University, and Farm Crops Specialist, Cooperative Extension Service, Oregon State University, Corvallis, Oregon, respectively.

Received for publication August 15, 1972.