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Sugar Metabolism in Developing Kernels of Wheat and Barley¹

The tissue distribution of two enzymes of sucrose cleavage, invertase (E.C.3.2.1.26) and sucrose synthase (E.C.2.4.1.13), was studied during the development of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) kernels. Kernels from the center of the ear were sampled every three or four days from anthesis to maturity and dissected into outer pericarp, inner pericarp, and endosperm. Each tissue was assayed for insoluble invertase, sucrose synthase, and starch. The amounts of sucrose, glucose, and fructose leached from the tissues into cold osmoticum in 60 min and remaining in the tissue after leaching were also determined. Insoluble invertase activity was predominantly associated with the outer pericarp tissue in wheat. The tissue location and activity of invertase was less clear in barley. Sucrose synthase activity was associated largely with the endosperm in both wheat and barley. Substantial amounts of sucrose were found in the teachable and non-leachable fractions of all tissues of both wheat and barley. Very little free glucose or fructose was found in any tissue at any stage of development. Starch was present in the outer pericarp immediately after anthesis, but disappeared as starch accumulation in the endosperm began. Sucrose in the leachable endosperm fraction of wheat increased as the activity of sucrose synthase declined. From the data, we suggest that sucrose is not cleaved as it moves from the phloem to the endosperm in wheat or barley. The duration of sucrose synthase activity in the endosperm may be important in determining the duration of grain filling.

Key Words: Phloem unloading • Invertase • Sucrose synthase • Sucrose • Endosperm • Triticum aestivum L. • Hordeum vulgare L.

² Assistant professor and former graduate student (current address: USDA, Metabolism and Radiation Research Lab, State Univ. Station, Fargo, ND 58105), Dep. of Agronomy and Soils, Washington State Univ., Pullman, WA 99164.

Received for publication April 5, 1982.

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