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Growth, Composition, and Metabolic Responses of Sugar Beet to Preharvest Application of Maleic Hydrazide¹

Maleic hydrazide, 8 x M, was applied to the foliage of mature plants of sugar beet (Beta vulgaris L.). Its effects on growth and various aspects of composition and metabolism were determined 7, 14, and 21 days after treatment. The striking result was a significant curtailment of growth of leaves and an increase in sucrose content of the storage root. Maximum inhibition of leaf growth, 66%, was evident 7 days after treatment. The sucrose content of the roots was significantly larger on all three days of measurement, while protein and total N were considerably less. Higher nitrate and lower nitrite contents paralleled a decrease in the activity of nitrate reductase.

Respiration in both leaves and roots was depressed, but the photosynthetic rate was greater by 49%, 21 days after treatment. The activity of nitrate reductase and transaminase in leaf and root fell 40% and 18% below values in control plants by the 21st day following treatment. Invertase activity was significantly reduced. The activities of glucose-1-, glucosed-, and fructose-6-phosphatases; phenyl phosphatase; and ATPase were depressed by treatment. On the other hand, the activities of the enzymes of sucrose biosynthesis, UDPG-pyrophosphorylase, sucrose synthetase, and sucrose phosphate synthetase, were significantly stimulated.

The responses suggest that "ripening" may be induced and that the late autumn growth, which often occurs at the expense of stored sucrose, may be considerably reduced by the foliar application of maleic hydrazide 7 to 21 days before the sugar beets are to be dug.

Key Words: Sucrose • Respiration • Photosynthesis • Enzymes • "Ripening" of sugar beet roots

² Professor of Plant Physiology and Postdoctorate Fellow, D of Botany, University of British Columbia, Vancouver 8, Canada.

Received for publication June 16, 1969.