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Foliar Regrowth of Alfalfa Utilizing ¹⁴C-Labeled Carbohydrates Stored in Roots¹

Alfalfa (Medicago sativa L.) at the bud to early-bloom stage was placed in a ¹⁴CO₂ atmosphere for 24 hr to label carbohydrates being synthesized. Labeled carbohydrates that were translocated to the crown and roots within the 24 hr were considered as "stored" therein. Immediately following the labeling period, shoots were harvested and discarded, leaving a 5-cm stubble. Thereafter, plants were harvested during the regrowth period at each of 5 stages — vegetative (15 and 30 cm), bud, first bloom, and 50% bloom and separated into shoots and roots plus crown.

Labeled carbohydrates initially stored in the root and crown were readily redistributed to developing shoots. Greatest redistribution occurred at the early vegetative stage and decreased with advancing maturity. Approximately 40% of the initially-stored nonstructural carbohydrates were found in the shoots at the early vegetative stage. Only 13% of these labeled carbohydrates remained in the shoots at 50% bloom. A maximum of 9% of the stored carbohydrates was utilized for the synthesis of structural tissues in the developing shoots. This maximum appeared at the 50% bloom stage. Respiratory utilization by the entire plant of labeled stored carbohydrates continued with advancing shoot maturity, ranging from 26% at the youngest vegetative stage to 71% at 50% bloom.

Key Words: Root reserves • Vegetative regrowth

² Professor in the Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, Minnesota 55101, and Research Agronomist, Crops Research Division, ARS, USDA, respectively.

Received for publication July 5, 1969.