Assimilation and Movement of Radioactive Carbon in Alfalfa and Reed Canarygrass1

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Abstract
Alfalfa (Medicago sativa L.) and reed canarygrass (Phalaris arundinacea L.) were exposed in the field to radioactive carbon dioxide. Plants were treated at several maturity stages and C14 was measured in various plant parts at several dates following exposure. Carbon‐14 moved to the tap root during the 15‐min exposure time and accumulated with time in the tap root to a greater extent when exposed in the more mature stage (bud stage of alfalfa and early head stage of reed canarygrass) than when exposed in the vegetative stage.Relative CO2 assimilation as influenced by several light intensities was measured by C14 in leaves after exposing plants to labeled carbon dioxide. Each reduction of light intensity resulted in reduced C14 in leaves. Light saturation (maximum C14O2 uptake) throughout the profile did not occur with 50% of full sun intensity. Measurable uptake of C14 occurred with 0.1% of full sun but may have been due in part to dark fixation.Labeled carbon fed into one tiller of a plant as fructose and glucose was translocated throughout the underground rhizome system of a reed canarygrass clone and throughout all parts of an alfalfa plant. Carbon‐14 from fructose, the predominant building block of nonstructural carbohydrates in reed canarygrass, moved to a lesser degree than C14 from glucose. Carbon‐14 from glucose, the predominant building block of nonstructural carbohydrates in alfalfa, moved to a lesser degree than C14 from fructose.

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