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Carbon Uptake and Distribution Before and After Defoliation of Alfalfa¹

The deposition and redistribution of ¹⁴C was followed during the growth of individual alfalfa plants. ¹⁴CO₂ was administered at 5 weekly intervals (plants were sampled 2 days after each addition of ¹⁴CO₂). The plants were then defoliated, and the redistribution of ¹⁴C was followed during the subsequent growth.

During the 1st week of ¹⁴CO₂ uptake, only 17% of the ¹⁴C in the ]plant was translocated to crown and roots. By the 5th week, 36% of the ¹⁴C in the plant was in crown and roots. It was not until the 3rd week that ¹⁴C started to accumulate (exponentially) in the "starch" fraction. This coincided with the time that large roots (> 1 mm in diameter) started to increase in dry weight.

After defoliation, ¹⁴C changes in the plant best fit a sigmoid curve. The greatest changes occurring between 3 and 15 days after defoliation. During the 28 days of regrowth, 45% of the ¹⁴C was lost (respiration, leaching, and sloughing) from the plant, 19% had moved into top growth and large roots had lost 70% of their original ¹⁴C content.

Key Words: Radioactive carbon • Carbohyrates • Medicago sativa L

² Plant Physiologist, Research Assistant, and Research Agronomist, respectively, Crops Research Division, Agricultural Research Service, U.S. Department of Agriculture. Beltsville, Md. 20705.

Received for publication May 5, 1969.