Carbon Uptake and Distribution Before and After Defoliation of Alfalfa¹

Abstract

The deposition and redistribution of 14 C was followed during the growth of individual alfalfa plants. 14 CO 2 was administered at 5 weekly intervals (plants were sampled 2 days after each addition of 14 CO 2 ). The plants were then defoliated, and the redistribution of 14 C was followed during the subsequent growth. During the 1st week of 14 CO 2 uptake, only 17% of the 14 C in the ]plant was translocated to crown and roots. By the 5th week, 36% of the 14 C in the plant was in crown and roots. It was not until the 3rd week that 14 C 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, 14 C 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 14 C was lost (respiration, leaching, and sloughing) from the plant, 19% had moved into top growth and large roots had lost 70% of their original 14 C content.