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Growth and Carbohydrate Changes in the Root Wood and Bark of Different Sized Alfalfa Plants during Regrowth after Cutting¹

Small, medium, and large alfalfa (Medicago sativa L.) plants were allowed to regrow 42 days in the greenhouse following removal of the herbage. Weight of new herbage and total top growth increased steadily during regrowth and was proportional to initial plant size and to numbers of shoots produced per pot. Total root weight for each plant size decreased during the first few weeks and then increased. Weight of root wood was always higher than weight of bark.

Minimum levels (g/pot) of total nonstructural carbohydrates (TNC) in the original plant (crown plus taproot) were reached at 7 days in the small plants and at 14 days in the medium and large plants, but weight of TNC in large plants was nearly as low at 21 as at 14 days. Proportion of the weight of TNC utilized until the minimum level was reached was the same for each plant size (27 to 33%). At the time of minimum TNC level, small, medium, and large plants had utilized 1.50, 0.75, and 0.58 g of TNC during the production of 1 g of herbage dry matter. TNC restoration (g/pot) in the original plant after it reached the minimum level was greatest for the large plants and least for the small plants.

Among the three original plant tissue fractions (crown, taproot wood and bark), highest percentage of TNC was found in the root wood, followed in order by the root bark and the crown. Nearly 55% of the TNC (g/pot) in the original plant was found in the root wood, 20% in the bark, and 25% in the crown at all stages of regrowth regardless of plant size. Of the weight of TNC utilized until the minimum level was reached, 46 to 52% was utilized from the root wood, 21 to 26% from the root bark, and 24 to 28% from the crown regardless of plant size

Key Words: Top growth yield • Plant yield • Shoot numbers • Reducing sugars • Sugars • Starch • Nonstructural carbohydrates

² Visiting Scientist and Professor of Agronomy, respectively, University of Wisconsin, Madison, Wis. 53706. The senior author's permanent address is Department of Forage Crops, National Institute Animal Industry, Aoba-Cho, Chiba-Shi, Japan. The senior author acknowledges the Japanese Government for their grant of travel funds.

Received for publication January 22, 1970.