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Changes in Nonstructural Carbohydrate Fractions at Intervals Down the Tap Root Bark and Wood of Alfalfa (Medicago sativa L.) During Regrowth¹

Changes in total nonstructural carbohydrates (TNC) at intervals down the tap root of alfalfa (Medicago sativa L.) during regrowth were studied to determine what depth of root sampling would adequately reflect readily available carbohydrate trends during regrowth. After the tops were harvested in mid-August, alfalfa roots were dug from the soil at weekly intervals to a 30-cm depth. Tap roots were cut into six segments of 5-cm length each, separated into bark and wood tissues, and dried at 70 C. Wood made up more of each tap root segment than bark, but ratio of bark to wood increased down the root. Content (grams) of carbohydrate fractions in the segments gave a clearer picture of changes down the root, and with time, than concentration (percentages). About 63% of the TNC used between 0 day and the date of lowest carbohydrate level during regrowth came from the root wood, while only 37% came from the root bark. Starch (ST) content accounted for 62% of the TNC used; 38'% was total sugars (TS). About 38% of the TNC content came from the top 5-cm segment and 23, 16, 10, 6, and 770 came from segments 2, 3, 4, 5, and 6, respectively. About 5S7, of the net gain in TNC content was accumulated in the root wood between the date of lowest carbohydrate level and 47 days, while 42Y0 was accumulated in the root bark. ST content accounted for 88% of the TNC content accumulated; 12% was TS. About 34% of the TNC content accumulated was in the top 5-cm segment and 22, 15, 12, 9, and S70 could be found in segments 2, 3, 4, 5, and 6, respectively.

Loss or gain of weight by each root tissue was almost equal to that of TNC. TNC trends were similar for each root segment, indicating that any depth of root sampling will adequately reflect carbohydrate changes during regrowth.

Key Words: Reducing sugars • Total sugars • Nonreducing sugars • Starch • Total nonstructural carbohydrates • Tap root segments

² Research Assistant and Professor of Agronomy, respectively, University of Wisconsin, Madison, Wis. 53706.

Received for publication February 19, 1972.