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Partitioning and Mobilization of Phoassimilate in Alfalfa Subjected to Water Deficits

Dep. of Plant, Soil and Entomological Science, Univ. of Idaho, Moscow, ID 83843
Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108
USDA-ARS and the Dep. of Agronomy and Plant Genetics Univ. of Minnesota, St. Paul, MN 55108

^* Corresponding author.

Faster regrowth of a stressed alfalfa (Medicago sativa L.) crop compared to an unstressed crop after rewatering has been reported. The bases of this compensatory response are unknown, but they may be important to understanding adaptation to water stress and to developing crop water management strategies. Our objective was to determine the effect of stress induced by water deficit on photoassimilate partitioning and the utilization of stored assimilates during regrowth of alfalfa. Field and greenhouse experiments were conducted using cultivars differing in winterhardiness. Plants were subjected to water stress (plant water potential [_W > –1.5 MPa), pulse-labeled with ¹⁴CO₂, and sampled following 0, 1, 14, 21, and 28-d translocation periods. Following the 14-d sampling, herbage was harvested and water stress was removed. Cultivars contrasting in winterhardiness responded similarly to water stress. Stressed and control plants (_W > –1.0 MPa) contained similar amounts of radiocarbon per unit mass of herbage at the onset of the translocation period. Water stress reduced herbage dry mass and did not alter root dry mass compared to the control. Stressed plant roots contained 73 and 114% more total plant radioactivity (TPR) than the control at the 1 and 14-d translocation periods, respectively. Water stress significantly increased root starch and TPR percentage in the starch fraction, but had much smaller effects on root soluble-sugar concentration and TPR percentage of the root sugar fraction. Herbage regrowth mass following harvest and rewatering of the waterstressed plants was similar to that of the control. Compared to the control, water-stressed alfalfa has greater total nonstructural carbohydrates in the roots, apparently due to increased photoassimilate partitioning to the roots. However, the greater root carbohydrate concentrations did not result in compensatory herbage regrowth following rewatering.

Received for publication December 2, 1987.

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