| There is a dearth of information on photoassimilate partitioning in water-stressed alfalfa (Medicago sativa L.). Our objective was to determine the effect of stress induced by water deficit on photoassimilate partitioning and the utilization of stored assimilates during regrowth. Field and greenhouse experiments were conducted using alfalfa cultivars differing in winterhardiness. Plants were subjected to water stress, pulse-labeled with {dollar}sp{lcub}14{rcub}{dollar}CO{dollar}sb2{dollar}, and sampled following 0, 1, and 14 (harvest and rewatering) d translocation periods. Subsequent samples were taken at 7 and 14 d after harvest and rewatering. Water stress (plant water potential (Yw) {dollar}<{dollar} {dollar}-{dollar}1.5 MPa) resulted in herbage and root dry mass of 65 and 119% of the control (Yw {dollar}>{dollar} {dollar}-{dollar}1.0 MPa), respectively, 14 d after labeling. Stressed plants had similar net carbon exchange and respiration rates but retained 10% greater percent total plant radioactivity (%TPR) in the leaves at the onset of the translocation period than did those of the control. Roots of water-stressed plants had 8% more starch and 12% greater %TPR in the starch fraction 14 d after labeling than did roots of control plants. The stressed plant roots contained 73 and 114% more %TPR than the control at the 1 and 14 d translocation periods, respectively. Water stress had no effect on individual or total root sugar concentration or the %TPR of the root sugar fraction. Alfalfa regrowth mass following harvest and rewatering of the water-stressed plants was similar to that of the control. Crown and axillary regrowth of previously stressed plants contained on average 4% more and 13% less %TPR, respectively, than control plants. Cultivars contrasting in winterhardiness were similar in partitioning and regrowth responses to water stress. The maintenance of net carbon exchange rate and the alteration of photoassimilate partitioning may play important roles in the ability of alfalfa to tolerate water deficits. |
