Photosynthate Partitioning in Alfalfa Before Harvest and During Regrowth

During the harvest regrowth cycle of alfalfa (Medicago sativa L.) plants, factors such as source to sink distance, sink size, and interorgan competition continually change. However, consequent changes in the pattern of photosynthate partitioning from leaves to other organs are poorly understood. Our objective was to examine photosynthate partitioning from upper and lower alfalfa leaves at intervals before herbage harvest and during regrowth after harvest. The uppermost or lowest fully expanded leaf on the longest or dominant stem was labeled with ¹⁴CO₂. After a 24-h translocation period, the plants were divided into various organs to determine distribution of the radiocarbon. Partitioning from upper and lower source leaves was similar before harvest and during the first 9 d of vegetative regrowth after harvest. During regrowth, photosynthate partitioning to the rapidly growing meristematic regions of the dominant stem progressively increased by three to fivefold at the expense of the crown shoots, crown, root, and nodules. At Day 21 following harvest, differences in the distribution of photosynthate from the leaf positions were pronounced. At that time, the upper leaf preferentially partitioned photosynthate to the shoot apex, unexpanded leaves and axillary shoots of the dominant shoot, whereas the lower leaf preferentially distributed photosynthate to the crown shoots, crown, root, and nodules. Expressions of ¹⁴C partitioning were affected differently by organ mass. While the smallest organs such as nodules and unexpanded leaves always ranked higher for ¹⁴C based on relative specific activity, the largest organs such as roots and crown shoots accumulated the largest percentage of total plant recovered radioactivity. The results illustrate the importance of growth stage and leaf position in photosynthate partitioning in alfalfa and the dominance of herbage meristems for current photosynthate during regrowth. Contribution from USDA-ARS and the Minnesota Agric. Exp. Stn. (Science Journal Series no. 15475. Supported in part by grant no. 59-2177-0-1-471-0 from the Competitive Research Grants Office of USDA and by a Doctoral Dissertation Fellowship to HTC from the Univ. of Minnesota Graduate School.