Root vs. Shoot Effects on Herbage Regrowth and Carbohydrate Metabolism of Alfalfa

Abstract

Physiological characteristics mediating shoot growth of alfalfa (Medicago sativa L.) following herbage removal are poorly understood. Our objective was to assess shoot vs. root effects on regrowth of alfalfa using reciprocally grafted and self‐grafted plants of two genotypes selected for contrasting shoot elongation rates (SER). Plants were propagated vegetatively from cuttings severed below the graft union. Plants were sampled weekly in a controlled environment and biweekly in the field during 4 wk of regrowth. Averaged across both environments, SER of plants with scions derived from the rapid SER (RSER) genotype was 88% greater than that of plants with slow SER (SSER) scions. By Week 4 of regrowth in the controlled environment, yield per shoot (YPS) of plants with RSER scions was 54% greater than that of plants with SSER scions. Differences in rates of single‐leaf net photosynthesis and leaf dark respiration were also associated with scion source, and like SER and YPS were not influenced by rootstock. In both environments, quantities of total nonstructural carbohydrates (TNC) in roots of plants with RSER scions were greater than those of plants with SSER scions. In the field environment, root TNC concentrations were 54% greater for plants with RSER scions when compared to plants with SSER scions, irrespective of rootstock. Results indicate that for these genotypes, characteristics of shoot tissues were more influential than root‐borne factors in mediating shoot regrowth and carbohydrate metabolism following herbage removal.