Shoot Growth, Dark Respiration, and Nonstructural Carbohydrates of Contrasting Alfalfa Genotypes K. L. Hendershot and J. J. Volenec* ABSTRACT The physiological processes associated with genetic differences in shootelongation rate (SER) and leaf areaexpansion rate (LAER) of alfalfa (Medlcago sativa L.) arepoorly understood. Oar objective was to examine the relationships among SER, LAER, andthe con- centration and utilization of nonstructural carbohydrates in roots and meristematic shoot tissues of alfalfa. Two genotypes selected for contrasting SER were sampled after 7 d of herbage regrowth in 15-hphotoperiods. Remaining plants were placed in continuous darkness and sampled periodically during the next 144h (Exp. 1) or 33 h (Exp.2) of regrowth. In Exp. 1, LAER, concentrations total nonstructural carbohydrate (TNC), and dark respiration rates (RD) of expanding leaves decreased during the first 48h of regrowth in continuous darkness. In Exp. 2, LAER declined to 40% of mid- photoperiod values after 17 h of continuous darkness, while RD of expanding leaves declined to 75% of mid-photoperiod rates. Con- centrations of TNC in expanding leaves declined by Hour 17 of darkness to 19% of mid-photoperiod concentrations, reflecting trends in Rn and especiallyLAER. In expanding leaves, TNC con- centrations greater than 28 g kg -~ drywt. were necessary for max- imum LAER andR~ to occur. Averaged overexperiments, root TNC concentrations were 185g kg -1 dry wt. during the initial 17 h of continuous darkness and did not declinesignificantly. Although rootscontained highconcentrations of TNC, LAER decreased rap- idly within 17 h of continuous darkness. Thisindicates that ex- panding leaves may rely upon sources of carbohydrate otherthan thatin roots for growth under C-limited conditions. ability and leaf growth has been a subject of recent investigations (11,14), few studies have specifically dealt with the relationship between carbohydrate con- centrations and growth in meristematic shoot tissues of dicotyledonous plants. In alfalfa, rapid elongation of internodes occurs after the leaf attached to the base of the elongating internode has attained near-maxi- mum area (3). This creates both a spatial and temporal separation of leaf area expansion and internode elon- gation on a growing shoot. Therefore, carbohydrate availability and utilization in these two meristematic tissues of shoots should be examined individually. Dark respiration rate, an estimate of the metabolic activity associated with growth in meristematic tissues (16), has been closely related to TNC concentrations in meristematic tissues of several species (1,17). examination of the relationship between TNCcon- centration and Rt) as determinants of genotypic dif- ferences in shoot growth, however, has not been made for most species, including alfalfa. Our objectives were to: (i) determine if SER and LAER were influenced by continuous darkness and if so, to estimate the crit- ical TNC concentration and Rt) rates required by mer- istematic shoot tissues to maintain maximum SER and LAER, and (ii) determine if root TNCcould utilized to support shoot growth of dark-grown alfalfa. G ENETIC DIFFERENCES in shoot regrowth rate of alfalfa have been documented (12,23), but the physiological factors influencing differences in shoot regrowth remain unclear. The rapid changes in source- sink relations that occur during regrowth and the com- plexity of their regulation have hindered the deter- mination of physiological mechanisms controlling shoot growth of this species. The amount of TNCaccumulated in the roots and crowns of alfalfa has been considered an important factor influencing regrowth after defoliation (15,19). May (13), however, suggested that root TNC may primarily utilized for root respiration rather than being used as biosynthetic precursors and energy to support shoot regrowth. Radioactive labelling studies indicated that root TNC may be important in re-es- tablishment of leaf area during the first 10 d of re- growth following defoliation (8). These studies con- sidered root TNC to be a possible limiting factor for shoot regrowth. However, Baysdorfer and Bassham (2) suggested that, in well-developed alfalfa plants, shoot regrowth is limited by the utilization of TNC in meristems; hence, a sink limitation on shoot regrowth mayexist. While the relationship between carbohydrate avail- Dep.of Agronomy, Purdue Univ., West Lafayette, IN 47907. Con- tribution from the Purdue Univ. Agric. Exp. Stn. Journal Paper no. 11751. Received 21 July 1988. *Corresponding author. Published in Crop Sci. 29:1271-1275 (1989). MATERIALS AND METHODS Plant Culture. Two genotypes of alfalfa selected for contrasting SER were used in these experiments. A genotype from germplasm NC- 83-2 (10) was selected for rapid SER (RSER), while a geno- type from PI 259522 (18) was selected for its slow SER (SSER). Plants were propagated using stem-tip cuttings, three plants per pot, in a greenhouse at 25 +_ 5 °C with a 15-h photoperiod. In addition to incident solar radiation, a photosynthetic photon flux density (PPFD) of 60 ~zmol -2 s -~ was provided by fluorescent and incandescent lamps. Plastic pots, 10-cm diam. by 30-cm deep, contained equal parts of Torontosilt loamtopsoil (fine-silty, mixed, mesic Udollic Ochraqualf) and coarse sand. This mixture con- tained 22 g m -2 available P and 56 g m -2 exchangeable K and had a pHof 7.1. Plants were inoculated with Rhizobium meliloti and received weeklyapplications of N-free Hoag- land’s solution and biweeklyapplications of 5 and 2 g m -2 K and P, respectively. Plants were watered with deionized water and insects were controlled. Experiment 1 Herbagewas removed to leave a 5-cm stubble on Day50, 86, and 113 after transplanting. Both genotypes were be- tween 50 and 100%bloom at each harvest. On Day 50, plants were placed in a controlled environment chamber that provided a 15-h photoperiod of 650 ~molm -2 s -~ PPFD and maintained an air temperature of 23 °C. Tissue Sampling. OnDay144 after transplanting, plants were blocked into three replicates, two pots per replicate, and defoliated to initiate the study (Day 0). Roots and 1271