Shoot Growth, Dark Respiration, and Nonstructural Carbohydrates of Contrasting Alfalfa Genotypes
The physiological processes associated with genetic differences in shoot elongation rate (SER) and leaf area expansion rate (LAER) of alfalfa (Medicago sativa L.) are poorly understood. Our objective was to examine the relationships among SER, LAER, and the concentration 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-h photoperiods. Remaining plants were placed in continuous darkness and sampled periodically during the next 144 h (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 48 h 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. Concentrations of TNC in expanding leaves declined by Hour 17 of darkness to 19% of mid-photoperiod concentrations, reflecting trends in RD and especially LAER. In expanding leaves, TNC concentrations greater than 28 g kg−1 dry wt. were necessary for maximum LAER and RD to occur. Averaged over experiments, root TNC concentrations were 185 g kg−1 dry wt. during the initial 17 h of continuous darkness and did not decline significantly. Although roots contained high concentrations of TNC, LAER decreased rapidly within 17 h of continuous darkness. This indicates that expanding leaves may rely upon sources of carbohydrate other than that in roots for growth under C-limited conditions.
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