Carbon and Nitrogen Reserve Remobilization Following Defoliation: Nitrogen and Elevated CO2 Effects

Abstract

ABSTRACTEarly regrowth following defoliation of forage species often depends on remobilization of nitrogen and non‐structural carbohydrate (TNC) reserves stored in roots and crowns. The degree to which TNC and N remobilization contribute to regrowth can depend on internal concentrations and on external CO2 and N supplies. We studied the effect of CO2 and N supply on reserve remobilization during the first 20 d following defoliation of 9‐wk‐old alfalfa (Medicago sativa L.), western wheatgrass [Pascopyrum smithii (Rydb) A. Love], and blue grama [Bouteloua gracilis (H.B.K.) Lag ex Steud]. plants. Reserve remobilization was studied in controlled‐environment chambers set at either ambient (350 μmol mol−1) or elevated (700 μmol mol−1) CO2 Plants were fertilized twice weekly with Hoaglands solution containing either 0 mg L−1 (low N) or 400 mg L−1 N (high N). Elevated CO2 increased the total amount and percent of available TNC that was remobilized in alfalfa, and the amount of remobilized TNC in western wheatgrass, but reduced TNC remobilization in blue grama. Nitrogen fertilization had little effect on TNC remobilization at ambient CO2, but increased remobilization in alfalfa and reduced remobilization in the two grasses under elevated CO2 Alfalfa remobilized a greater percentage of its root and crown N reserves than either grass species. Nitrogen remobilization was highest under high N and ambient CO2 conditions for all species. Nitrogen deficiency and elevated CO2 reduced N remobilization and the contribution of remobilized N to shoot regrowth.