Carbon and Nitrogen Reserve Remobilization Following Defoliation: Nitrogen and Elevated CO 2 Effects

ABSTRACT Early 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 CO 2 and N supplies. We studied the effect of CO 2 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 ) CO 2 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 CO 2 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 CO 2 , but increased remobilization in alfalfa and reduced remobilization in the two grasses under elevated CO 2 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 CO 2 conditions for all species. Nitrogen deficiency and elevated CO 2 reduced N remobilization and the contribution of remobilized N to shoot regrowth.