Carbon and nitrogen reserve remobilization following defoliation: Nitrogenand elevated CO2 effects

Early regrowth following defoliation of forage species often depends on rem obilization of nitrogen and non-structural carbohydrate (TNC) reserves stor ed in roots and crowns. The degree to which TNC and N remobilization contri bute to regrowth can depend on internal concentrations and on external COL and N supplies. We studied the effect of CO2 and N supply on reserve remobi lization during the first 20 d following defoliation of 9-wk-old alfalfa (M edicago 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 eithe r ambient (350 mu mol mol(-1)) or elevated (700 mu mol mol(-1)) CO2. Plants were fertilized twice weekly with Hoaglands solution containing either 0 m g L-1 (low N) or 400 mg L-1 N (high N). Elevated CO, increased the total am ount and percent of available TNC that was remobilized in alfalfa, and the amount of remobilized TNC in western wheatgrass, but reduced TNC remobiliza tion in blue grama. Nitrogen fertilization had little effect on TNC remobil ization 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 spe cies. Nitrogen remobilization was highest under high N and ambient CO2 cond itions for all species. Nitrogen deficiency and elevated CO2 reduced N remo bilization and the contribution of remobilized N to shoot regrowth.