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CROP PHYSIOLOGY & METABOLISM

Nitrogen and CO₂ Affect Regrowth and Biomass Partitioning Differently in Forages of Three Functional Groups

^a USDA, ARS Rangeland Resources Research Unit, 1701 Centre Ave., Fort Collins, CO 80526
^b USDA, ARS, Pasture Systems and Watershed Management Research Laboratory, Bldg. 3702 Curtin Road, University Park, PA 16802
^c USDA, ARS, Northern Great Plains Research Laboratory, PO Box 459, Mandan, ND 58554

Corresponding author (morgan{at}lamar.colostate.edu)

Little work has been done to assess the impact of elevated CO₂ on responses of forages to defoliation. This study examines regrowth, biomass partitioning, and labile C and N metabolites in three functional plant-types: a C₃ grass [Pascopyrum smithii (Rydb.) A. Love], a C₄ grass [Bouteloua gracilis (H.B.K.) Lag.], and a forage legume (Medicago sativa L.). Plants were grown from seed, defoliated twice, and grown in a controlled environment under a factorial arrangement of two CO₂ [low CO₂ (LC), 355 µmol mol^-1, and high CO₂ (HC), 700 µmol mol^-1] and two N nutrition regimes [low N (LN), watered twice weekly with half-strength Hoagland's containing 0 N, and high N (HN), half-strength Hoagland's containing 14 mM N]. High N enhanced regrowth in all three species, while high CO₂ enhanced regrowth only in the two C₃ species. In M. sativa, CO₂ and N treatments had no significant effect on k, the allometric growth coefficient. In contrast, k was reduced in P. smithii plants grown under LN (0.63) compared with HN (0.99). In B. gracilis, low N also reduced k, but it interacted with CO₂ so that k was greatest for plants grown at HN/HC (0.95) and HN/LC (0.89), intermediate at LN/LC (0.58), and least at LN/HC (0.44). These results indicate greater partitioning to below-ground organs (reduced k) when N is limiting, particularly under elevated CO₂. Significant correlations were established between k and several measures of plant N status, suggesting that the effects of CO₂ on plant biomass partitioning involve N status.

Abbreviations: C_lab, labile C compounds • DW, dry weight • HC, high CO₂ • HN, high N • k, allometric growth coefficient • LC, low CO₂ • LN, low N • N_lab, labile N compounds • SDW, structural dry weight • TNC, total nonstructural carbohydrates

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