Nitrogen and CO2 affect regrowth and biomass partitioning differently in forages of three functional groups

Little work has been done to assess the impact of elevated CO2 on responses of forages to defoliation, This study examines regrowth, biomass partition ing, and labile C and N metabolites in three Functional plant-types: a C-3 grass [Pascopyrum smithii (Rydb.) A, Love], a C-4 grass [Bouteloua gracilis (H,B,K,) Lag.], and a forage legume (Medicago sativa L,), Plants were grow n from seed, defoliated twice, and grown in a controlled environment under a factorial arrangement of two CO2 [low CO2 (LC), 355 mu mol mol(-1) and hi gh CO2 (HC), 700 mu mol mol(-1)] and two N nutrition regimes [low N (LN), w atered 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 regro wth in all three species, while high CO2 enhanced regrowth only in the two C-3 species. In M. sativa, CO2 and NY treatments had no significant effect on k, the allometric growth coefficient, In contrast, k was reduced in P, s mithii 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 f or 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 partitionin g to belowground organs (reduced k) when N is limiting, particularly under elevated CO2. Significant correlations were established between k and sever al measures of plant N status, suggesting that the effects of CO2 on plant biomass partitioning involve N status.