Redistribution of labile nitrogen between growing and non-growing regions: Implications for differential responses of plant growth and N uptake to high CO2

Background/Question/Methods
Rising atmospheric CO₂ concentration has been shown to, at least initially, enhance plant growth despite a substantial decrease in nitrogen (N) concentration. Here we suggest that despite having a lower proportional N content, plants grown under high CO₂ may be able to allocate a disproportionately larger amount of their available N to zones of cell division and expansion and away from mature tissues. We examined this hypothesis by evaluating concentration and pools of labile N in various zones of development in barley (Hordeum vulgare) seedlings grown under ambient (400ppm) and elevated (800ppm) CO₂ concentrations. Grasses are particularly suitable for such studies because they are characterized by distinct meristematic, expanding and mature zones both in roots and shoots. Nitrogen species examined were, nitrate, ammonium and total amino acids.

Results/Conclusions
Elevated CO₂ enhanced total plant and root biomass by 26 and 20% respectively. We found that whole root concentration of nitrate, ammonium and AAs dropped by 20, 19, and 10% respectively in response to CO₂ enrichment. However, % allocation of nitrate, ammonium and AAs to the root meristem increased by 93, 68 and 106% respectively. The data suggests that fractional allocation of labile pools of N may be an important internal redistribution mechanism that would regulate growth responses to elevated CO2 despite an overall drop in N budget of plants.