Responses of Noogoora Burr (Xanthium occidentale Bertol.) to Nitrogen Supply and Carbon Dioxide Enrichment

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Abstract
We studied the responses of Xanthium occidentale (Bertol.) (cocklebur or Noogoora burr), a noxious weed, to atmospheric CO2 enrichment and nitrate-N concentrations in the root zone ranging from 0.5 to 25 mM. CO2 enrichment (1500 cm3 m−3) increased dry-matter production to about the same extent (18 per cent) at all levels of supplied N: most of the increment in dry matter was distributed equally between leaves and roots so that there was little effect on shoot-to-root dry-weight ratios. Growth was stimulated greatly by N and plateaued at 12 mM supplied N. Shoot-to-root dry-weight and total N ratios increased with increasing N supply. CO2 enrichment had no effect on the total amount of N accumulated by plants, but increased the N-use efficiency of leaves. Enriched plants had lower concentrations and quantities of N in their leaves than controls, and therefore lower shoot-to-root total N ratios. Little free NO3− accumulated in organs of control or enriched plants. NO3− was the major form of N in xylem sap from detopped plants at low supplied NO3-N, but amino N was equal in importance at high supplied NO3-N in control and enriched plants. Concentrations of NO3− were lower in the xylem sap of CO2 enriched plants. It was concluded that the better N-use efficiency of CO2 enriched plants could result in increased growth of X. occidentale in regions of marginal soil fertility as atmospheric levels of CO2 increase.