Polygonum bistorta N uptake and storage data for Saddle, 1994.

The patterns of luxury consumption and storage of nitrogen were studied in the rhizomatous alpine herb, Polygonum (Bistorta) bistortoides (Pursh.). Emphasis was placed on understanding the mechanisms, carbon costs, and possible benefits to growth of assimilating and storing excess nitrogen. Nitrogen pools in the rhizome and shoot were measured in fertilized and non-fertilized plants over two growing seasons (the 3rd and 4th growing seasons after initiation of fertilization). Fertilized plants stored more N in the rhizome throughout both seasons and exhibited greater mobilization of N reserves to support growth, compared to non-fertilized plants. Flower, foliar, and preformed bud N concentrations and biomass production rate did not increase in response to fertilization. The increased storage of N without a concomitant influence on biomass production demonstrates that luxury consumption occurs in this species. Fertilized plants relied more heavily on stored N to drive seasonal growth, with translocation from reserves accounting for 56% and 100% of the aboveground N requirement in non-fertilized and fertilized plants, respectively. In fertilized plants, the complete reliance on stored N indicates that the increase in utilization of N from reserves is accompanied by a decrease in utilization of N from current-season uptake. Total plant N content in fertilized plants did not change throughout the entire growing season, demonstrating the absence of net N uptake from the soil. Thus, the uptake and storage of excess N was accompanied by negative feedback to further soil N uptake. Such feedback would constrain the continued accumulation of excess N and eliminate any long term benefits of luxury N uptake in this species. Increased N storage in fertilized plants was accommodated by an increase in the amino acids arginine, the non-protein amino acid, delta-acetyl-ornithine, glutamine, and glutamate. It was found that the entire length of the rhizome, including tissues that are several decades old, and both the pith and the cortex are active in N storage. Luxury uptake was accommodated entirely through filling of existing rhizome cells; no increased cell proliferation was found. Sucrose concentration was significantly lower in the recently produced rhizome tissue of fertilized plants, reflecting the increased, direct carbon costs of storing the additional N. There do not appear to be growth benefits associated with luxury N uptake in this species. In response to luxury uptake a switch occurs, such that growth is driven entirely by stored, excess N, rather than a combination of stored N and N assimilated during the current season. By relying entirely on stored N, the cost of root construction and nutrient assimilation should be reduced. However, this apparent benefit is offset by (1) inherent developmental constraints to the rate of growth in this species which prevents the utilization of excess N to drive increased growth, (2) negative feedback between luxury uptake and future N assimilation, and (3) substantial direct carbon costs of storing the excess N. These costs and constraints appear to balance any benefits to luxury uptake, resulting in no net response of plant growth. Delta-acetyl-ornithine was found to vary seasonally and in response to nitrogen availability in the rhizomes of the alpine herb, Polygonum bistortoides. Delta-acetyl-ornithine accounts for 38% of the free amino acid pool and up to 12% of the total nitrogen in rhizomes. It is concluded that this compound plays an important role in nitrogen storage in this species.