The Allometry of Nitrogen to Growth and an Inducible Defense under Nitrogen‐Limited Growth

Abstract

As plants become nitrogen—limited in their growth, carbon/nutrient (C/N) theory predicts a decreasing allocation of nitrogen to nitrogen—intensive defense compounds, while optimal defense (OD) theory predicts an increasing allocation to defense; CN ratio of the plant does not change, while OD theory does. We compare C/N and OD theory predictions for the inducible nicotine production of a native tobacco, Nicotiana sylvestris. We define allocation as the slope of the ln—ln allometric relationship between whole—plant nicotine and nicotine—free biomass in undamaged and damaged plants, harvested every other day in eight consecutive harvests. Relative growth, nitrogen, and nitrogen in nicotine accumulation rates (RGRs, RNARs, and RNicARs) were calculated to estimate the instantaneous investment of nitrogen in nicotine and the growth of plant tissues. Nicotine accumulation continues during nitrogen stress, resulting in a 3.29—fold increase in the nicotine pools of undamaged plants, and is accelerated in response to leaf damage, resulting in 4.06 fold increase in the nicotine pools of damaged plants. Nitrogen pool size did not change after day 2, and both undamaged and damaged plants allocated an increasing proportion of their nitrogen pool to nicotine throughout the experiment. Damage increased the allocation of nitrogen to nicotine; the slope of the ln nicotine vs ln nicotine—free biomass regression for damaged plants was significantly greater (16.9%; P < .041) than the slope for undamaged plants as measured during the period of damage—induced nicotine accumulation (days 2—10 damage). Allocation of nitrogen to nicotine accumulating is clearly not a passive function of the nitrogen in excess of growth requirements, and damage increases this allocation. Trade—offs between biomass and nicotine production over the course of the experiment between damaged and undamaged plants were not apparent. However, the short—term estimates indicate otherwise. On day 2, there were a significant decrease in RGR, with a correlated increase in RNicAR in response to damage, suggesting that there was a short—term trade—off between allocation to growth and nicotine accumulation or other induced processes. By the end of the experiment, each damaged plant had accumulated 0.595 mg N in nicotine above that accumulated by undamaged plants, and additional investment representing 1.94% of the total nitrogen pool. However 2 d after damage, damaged plants allocated 0.131 mg more N to nicotine than undamaged plants, representing a short—term investment of 17.7% of the nitrogen potentially made available by the decrease in RGR (.0740 mg). Plants invest substantial portions of their nitrogen in an induced defense, but our estimates do not include the nitrogen costs of production, storage, transport, detoxification, or the indirect cost of this induced defense.