A comparison of the responses of two Plantago species to nitrate availability in culture experiments with exponential nutrient addition

Juvenile plants of Plantago lanceolata and P. major ssp. major were grown on nutrient solution at optimal availability of nitrate as well as at various degrees of suboptimal nitrate availability. In the optimal treatment the nutrient solution contained 7.5 mmol NO ₃ ^- per 1. In the various suboptimal treatments nitrate of the basic solution was replaced by sulphate, and nitrate was continually added during the experimental period. The rate of addition was exponential, according to the formula: $$NO_3 - N = N_0 \cdot {\text{ (e}}^{{\text{RAR}} \cdot t} - 1)$$ (N_o=initial content of nitrogen in the seedling, RAR=relative addition rate, t=time in days). There were four suboptimal treatments corresponding to RAR's of 0.25, 0.20, 0.15 and 0.10 per day. In all cases plants were in steady state. The plant parameters as varied in the experimental treatments were related to the N concentrations (% fresh weight). Most relationships were linear. P. major attained a higher RGR than P. lanceolata at equal N concentrations. The root weight ratio was inversely proportional to the N concentration, and varied between 0.55 and 0.25. The N productivity (mg·(mg N)^-1·d^-1) with respect to shoot biomass was proportional to the N concentration, and higher in P. major. This difference is related to the lower N concentration of P. major. The N productivity with respect to root biomass was highest when the N concentration was reduced by about 50 per cent. The length of the root axis was inversely proportional to the N concentration, and greater in P. lanceolata. The ecological implications of the experimental results were discussed.