The relationship between the specific absorption rate and extremely low ambient nitrate concentrations under steady-state conditions

Some water-culture experiments were carried out to measure the ambient nitrate concentration under conditions of exponential addition of nitrate. Juvenile plants ofPlantago lanceolata were therefore grown on an agitated nutrient solution. Nitrate was continually added to the culture vessels during the experimental period. The amount added per plant increased exponentially with time, in accordance with the formula: $$NO_3 - N = N_0 .(e^{(RAR.t)} - 1)$$ (N₀=initial content of N in the seedling, RAR=relative addition rate, t=time in days). There were three treatments corresponding to RAR's of 0.20, 0.15 and 0.10 d^?1. All of them were suboptimal forP. lanceolata. Nitrate concentrations of the culture solution were measured weekly. In all cases plants were in steady state after two weeks. The relative growth rate was about equal to the RAR within treatments. The root weight ratio was inversely proportional to the RAR, and the internal N concentration proportional. Ambient nitrate concentrations were relatively high during the first two weeks. Ambient concentrations in the second half of the culture periods ranged from 0.35 to 1.8 μM. Linear and multiple regressions of the external nitrate concentration on the specific absorption rate (SAR), and on SAR and some other variables, respectively, were carried out. Most of its variation could be explained by the SAR (r²=0.9827); root size had a minor effect. The experimental results lead to the conclusion that ambient concentrations in culture experiments with exponential nutrient-addition are a result of the treatment. They also indicate which external concentrations may be expected to create strongly suppressed growth. Furthermore, the experiments suggest that the lower limit for nitrate absorption is a zero concentration.