A model relating the maximum nitrate inflow of lettuce (Lactuca satvia L.) to the growth of roots and shoots

The net inflow of nitrate can be calculated from the nitrate concentration at the root surface by means of the Michaelis-Menten equation. Because of maximum inflow (Imax) is not constant but varies with plant age and growing conditions, a model for calculating Imax during plant growth was derived. Lettuce was grown in nutrient solution. Variations in temperature, radiation and plant age were used to vary growth rates and N-demand of plants. There was a linear relationship between relative growth rates (RGR) and maximum nitrate inflow (Imax), that could be described by the following regression function: Imax = 0.24 + 6.57 RGR. A residual analysis showed a further influence on Imax from the root:shoot-ratio (RSR), the effects of which could be accounted for by including an e-function in the relationship: Imax = (0.27 + 10.63 RGR) e^{(–0.0017 RSR)}. This model for calculating Imax was validated in two further experiments.