STUDIES OF THE RELATIONSHIP BETWEEN THE GROWTH-RATE OF YOUNG PLANTS AND THEIR TOTAL-N CONCENTRATION USING NUTRIENT INTERRUPTION TECHNIQUES - THEORY AND EXPERIMENTS

Alternative assumptions about the utilization of stored nitrogen are used to derive two different models for predicting how the growth rate of both the whole plant and its shoot vary with their respective total-N concentrations following interruption of the external N supply. Model 1 predicts that plant growth should follow monomolecular kinetics after the supply is interrupted, with the resulting relative growth rates linearly related to total-N concentration. Model 2 predicts that plants grow logistically once N is withheld, with their relative growth rates varying linearly with the reciprocal of total-N concentration. The versions of the models derived for the shoot are similar to those for the whole plant, but include an additional term to allow for transfer of N to the roots as deficiency increases. Tests of the models were carried out using data from N interruption experiments with young cabbage and lettuce plants (containing either high or low nitrate concentrations) which were grown hydroponically in nutrient recirculating units. The results showed that there was little statistical difference between the fits of the two models to the growth data over the range tested, but that model 1 was unsatisfactory because the estimates of its parameters were inconsistent with assumptions about the physiological processes controlling growth, and because its predictions became unrealistic when extrapolated to conditions of acute N deficiency. Model 2 did not suffer from either of these problems and provided a better mechanistic interpretation of the data, yielding predictions that were in close agreement with the observed relationship between relative growth rate and total-N concentration for both the whole plant and its shoot. The curvilinear form of this relationship for model 2 differs from the linear form of other models derived from measurements in experiments where there was a continuing but restricted supply of external N to plants. This implies that the relationship between relative growth rate and total-N concentration may vary depending on whether or not a plant has to rely entirely on its internal reserves of N in times of shortage. The results also showed that the size of these reserves governed the amounts of N transferred to the roots as deficiency developed. Transfer of N was greater in cabbage than lettuce because of a greater capacity to adapt by increasing root growth at the expense of the shoot.