A Mechanistic Theory for the Relationship between Growth Rate and the Concentration of Nitrate-N or Organic-N in Young Plants derived from Nutrient Interruption Experiments

Abstract

A simple assumption about nitrate assimilation (incorporating a single parameter to represent the conversion of nitrate into organic-N) has been used to derive mechanistic equations to describe the interrelationships between the concentrations of nitrate-N and organic-N, and dry weight for both the whole plant and its shoot in nutrient interruption experiments. These equations have been combined with a logistic growth model, which was derived from initial assumptions about the way in which plants use stored N under these conditions (Burns, 1994), to quantify effects of nitrate-N and organic-N concentrations on relative growth rate. The models were tested by fitting equations for the predicted relationships to data for young cabbage and lettuce plants, from which estimates of the N assimilation parameter were obtained. The tests showed that predictions of relative growth rate were generally in good agreement with the data over the whole range, as were those for the corresponding relationships between dry weight and either nitrate-N or organic N- concentration, and for the interrelationships between the two forms of N. The most reliable estimates of the N assimilation parameter were obtained from relationships where nitrate-N concentration was the explanatory (independent) variable, because the fits of the corresponding relationships with organic-N were relatively insensitive to large changes in its values. The results showed no evidence of any consistent variation in the size of this N assimilation parameter with the nitrate status of the plant. However, small between-crop differences in its value suggest that shoot nitrate may have been assimilated slightly more efficiently in cabbage than in lettuce.The new model predicts that dry matter production is restricted as soon as the external N supply is withheld (irrespective of the plant nitrate status), producing a slow but consistent decline in relative growth rate which is maintained until nitrate is almost depleted, whereupon it falls rapidly. This implies that the rate of chemical reduction of stored nitrate was not sufficient to maintain an adequate supply of organic-N for the production of new dry matter (even when its concentration in the plants is still high). The results show that nitrate concentrations in excess of 0·1 mmol g-1 are required in plants to avoid serious reductions in growth rate when N is in short supply.