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

A simple assumption about nitrate assimilation (incorporatinga single parameter to represent the conversion of nitrate intoorganic-N) has been used to derive mechanistic equations todescribe the interrelationships between the concentrations ofnitrate-N and organic-N, and dry weight for both the whole plantand its shoot in nutrient interruption experiments. These equationshave been combined with a logistic growth model, which was derivedfrom initial assumptions about the way in which plants use storedN under these conditions (Burns, 1994), to quantify effectsof nitrate-N and organic-N concentrations on relative growthrate. The models were tested by fitting equations for the predictedrelationships 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 weregenerally in good agreement with the data over the whole range,as were those for the corresponding relationships between dryweight and either nitrate-N or organic N- concentration, andfor the interrelationships between the two forms of N. The mostreliable estimates of the N assimilation parameter were obtainedfrom relationships where nitrate-N concentration was the explanatory(independent) variable, because the fits of the correspondingrelationships with organic-N were relatively insensitive tolarge changes in its values. The results showed no evidenceof any consistent variation in the size of this N assimilationparameter with the nitrate status of the plant. However, smallbetween-crop differences in its value suggest that shoot nitratemay have been assimilated slightly more efficiently in cabbagethan in lettuce. The new model predicts that dry matter production is restrictedas soon as the external N supply is withheld (irrespective ofthe plant nitrate status), producing a slow but consistent declinein relative growth rate which is maintained until nitrate isalmost depleted, whereupon it falls rapidly. This implies thatthe rate of chemical reduction of stored nitrate was not sufficientto maintain an adequate supply of organic-N for the productionof new dry matter (even when its concentration in the plantsis still high). The results show that nitrate concentrationsin excess of 0·1 mmol g^-1 are required in plants to avoidserious reductions in growth rate when N is in short supply.Copyright1994, 1999 Academic Press Cabbage, concentration, deficiency, hydroponics, lettuce, model, nitrogen, nitrate, nutrient interruption, organic-N, relative growth rate, shoot, whole plant