A mathematical model for nitrogen demand quantification and a link to broccoli ( Brassica oleracea var. italica) glutamine synthetase activity

Abstract

Determination of crop nitrogen requirement is at the basis of crop nitrogen fertilizer recommendation. This study aimed at determining the nitrogen demand of broccoli plants ( Brassica oleracea var. italica). A mathematical model predicting nitrogen demand was developed and successively tested in an experiment involving a broccoli cultivar at four stages of growth (15; 30; 45; and 60 days after transplanting), in six complete nutrient solutions differing only by their nitrogen rates. The daily critical nitrogen demand was bell-shaped with its maximum at 30 days after transplanting. This pattern paralleled the time course of broccoli growth rate. The critical nitrogen rates of supply were 100 mg l −1 N from the day of transplantation until 30 days, and 175 mg l −1 N from 30 days until 60 days. Any positive or negative deviation from these critical rates of supply led to excessive or deficient tissue nitrogen contents, respectively, which in turn resulted in significant reductions of growth rate. Unlike the cases where nitrogen was non-limiting, root glutamine synthetase activity was linearly correlated to nitrogen deficit rate ( R 2=0.99) under limiting nitrogen conditions, suggesting that under such conditions, glutamine synthetase activity is under the developmental control of nitrogen demand.