NITROGEN-UTILIZATION IN N-LIMITED BARLEY DURING VEGETATIVE AND GENERATIVE GROWTH .1. GROWTH AND NITRATE UPTAKE KINETICS IN VEGETATIVE CULTURES GROWN AT DIFFERENT RELATIVE ADDITION RATES OF NITRATE-N

Growth and nitrate uptake kinetics in vegetatively growing barley (Hordeum vulgare L., cvs Laevigatum, Golf, and Mette) were investigated in solution culture under long-term limitations of external nitrogen availability. Nitrate was fed to the cultures at relative addition rates (RA) ranging from 0.02 to 0.2 d-1. The relative growth rate (RG, calculated for total plant dry weight) correlated well with RA in the range 0.02 to 0.07 d-1. In the RA range from 0.07 to 0.2 d-1 RG continued to increase, but an increasing fraction of nitrogen, added and absorbed, was apparently stored rather than used for structural growth. The RG of the roots was less affected by RA. V(max) for net nitrate uptake increased with RA up to 0.11 d-1, but decreased at higher RA. The decline in V(max) coincided with a build-up of nitrate stores in both roots and shoots. V(max), expressed per unit nitrogen in the plants (the relative V(max)), was higher than required for maintenance of growth (up to 30-fold) at low RA, whereas at higher RA the relative V(max) decreased. Kinetic predictions of steady-state external nitrate concentrations during N-limited growth ranged from 0.2 to 5.0 mmol m-3 over the RG range 0.02 to 0.11 d-1. It is suggested that the nitrate uptake system is not under specific regulation of nitrate uptake, possibly via root nitrate pools, become important. The three cultivars showed very similar growth and nitrate uptake characteristics.