GROWTH AND DEVELOPMENT OF SEMINAL AND CROWN ROOT SYSTEMS IN N-LIMITEDBARLEY, AND THEIR CONTRIBUTIONS TO NITRATE ACQUISITION DURING VEGETATIVE AND GENERATIVE GROWTH

Barley (Hordeum vulgare L., cvs Golf and Laevigatum) was grown under n itrogen limitation, controlled by the relative rate of nitrate-N addit ion (RA), in solution culture. The seminal and crown root systems were kept apart, but in contact with the same nutrient solution throughout culturing. Growth, nitrate uptake, and in vitro nitrate reductase (NR ) activity in the different root parts were studied at plant ages from 40 (late vegetative stage) to 110 (mid grain-filling) days. The RA wa s during this time interval stepwise decreased from 0.08 day-1 to 0.00 5 day-1. The ratio between seminal root dry weight and total plant dry weight decreased drastically during post-anthesis growth, whereas the contribution by crown roots remained unchanged. Tissue nitrogen conce ntrations in seminal roots did not change with time, but decreased in crown roots after day 80. The NR activity decreased with age in both s eminal and crown roots. The V(max) for net nitrate uptake decreased th roughout the experiment in the seminal root system, but not in the cro wn root system. The kinetic properties (V(max) and K(M)) were used to calculate the nitrate concentration required to maintain a relative ra te of nitrate-N uptake that equals the relative addition rate. These c oncentrations (2 to 5 mmol m-3) were found to closely match actually m easured nitrate concentrations in the nutrient solution (1 to 6 mmol m -3). From uptake kinetics, it was deduced that the contribution by sem inal roots to total nitrate uptake at these concentrations decreased f rom more than 50% in vegetative plants, to about 20% just after main s hoot anthesis, and to less than 5% during grain-filling.