THE EFFECTS OF LIGHT ON INDUCTION, TIME COURSES, AND KINETIC PATTERNSOF NET NITRATE UPTAKE IN BARLEY

Barley seedlings (Hordeum vulgare L.) were grown hydroponically with ( induced) or without (uninduced) nitrate in a light/dark cycle with hig h photon flux density to determine the effects of light on time course s, induction and kinetics of net nitrate uptake. Nitrate uptake was in duced by external nitrate in both light and dark and was prevented by 1 mol m(-3) p-fluorophenylalanine. In high light, nitrate uptake was a bout 2-fold higher than in low light. During time course experiments t he uptake rates oscillated due to daily light-dark changes. Rates of n itrate uptake also increased at about 2200 h during continuous darknes s. This increase coincided approximately with the time at which the da rk period started during the previous culture of the plants, indicatin g that it was due to a mechanism associated with an endogenous diurnal rhythm. When calculating the kinetics of nitrate uptake, a model with two saturable systems, including a high-affinity system (HATS) and a low-affinity system (LATS), gave the best fit to data in all treatment s. The apparent affinity of the HATS ranged from 7.7 to 12.2 mmol m(-3 ) in induced plants in all light conditions. The effect of light on th e HATS was mainly an increase of apparent V-max in the step from low t o high light. In uninduced plants the HATS operated at a very low acti vity which was strongly enhanced during induction, Interpretation of t he calculated kinetics of the LATS was much more difficult on the basi s of net uptake data. The apparent affinity of the LATS increased from 24.3 mol m(-3) in low light up to 0.17 mol m(-3) after acceleration i n high light. These extreme changes in apparent affinity of the LATS c ould not be explained satisfactorily, and the nature of this system is also discussed with respect to the method used.