The effects of light on induction, time courses, and kinetic patterns of net nitrate uptake in barley

Abstract

Barley seedlings (Hordeum vulgare L.) were grown hydroponically with (induced) or without (uninduced) nitrate in a light/dark cycle with high photon flux density to determine the effects of light on time courses, induction and kinetics of net nitrate uptake. Nitrate uptake was induced by external nitrate in both light and dark and was prevented by 1 mol m -3 p-fluorophenylalanine. In high light, nitrate uptake was about 2-fold higher than in low light. During time course experiments the uptake rates oscillated due to daily light-dark changes. Rates of nitrate uptake also increased at about 2200 h during continuous darkness. This increase coincided approximately with the time at which the dark period started during the previous culture of the plants, indicating 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 treatments. 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 the HATS was mainly an increase of apparent V max in the step from low to high light. In uninduced plants the HATS operated at a very low activity which was strongly enhanced during induction. Interpretation of the calculated kinetics of the LATS was much more difficult on the basis 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 in high light. These extreme changes in apparent affinity of the LATS could not be explained satisfactorily, and the nature of this system is also discussed with respect to the method used.

Nutrient translocation in the xylem of poplar ? diurnal variations and spatial distribution along the shoot axis

“…Nutrients can be transferred from the phloem into the xylem, which results in circulation of most of the nutrients. Exceptions are Ca 2+ and NO 3 ) (Peuke and Jeschke 1998), which are almost phloem-immobile. In wheat and rice, more than 60% of the amino-N and 26% and 36% respectively of K + in the xylem originates from the phloem (Grignon and Sentenac 1991).…”

Diurnal changes in nitrogen and potassium absorption rates of plants grown in a greenhouse

“…In these studies, the change in nutrient absorption rates throughout the day has been one of the most frequently researched topics, and it was found that the daily changes in absorption rates follow different patterns: a single peak during the daytime (Adams, 1980;Keltjens and Nijenstein, 1987;Pearson and Steer, 1977) or double peaks, one in the daytime and another in the nighttime (Hansen, 1980;Le Bot and Kirkby, 1992;Pan et al, 1987;Peuke and Jeschke, 1998;Steingröver et al, 1986). Currently, these different absorption patterns are explained by the variations in growth conditions maintained during experiments, such as those in a greenhouse versus those in an artificial climate room.…”

The effect of light on nitrate uptake by wheat roots

“…For instance, nitrate uptake increases 20% to 40% within a few hours and then becomes steady when plants are transferred from dark to light in barley [4] and decreases by a similar amount on transfer from light to dark in tobacco and soybean [5,6]. In prolonged darkness, the nitrate uptake rate may subsequently decline even further [7,8]. In tomato the initial adjustment of nitrate uptake takes longer, increasing in the light and decreasing in darkness continuously over a period of 6 h or more [9].…”

“…Measurements of nitrate uptake with a time resolution of minutes are therefore needed. However, in most published work on nitrate responses to light, time courses have generally been measured with relatively few points, none less than 1 h apart, and often after 4 or 6 h. Peuke and Jeschke have provided systematic time courses with more frequent measurement, but a lag of the order of minutes would not be visible in their system because of the variability of the measured uptake rates [8].…”

“…On the other hand, the low affinity transport systems, with Km above 1 mM, will transport little at 0.3 mM nitrate. For example, with a Km of 3.7 mM as in barley [8] the low affinity transport system would be operating at only 4% of its maximum capacity and add only a few percent to the total measured nitrate uptake in these experiments. The results of the paper therefore concern the characteristics of the fully induced, high affinity nitrate transport systems.…”