From 10 to 60% of the nitrate present in plant tissue extracts and stem exudates of corn (Zea mays L.) was found to be reduced during Kjeldahl digestion, even in the absence of added reducing agents. This reduction is of particular concern in [(15)N]nitrate assimilation studies, because it results in an overestimate of nitrate reduction. To overcome this problem, a method was developed for removing nitrate prior to Kjeldahl digestion, thereby preventing nitrate reduction. The procedure utilizes hydrogen peroxide for partial oxidation of organic matter in order to minimize the nitration of organic compounds. The free nitrates are then volatilized as nitric acid from concentrated sulfuric acid at 95 degrees C. When the proposed method was used as a pretreatment to Kjeldahl digestion, less than 0.5% of the applied nitrate was recovered in the reduced nitrogen fraction of plant tissue extracts and stem exudates.
The effects of accumulated [14N]nitrate and its utilization in decapitated, 5-day-old dark-grown corn roots on influx, accumulation, xylem deposition, and reduction of concurrently absorbed nitrate during an 18-hour exposure to 0.5 millimolar K15NO3 nutrient solution were examined. A 20-hour pretreatment in 15.0 millimolar K14NO3 high nitrate (HN) resulted in a 2-fold greater tissue nitrate level than pretreatment in 0.5 millimolar K14NO3 low nitrate (LN). Upon transfer to the 0.5 millimolar K15NO3 solution, the net nitrate uptake rate in HN roots after 2 hours was 52% of the LN rate, but increased to 93% at the end of the uptake period. Despite an enhanced [14N]nitrate efflux from HN roots to the uptake solution, the efflux differences between the two pretreatments did not compensate for the decrease in net nitrate uptake. The [15N]nitrate influx rate was initially restricted by 33% in the HN roots compared to LN roots, but it had decreased to 7% by the end of the 18-hour uptake period. At this time, the total tissue nitrate levels were similar for both pretreatments. The rate of accumulation of [15N]nitrate in the tissue was relatively constant for both pretreatments, but was 25% less in HN roots. Of the previously accumulated [14N]nitrate, 52 and 46% remained after 18 hours in the LN and HN roots, respectively. The [14N]nitrate decline for HN roots was initially more rapid than in the LN roots which was linear over time. Xylem transport and efflux more than accounted for the decline in [14N]nitrate of LN roots and all but 4% in the HN roots which was attributed to reduction. Compartmentation of the previously accumulated nitrate was evident from the higher atom per cent 15N of xylem nitrate compared to that of the tissue nitrate of both LN and HN roots. During the first 2 hours, xylem transport of [14N]nitrate by the HN roots was 49% greater than for LN roots, while [15N]nitrate transport was 9% less in HN roots compared to LN roots. Even though the reduction of [15N]nitrate in HN roots was 31% less than LN roots during the first 2 hours, [15N]nitrate was reduced more rapidly than the previously accumulated [14N]nitrate. After the first 4 hours, the relative partitioning of absorbed [15N]nitrate between accumulation, reduction, and translocation was similar regardless of pretreatment.
Abstract Growth of bean plants was limited when nitrogen was supplied as (NH 4 ) 2 SO 4 in sand culture in comparison to the growth of plants receiving nitrate nitrogen. Mixing relatively insoluble carbonates with the sand, using (NH 4 ) 2 CO 3 instead of (NH 4 ) 2 SO 4 or maintaining the pH of the nutrient solution near neutrality with NaOH all prolonged the life and increased the growth of plants. Experiments in which C 13 ‐labeled carbonate was used revealed no absorption or incorporation of C 13 into the plant tissues. The changes in mineral element contents of plant tissue induced by the treatments did not account for the increased growth. In the absence of pH control, ammonium, amide, and amino acid accumulation was very high in the leaves. Maintaining the pH of the nutrient solution near neutrality stimulated N assimilation in roots, especially the synthesis of amino acids and amides. Acidity control thus prevented the accumulation of ammonium in the tops and hence lessened the possibility of toxic effects from ammonium. The increased conversion of ammonium to nontoxic metabolites in roots is considered to be thedominant process by which acidity control improved the growth of plants on ammonium nutrition.
Abstract Previous investigations revealed that nitrogen‐depleted wheat absorbed nitrate slowly on first exposure to nitrate solutions. Subsequently, however, the rate of nitrate uptake increased. The present studies were initiated to more thoroughly characterize this response in wheat, and to determine if other species responded similarly. Nitrate uptake of various species was determined from changes in the amount of nitrate in the external solution. Both chemical and isotopic procedures were employed. With nitrogen‐depleted wheat, the characteristic two‐phase pattern was observed even when nitrate uptake was severely restricted by presence of ammonium. Chloride uptake differed from nitrate uptake in two ways; the initial slow phase was not observed and the tissue reached chloride flux equilibrium at about six hours. Corn seedlings previously grown with ammonium showed the two phase pattern of nitrate uptake whereas linear rates were observed when they were previously grown with nitrate. After being deprived of nitrate, cotton and tobacco both exhibited the two‐phase pattern of nitrate uptake.
Abstract Two‐week‐old nitrogen‐deficient wheat plants attained a high rate of nitrate uptake on the first day of exposure to nutrient solutions supplemented with KNO 3 . Ammonium uptake from similar solutions supplemented with NH 4 NO 3 was also high during the first day of exposure, but nitrate uptake from this solution was lower than from the KNO 3 treatment. During the next two to three days there was a progressive decrease in uptake of both nitrogen ions. A steady increase in uptake then occurred as the plants fully recovered from the nitrogen‐deficient state. The transient low nitrate uptake after three or four days of exposure to KNO 3 was not due to an excessive accumulation of nitrate in the tissue, nor to a failure in nitrate reduction as indicated by the rate of nitrate accumulation relative to the uptake rate. Nitrogen supplied as 15 N‐nitrite during the low uptake period was effectively incorporated into organic forms and effectively translocated to the shoots. Failure of the root tissue to increase in soluble carbohydrates during illumination was characteristic of the low uptake period. This contrasted with an increase in root soluble carbohydrates in the light during rapid uptake associated with full recovery from the nitrogen‐deficient state. It is concluded that carbohydrate translocation to the root system was insufficient during the intermediate recovery period for optimal nitrate uptake, although it was sufficient for effective reduction and translocation of nitrate and reduced nitrogen. Ammonium uptake from NH 4 NO 3 was restricted during darkness by the third day whereas there was little difference between light and dark periods in nitrate uptake from KNO 3 until about the sixth day of recovery. The extent to which ammonium restricted nitrate uptake increased progressively for two or three days following which a lessening influence seemed evident, and the effects were not directly associated with the rate of ammonium uptake.
