It has been established with considerable certainty that nitrate is assimilated via ammonia in higher plants. Evidence for this comes from several different investigations in which N15-labeled compounds have been utilized. Firstly, it has been shown that nitrate is reduced to anmmonia (7,17). Secondly, it has been shown that nitrate and ammonia have essentially the same effect on the production of free amino acids and proteins (17,26). Finally, ammonia has been shown to be directly incorporated into organic nitrogenous substances via glutamate and glutamine (4,26).It has come to be generally accepted that nitrate reductase is the first enzynme to act on nitrate during its assimilation in higher plants (8). However, the physiological importance of this enzyme must rest, ultimately, on the demnonstrationi that enzymes capable of reducing nitrite to the level of amiiino-nitr-ogen, also exist in higher plants (20).In this investigation a stu(ly wras mlade of the enzymic systems responsible for the entire process of nitrate assimilation in tomato plants. The which was designated "Normal NO,-Level Solution," was 3.2 mA. One set of cultures was growvn in the above nutrient solution nmodified by increasing the level of nitrate containing salts sixfold. The concentration of nitrate in these cultures, which was designatedl "6 X NO,-Level," was 19.2 mai. In these studies the seecds wvere sterilized before germination and all subsequent manip)ulations were carriedI out un(ler aseptic con(litionis.Preparation of E;izvy;ic. Plant tissue wvas ground in 4 times its wseight of 0.1 Mr Tris HCl buffer (pH 7.5) containinig 10 " cysteine. Grinding was done with a cold nmortar and pestle containing approximately as mluclh cold acid washedl sand by weight as plant mlaterial. The macerate wvas pressed through cheese cloth and the filtrate was centrifuged at 1750 X g for 20 minutes. The supernatant solution was either used directly as the crude enzyme preparation or after purification by Sephadex treatment.Treatmlent with Sephadex was carried out by passing 4.0 ml of crude enzyme preparation through a colunin containing 6.0 g of Sephadex G-25 (A. B. Pharmacia, Uppsala, Sweden). Elution was carried out with 0.01 Ni Tris HCl buffer (pH 7.5) containing 10-3 AI cysteine. From 80 % to 100 % of the enzyme was normally recovered virtually free of nitrate in the macerate in the 6 ml of eluate following the first 12 ml. The enzyme containing eluate was used as the purified enzyme preparation. All of the above operations were carried out at 00 to 40 using cold materials and reagents.Enzymitic Assay. Nitrate reductase was assayed using the following reaction mixture: 0.5 ml of 0.1 WI potassium phosphate (pH 7.5), 10,umoles KNO3, 0.27 ,umole DPNH, 0.05 to 0.20 ml enzyme preparation and distilled water to make a final volume of 0.8 ml.Incubations were for 20 minutes at 270. Incubations were stopped by adding 0.2 ml M zinc acetate followedl by 6.0 ml 95 % ethanol. The treated reaction mixtures were centrifuged at 1500 X g for 5 minutes. A suitabl...

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Keywords:Modelling HYDRUS Mandarin Fertigation Nitrate leaching Soil salinity s u m m a r y Estimation of all water fluxes temporally and spatially within and out of the crop root zone, and evaluation of issues like salinity and nutrient leaching, are necessary to fully appraise the efficiency of irrigation systems. Simulation models can be used to investigate these issues over several seasons when the cost of long term monitoring is prohibitive. Model results can be used to advise growers if improvements are required to various aspects of irrigation system operations. In this study, HYDRUS-2D was used to evaluate data measured during one season in a young mandarin (Citrus reticulata) orchard, irrigated with an intensive surface drip fertigation system. Water contents, salinities, and nitrate concentrations measured weekly in the field were compared with model predictions.The temporal mean absolute error (MAE) values between weekly measured and simulated water contents ranged from 0.01 to 0.04 cm 3 cm À3. However, modelling error (MAE) was slightly larger at 10 cm depth (0.04 cm 3 cm À3), as compared to greater depths (0.02-0.03 cm 3 cm À3). Similarly, the errors were larger in the surface soil layer (25 cm depth) for nitrate-nitrogen, NO 3), as compared to greater depths. The spatial and temporal soil solution salinity (EC sw ) and NO 3 À -N data showed accumulation of salts and nitrate within the soil up until day 150 of the simulation (December, 2006), followed by leaching due to high precipitation and over irrigation at later times. Only 49% of applied water was used by the mandarin trees, while 33.5% was leached. On the other hand, the simulation revealed that a significant amount of applied nitrogen (85%) was taken up by the mandarin trees, and the remaining 15% was leached. The results indicate that the irrigation and fertigation schedule needs modifying as there was overwatering from December onwards.Different permutations and combinations of irrigation and fertigation scheduling were evaluated to optimise the water and nitrogen uptake and to reduce their leaching out of the crop root zone. Slightly higher nitrogen uptake (1.73 kg ha À1) was recorded when fertigation was applied second to last hour in an irrigation event, as compared to applying it earlier during an irrigation event. Similarly, a 20% reduction in irrigation and N application produced a pronounced reduction in drainage (28%) and N leaching (46.4%), but it also decreased plant N uptake by 15.8% and water uptake by 4.8%, and increased salinity by 25.8%, as compared to the normal practice. This management would adversely impact the sustainability of this expensive irrigation system. However, reducing only irrigation by 30% during the 2nd half of the crop season (January to August) reduced drainage and N leaching by 37.2% and 50.5%, respectively, and increased N uptake by 6.9%. Such management of irrigation would be quite promising for the sustainability of the entire system. It is concluded that judicious manipulations of irrigation and fer...

SUMMARY: 14C‐amino acids were added to fresh tea‐leaf homogenate undergoing conversion to black tea. After conversion (30 min, 25°C), the volatile compounds present in the headspace over the reaction mixture were collected and analyzed by gas chromatography. Results showed that leucine, isoleucine, valine and phenylalanine were partially converted to the aldehydes expected from a Strecker degradation. These aldehydes are constituents of black tea aroma. Further, drying of the fermented mixture caused an additional amount of the aldehydes to be formed. In contrast, no detectable volatile compounds were formed from aspartic acid, glutamic acid, glutamine, arginine, threonine. serine or theanine under the same conditions. Production of aldehydes from amino acids was shown to be dependent on the enzymic conversion process: Tea leaf which had been inactivated by steam treatment was not effective in causing formation of volatile aldehydes from the amino acids. Identical results were obtained in a model tea fermentation system composed of a crude soluble enzymes extract from tea leaves, purified epigallocatechin gallate and 14C‐amino acids. Ascorbic acid was found to inhibit formation of aldehydes from amino acids in this model tea fermentation system; dehydroascorbic acid by itself was found to be effective in causing formation of volatile aldehydes from amino acids.

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