Changes in amino acid and organic acid composition in tomato and cucumber plants in relation to salinity and nitrogen nutrition

Under conditions of salt stress, plants show qualitative and quantitative alterations in various organic compounds, such as nitrogen (N) compounds and organic acids. In this work, the effect of different saline levels as well as various N levels, supplied as nitrate (NO₃) or as ammonium (NH₄)+NO₃ on the concentration of amino acids and organic acids in the leaves of tomato and cucumber plants has been studied. The effect of the source of N on individual amino acid contents varied with plant species. Most of the amino acids increased when the concentration of N in the nutrient solution was increased, except when N was added as NH₄+NO₃ for tomato. The effect of salt stress depended on which amino acid was considered. The data also indicate that the effect of salinity on each particular amino acid was greatly dependent on the plant species and N source. Organic acids were differently affected by salinity and by the N source, depending on the plant species. In tomato, the concentrations of short‐chain organic acids were 2–3 times higher in NO₃‐supplied plants than in those grown with NH₄+NO₃. Finally, in cucumber, malic acid concentration increased as a function of the saline level in the medium.     

Accumulation and partitioning of biomass and soluble carbohydrates in maize seedlings as affected by source of nitrogen,nitrogen concentration,and cultivar 1

Abstract

Seedlings of four maize hybrids were grown hydroponically to investigate the impact of different N sources (Ca(NO₃)₂, (NH₄)₂SO₄ and a 1:1 mixture of both) on (i) production and partitioning of root and shoot dry matter, (ii) concentration of soluble carbohydrates in roots and shoots and their partitioning to these plant parts, (iii) concentration of starch in the shoot, and (iv) N uptake. During the main phase of the experiments (duration 14d), the plants were grown in a greenhouse at 25/22°C day/night temperatures and a photoperiod of 16h. Nitrogen was supplied at three concentrations (2.8, 28, and 280 ppm). The root‐zone pH was 6.5. Under the lowest N supply, the N sources produced similar root and shoot dry matters. At the highest N level (280 ppm), NO₃‐fed plants were superior. In contrast, the mixture of NH₄ and NO₃ ^? was optimum at 28 ppm. More or less pronounced N form by N concentration interactions were also found in the concentration and distribution of soluble carbohydrates and in all remaing traits. There were almost statistically significant cultivar by N form interactions in shoot dry matter (P = 0.07) and total dry matter (P = 0.06), indicating the existence of considerable genotypic variation in sensivity to NH₄‐N.     

Tillering,nutrient accumulation,and yield of winter wheat as influenced by nitrogen form 1

When grown with mixtures of nitrate‐nitrogen (NO₃‐N) and ammonium‐nitrogen (NH₄‐N) (mixed N) spring wheat (Triticum aestivum L.) plants develop higher order tillers and produce more grain than when grown with only NO₃. Because similar work is lacking for winter wheat, the objective of this study was to examine the effect of N form on tillering, nutrient acquisition, partitioning, and yield of winter wheat. Plants of three cultivars were grown to maturity hydroponically with nutrient solutions containing N as either all NO₃, all NH₄, or an equal mixture of both forms. At maturity, plants were harvested; separated into shoots, roots, and grain; and each part analyzed for dry matter and chemical composition. While the three cultivars varied in all parameters, mixed N plants always produced more tillers (by a range of 16 to 35%), accumulated more N (28 to 61%), phosphorus (P) (22 to 80%), and potassium (K) (11 to 89%) and produced more grain (33 to 60%) than those grown with either form alone. Although mixed N‐induced yield increases were mainly the result of an increase in grain bearing tillers, there was cultivar specific variation in individual yield components (i.e., tiller number, kernels per tiller, and kernel weight) which responded to N form. The presence of NH₄ (either alone or in the mixed N treatment), increased the concentration of reduced N in the shoots, roots, and grain of all cultivars. The effect of NH₄ in either treatment on the concentrations of P and K was variable and depended on the cultivar and plant part. In most cases, partitioning of dry matter, P, and K to the root decreased when NH₄ was present, while partitioning of N was relatively unaffected. Changes in partitioning between the shoot and grain were affected by N treatment, but varied according to cultivar. Based on these data, the changes in partitioning induced by NH₄ and the additional macronutrient accumulation with mixed N are at least partially responsible for mixed‐N‐induced increases in tillering and yield of winter wheat.     

Effect of NO3:NH4 ratio and light intensity on nitrogen partitioning in pepper plants

Pepper plants were hydroponically grown in a growth chamber with two levels of light intensity (300 and 240 w/m²) and two NO₃:NH₄ ratios (100:0 and 80:20). Plants grown with both N forms (80:20) under high light regime displayed greater concentrations of free NH₄, organic soluble N and insoluble N than NO₃, treated plants, this effect was more pronounced for stem, petioles and root. By contrast, nitrate‐ammonium fed plants with light reduced over 25% presented similar concentrations of organic N (soluble and insoluble) as compared to plants supplied with NO₃ as sole source of N, however, the amount of free NH₄ in stem and root was increased. Results indicat that NH₄ supplied at low concentration was more deletereous for pepper plants grown under high light conditions, when light was reduced, plants presented an alleviation of toxic effect of NH₄ supply. Finally, nitrogen use efficiency is related to form of N nutrition and the light supplied to crop.     

Tomato growth,nitrogen fraction and mineral composition in response to nitrate and ammonium foliar sprays 1

Tomato plants were grown in sand culture with NO₃ or NH₄ N at two levels of light. Foliar sprays at three levels of N as well as combinations of foliar and root feeding were used.

Shade increased NH₄ toxicity in plants sprayed with NH₄ but decreased the toxicity in plants receiving NH₄ through the roots. NH₄‐N greatly reduced growth and cation uptake when supplied through the roots but not with foliar application. Plants sprayed with NH₄ showed better growth, higher K, Ca, and Mg content and lower free NH₄ in shoot, compared to plants receiving NH₄ through the roots.

The overall free amino acid contents of shoots was higher for NH₄‐fed plants regardless of how the N was applied. Plants sprayed with NH₄ incorporated a greater amount of N into insoluble compounds compared with NO₃ nutrition. The N uptake per unit of leaf area was higher for plants grown under full sun light whereas N content was higher for plants grown under hade. N content in tissue increased with N concentration in foliar spray, although plants supplied with N through the roots had higher levels of free amino acids and total nitrogen.     

Changes in growth and nitrogen assimilation in barley seedlings under cadmium stress

Barley (Hordeum vulgare L. cv. Martin) plants grown in solution culture, were exposed to increasing cadmium (Cd) concentration (0, 5, 10, 25, 50, and 100 μM) for a duration of 12 days. The sequence of important biochemical steps of nitrate (NO₃) assimilation were studied in roots and shoots as a function of external Cd concentration. Cadmium uptake in roots and shoots increased gradually with Cd concentration in the medium. This Cd accumulation lowered substantially root and shoot biomass. The nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NiR, EC 1.6.6.4) activities declined under Cd stress. Concurrently, tissue NO₃ contents and xylem sap NO₃ concentration were also decreased in Cd‐treated plants. These results suggest that Cd could exert an inhibitory effect on the assimilatory NO₃ reducing system (NR and NiR) through a restriction of NO₃ availability in the tissues. We therefore examined, in short‐term experiments (12 h), the impact of Cd on NO₃ uptake and the two reductases in nitrogen (N)‐starved plants that were pretreated or not with Cd. It was found that Cd induced inhibition of both NO₃ uptake and activities of NR and NiR, during NO₃ induction period. The possible mechanisms of Cd action on NO₃ uptake are proposed. Further, in Cd‐grown plants, the glutamine synthetase (GS, EC 6.3.1.2) showed a decreasing activity both in shoots and roots. However, increasing external Cd concentration resulted in a marked enhancement of glutamate dehydrogenase (NADH‐GDH, EC 1.4.1.2) activity, coupled with elevated levels of ammonium (NH₄ in tissues. On the other hand, the total protein content in Cd‐treated plants declined with a progressive and substantial increase of protease activity in the tissues. These findings indicate that under Cd stress the usual pathway of NH₄ assimilation (glutamine synthetase/glutamate synthase) can switch to an alternative one (glutamate dehydrogenase). The changes in all parameters investigated were concentration‐dependent and more marked in roots than shoots. The regulation of N absorption and assimilation by Cd in relation to growth and adaptation to stress conditions are discussed.     

Soil nitrogen fractions as influenced by sample preparation and extraction

Abstract

In order to evaluate the influence of extraction procedure on extractable nitrogen (N) fractions, fresh as well as dried soil samples were extracted with CaCl₂ at various temperatures (20,40,60, 80°C) for 30–120 minutes. Data obtained were compared with those from the electro‐ultra‐filtration (EUF) method. Increasing the drying temperature as well as the extraction temperature led to an increase in N_org content. The EUF and CaCl₂‐method produced comparable results for all N‐fractions (NO₃ ^‐, NH₄ ⁺, N_org) when an extraction temperature of 80°C was applied for two hours. Data presented suggested that the N_org fraction represented mainly the microbial biomass and may thus be considered as being easily available to plants.     

Potassium,nitrogen, ammonium/nitrate ratio,and sodium chloride effects on wheat growth

Fertigation with KNO₃ as a means of reducing salinity hazards was tested with peanut (Arachis hypogaea) plants grown on dune sand, resulting in a reduction of plant growth and yield. The objective of this work was to study the interactions between N, K⁺ and NaCl as well as the effects of the NH₄ ⁺/NO₃ ^‐ ratio on vegetative and reproductive growth. Wheat (Triticum aestivum L.) plants were grown in polyethylene pots with fine calcareous dune sand with different proportions of NH₄ ⁺ and NO₃ ^‐, under saline (60 mM NaCl) and non‐saline conditions. Three replicates were harvested at the beginning of flowering, and one was grown to grain maturity. NaCl reduced shoot dry weight in all the treatments. Increasing the NH₄ ⁺ proportion in the total of 6 mM N in the nutrient solution, increased shoot dry weight, did not change nitrogen concentration in the dry mass but increased P percentage, either with or without 60 mM NaCl. The number of tillers produced in each treatment was correlated with dry matter yield. The effect of the NH₄ ⁺/NO₃ ^‐ ratio may be explained by alteration of the cation‐anion balance on the nutrient uptake by roots, which lowered pH of the nutrient solution with increasing NH₄ ⁺ concentration, by alteration of the cation‐anion balance on the nutrient uptake by roots, which lowered pH of the nutrient solution with increasing NH₄ ⁺ concentration.     

An inverse abundance approach to separate soil nitrogen pools and gaseous nitrogen fluxes into fractions related to ammonium,nitrate and soil organic nitrogen

The soil nitrogen (N) cycle exhibits a variety of complex biochemical reactions in which N species such as NO₂⁻, NO and N₂O are produced and consumed by co‐existing processes that respond differently to the local environmental conditions. Key to understanding the soil N cycle in its full complexity is the development and application of methods that allow a quantification of individual pathways and processes that are responsible for the build‐up and/or emission of N compounds. Triplet ¹⁵N tracer experiments (TTE) have been developed and applied to allow a source‐related quantification of N species such as NO₂⁻, and N₂O by different biochemical pathways (e.g. ammonia oxidation and nitrate reduction) that are related to multiple N sources (NH₄⁺, NO₃⁻ and N_org). An analysis of a TTE requires the application of either a numerical or analytical model. Because of the ease of application it is desirable to use analytical models. However, available analytical solutions suffer from serious drawbacks concerning the quantification of N fluxes related to soil organic N. In this paper we describe the development and application of a new inverse abundance approach (IAA) to analyse a TTE. Theoretical and experimental data sets of soil N₂O release were analysed by the new method. The IAA was also applied to an already existing data set to identify fractions of the soil nitrite pool related to NH₄⁺, NO₃⁻ and N_org. We show that the IAA provides a reliable and comprehensive data evaluation of a TTE.     

The influence of the form of nitrogen nutrition on uptake and distribution of cadmium in lettuce varieties

Four lettuce (Lactuca saliva L.) varieties ('Benita’, ‘Wendy’, ‘Mirena’, and ‘Jacky') were grown for 43 days in a pH‐controlled complete nutrient solution without cadmium (Cd) or with either 0.01 or 0.03 mg Cd/1 and with NH₄ or NO₃ as the form of nitrogen nutrition. Cadmium did not affect dry matter yield. ‘Wendy’ had a significantly higher total dry matter production when grown on NO₃ compared to NH₄, whereas growth of ‘Mirena’ was best on NH₄ (P<0.05). Dry weights of ‘Benita’ and ‘Jacky’ were not affected by the N source. Cadmium concentrations in shoots (and roots) of plants grown on NH₄ were significantly higher than in plants grown on NO₃. The Zn concentrations in the shoots were also enhanced. The distribution of Cd in the lettuce varieties was independent of the form of N nutrition. It is concluded that the N source directly affects the amount of Cd taken up, without influencing the Cd distribution.     

Tomato growth and nutrient uptake patterns as influenced by nitrogen form and light intensity 1

Tomato plants were grown in sand culture with NH₄ or NO₃ forms of N and at two levels of light. Plants were harvested at 0, 5, 9, or 12 days after starting treatments. NH₄‐N nutrition reduced growth, suppressed K, Ca, and Mg accumulation in shoot, increased P and N content and markedly reduced K, Ca, and Mg uptake per unit of root surface. Reduced light level decreased the toxic effects of NH₄ and markedly decreased NH₄ accumulation in shoots.     

Drought and nitrogen source effects on nitrogen nutrition,seed growth,and yield in soybean

Drought in soybean [Glycine max (L.) Merr.] decreases yield‐related processes and N₂ fixation is more sensitive to drought than are many other of these processes. Therefore, application of nitrogen (N) fertilizer may increase drought tolerance over those plants primarily dependent on N₂ fixation. In a field experiment, NH₄NO₃ applications (+N) to drought‐stressed soybean resulted in biomass and N accumulation rates similar to those rates for an irrigated treatment without N fertilizer (‐N). In contrast, biomass and N accumulation rates were decreased for the ‐N treatment. N fertilization increased seed growth rate and decreased seed fill duration for irrigated and drought treatments. In the drought treatment, N application increased seed number per unit area, which resulted in higher yields. In a greenhouse experiment, fertilization with either KN0₃ or NH₄C1 increased biomass and N accumulation rates during drought over those of plants dependent solely on N₂ fixation. It was concluded that application of N fertilizer to soybean increases drought tolerance because of the extreme sensitivity of N₂ fixation to drought.     

Ammonium and nitrate influence on artichoke growth rate and uptake of inorganic ions

Artichoke plants (Cynara scolymus L.) were grown in a growth chamber in a modified Hoagland solution for seven weeks to determine the influence of ammonium:nitrate (NH₄:NO₃) ratio (100:0, 70:30, 30:70 and 0:100) on growth, water use, and the uptake of nitrogen (N) and inorganic anions and cations. Typical pH changes were recorded: the nutrient solution became acidified with NH₄ or NH₄:NO₃ nutrition; pH increased when NO₃ was the only N source. Ammonium‐fed plants (100:0 ratio) were stunted, with signs of marginal leaf necrosis, progressive wilting of leaves and poor root growth. After 49 days, leaf area was 77, 998, 2,415, and 1,700 cm² and dry weight was 1.0, 12.9, 38.0, and 26.0 g/plant, with NH₄:NO₃ 100:0, 70:30, 30:70, and 0:100, respectively. Leaf area ratio (LAR) was lower in plants supplied solely with NO₃ than in those with mixed NH₄‐NO₃. Increasing NO₃‐N percentage in the nutrient solution increased water use efficiency (WUE): 623, 340, and 243 mL of water were necessary to produce 1 g of dry matter in 100:0, 70:30, 30:70 or 0:100 NH₄:NO₃ ratio, respectively. Increasing NO₃ from 0 to 100% of the total N supplied in the nutrient solution, the shoot content of inorganic cations increased on an equivalent basis by 30% and organic anions (estimated by the difference between inorganic anions and inorganic cations) increased by 2.3 times. These results suggest that leaves are the most important site of NO₃ assimilation in artichoke. By increasing NH₄ percentage in the nutrient solution, the tissue content of inorganic anions was generally increased, except for NO₃, and the same figure was observed for the percentage of reduced N. Results from this study suggest that NO₃ is the N‐form preferred by artichoke.     

Dynamic of different C and N fractions in a Cambisol under five year succession fallow in Saxony (Germany)

The dynamic of different soil C and N fractions in a Cambisol under succession fallow was investigated from June 1996 until May 2001. Mineral soil samples (0 – 10 and 10 – 30 cm) were analyzed for their concentrations of organic C (C_org), total N (N_t), hot water extractable C and N (HWC and HWN), and KCl extractable C and N (C_org(KCl), N_org(KCl), NH₄⁺‐N, NO₃^–‐N). The values of all C and N fractions revealed a distinct depth gradient. While the concentrations of C_org increased after set aside significantly from 7.7 to 8.9 g kg^–1 at 0 – 10 cm, those at 10 – 30 cm depth decreased from 7.2 to 6.1 g kg^–1. N_t remained rather constant throughout the whole observation period. The HWC concentrations increased from 0.33 to 0.49 g kg^–1, while HWN decreased slightly at 0 – 10 cm with time. In contrast, both HWC and HWN increased at 10 – 30 cm soil depth. HWC showed close significant correlations to C_org, and HWN to N_t as well as to NH₄⁺‐N and NO₃^–‐N, respectively. In comparison to hot water‐extractable C and N, C_org(KCl) and N_org(KCl) accounted only about one tenth of those and showed a decreasing trend with time of succession. C : N ratio of the KCl fraction was in the same order of magnitude as the HWC : HWN ratio, except the last phase of the experiment where hot water extract values increased above 10.     

Ammonium (15N) metabolism in cotton under salt stress

Plant growth and metabolism is impaired under stress conditions, resulting in decreased crop yields. The purpose of this investigation was to evaluate the NaCl stress effects on NH⁺ ₄ metabolism in cotton plants at vegetative and reproductive stages of growth.

Cotton (Gossypium hirsutum L.) plants grown in normal (control) and NaCl treated Hoagland solutions were analyzed for distribution of N¹⁵ in NH⁺ ₄ plus amide‐N, free α‐amino‐N, total soluble‐N and protein‐N after the plants were provided ¹⁵NH₄NO₃ in nutrient solutions for 6, 12 and 24 h. The concentration of protein‐¹⁵N was enhanced under a low level of NaCl (‐0.4 MPa osmotic potential) at the vegetative growth stage. The difference between the protein‐¹⁵N concentration of the moderately salinized (‐0.8 MPa) plants and the controls was not significant. A high level of NaCl (‐1.2 MPa) significantly decreased protein‐N content of plants compared with the controls and any other level of salinity. The NaCl increased accumulation of NH₄ ⁺ plus amide‐N, free (α‐amino‐N, and total soluble‐N in cotton shoots, at both stages of growth. Low osmotic potential (high osmotic pressure) of the nutrient solution induced by excessive amounts of NaCl in nutrient solution inhibited NH⁺ ₄ metabolism and decreased protein synthesis, thus resulting in accumulation of soluble N‐compounds. The ionic effect probably contributed also to inhibition of protein synthesis.     

Effect of nitrogen form during the flowering period on zucchini squash growth and nutrient element uptake

Zucchini squash (Cucurbita pepo L. cv. Green Magic) plants were grown hydroponically with nitrate (NO₃):ammonium (NH₄) ratio of 3:1 until the onset of flowering when the plants were assigned to four NO₃:NH₄ ratio (1:0, 1:1, 1:3, or 3:1) treatments. Changing the original nitrogen (N) form ratio significantly affected plant growth, fruit yield, nutrient element, and water uptake. Growth of plants was better when NO₃‐N (1:0) was the sole form of N than when NH₄‐N was part of the N treatment. Fruit yields for plants fertilized with 1:0 or 1:3 N‐form ratio were double those of plants grown continuously with 3:1 N ratio. The largest leaf area and plant water use were obtained with 1:0 N ratio treatment Total uptake of calcium (Ca), magnesium (Mg), and potassium (K) decreased with increasing NH₄‐N proportion in the nutrient solution which suggest NH₄‐N was competing with these cations for uptake. The results also demonstrated that growers may increase fruit yield by using a predominantly NO₃‐N source fertilizer through the vegetative growth stage and by shifting the NO₃:NH₄ ratio during the reproductive phase.     

Potassium and nitrogen effects on carbohydrate and protein metabolism in alfalfa roots

An investigation was conducted to determine the effect of potassium (K) nutrition on alfalfa (Medicago sativa L.) growth and metabolism of root total nonstructural carbohydrates (TNC) and proteins, and to study whether nitrogen (N) fertilization overcomes N deficiency and low root protein concentrations caused by K deficiency. In Experiment 1, nodulated alfalfa plants were grown in plastic pots containing washed quartz sand and provided minus‐N Hoagland's solution containing 0, 0.6, or 6.0 mM K. Shoot and root K concentrations increased with increasing solution K. Root N concentrations were higher in plants receiving 6.0 mM K than in plants receiving 0.6 or 0 mM K, but shoot N concentrations were similar for all treatments. Plant persistence, shoots per plant, and shoot mass increased as solution K levels increased. Root starch concentration and utilization were positively associated with K nutrition. Total amylase activity was higher, but endoamylase activity was lower in roots of plants receiving 6.0 mM K compared to plants receiving 0.6 or 0 mM K. Root soluble protein concentrations were significantly higher in plants receiving 6.0 mM K than in plants receiving 0 or 0.6 mM K. In Experiment 2, plants were supplied with Hoagland's solution containing 10 mM N as ammonium (NH₄ ⁺) or nitrate (NO₃) with 0,3, or 6.0 mM K. The addition of N increased root N concentrations only in plants receiving 0 mM K. Plant persistence was reduced by NH₄ ⁺ application, especially in plants receiving 0 or 3 mM K. Root starch concentrations were markedly reduced in plants receiving NH₄ ⁺ at all K levels. The addition of NO₃ ^‐ had little effect on alfalfa root carbohydrate and protein metabolism and subsequent shoot growth. Potassium deficiency reduced starch and protein concentrations in roots; factors that were associated with poor persistence and slow shoot regrowth of alfalfa.     

Intraspecific differences in nitrogen assimilating enzymes in spinach under contrasting forms of nitrogen supply 1

Intraspecific differences in the activities of nitrate reductase (NR), glutamine synthetase (GS), NADH dependent glutamate synthase (NADH‐GOGAT), and glutamate dehydrogenase (GDH) under contrasting forms of nitrogen (N) supply were studied in tissues of three spinach (Spinacia oleracea L.) cultivars. The varieties (Viroflay, Butterflay, and Giant) were smooth, curly and semicurly leaved, respectively. The plants were grown in nutrient solutions containing NO₃ as the sole source of N (100:0) and NO₃ plus NH₄ (80:20). Giant, the NH₄ tolerance of which had been evaluated in growth and on the basis of nutrient uptake, had much higher GS and GDH activities in the roots and higher NR and NADH‐GOGAT levels in the leaves of plants grown on NO₃ and NH₄ than that grown on NO₃ alone. On the level of N assimilating enzymes of Butterflay, mixed N nutrition caused an increase of GDH and NADH‐GOGAT in leaves and roots and at the same time a decrease of GS in the roots and NR in the leaves. An inverse relationship between GS and GDH activities was detected in the leaves and foots of Virofiay grown with both N sources. Finally, Viroflay gave the highest levels of GDH irrespective of the NO₃:NH₄ assayed, whereas the leaves of Giant were GDH deficient in comparison with the other cultivars. In addition, the GS activity approached zero in the roots of spinach cultivars characterized by hardly any NH₄ tolerance, whereas in those of Giant it increased remarkably with the supply of NO₃ plus NH₄.     

Effects of atmospheric nitrogen dioxide on uptake and assimilation of ammonium in soybean plants

Twelve‐day‐old soybean plants were supplied with 1 mM ammonium chloride (NH₄C1) to roots and exposed to 0.2–0.25 μL.L^‐1 nitrogen dioxide (NO₂) for seven days. Amount and rate of ammonium uptake were decreased by NO₂ exposure. However, the ammonium concentration in leaves and leaf pH of exposed plants were increased by NO₂ exposure. These results suggest that the decrease in ammonium uptake may be due to the decline in hydrogen (H⁺) ion concentration in exposed plants which resulted from the reduction of the nitrate and nitrite from NO₂ absorption. They also suggest that the decrease in ammonium uptake and the competition for energy between nitrate reduction and ammonium assimilation may limit ammonium assimilation to organic nitrogen (N) which would further inhibit acidity increase in exposed plants and ammonium uptake by roots.     

Effect of intensively cropping greenhouses in semiarid regions on soil salinity and nitrogen fertilizer requirements of cucumber

The soil of a greenhouse located in the semiarid Lebanese coast of the Mediterranean sea, and which has been in intensive crop production for the past ten years, was chemically analyzed. Results were: NO₃‐N = 225 ppm, NH₄‐N = 56 ppm, pH = 7.0 and salinity (ECe) = 2.5 dS/m. Irrigation water salinity (ECw) = 0.4 dS/m. Cucumber (Cucumis sativus L. cv. Lolita), a parthenocarpic beit‐alpha type plant, was grown in this greenhouse, and was treated with nitrogen (N) fertilizer at the rates of 0, 81 and 162 kg N/ha. The N was split into six equal weekly applications, with the first application made on the first week of fruit production. Total fruit yield for the first 8 weeks of harvest was 71.4, 63.4 and 60.2 ton/ha for the plots receiving 0, 81 and 162 kg N/ha, respectively. Leaf petiole NO₃‐N concentration was higher than the recommended level in all treatments throughout the experiment. At last harvest, NO₃‐N concentration in leaf petiole was 12500, 15500 and 19500 ppm in plants receiving 0, 81 and 162 kg N/ha, respectively. Soil salinity has sufficient to cause yield reduction for many greenhouse crops. In contrast, soil mineral N has sufficient to meet the N requirement for an entire season for many annual crops.