Nitrate Accumulation In Lettuce Research Articles

Impact of substrate depth and fertilizer type on growth, production, quality characteristics and heavy metal contamination of tomato and lettuce grown on urban green roofs

Urban farming on rooftops is a contemporary approach that combines the benefits of green roofing with food production within the city sprawl. The present study aims at assessing plant growth, nutritional value, and consumer safety of two vegetables, lettuce (Lactuca sativa L.) and tomato (Solanum lycopersicum L.) grown on urban green roof. Treatments included: (a) two substrate depths (7.5 and 15 cm) and (b) three types of fertilization (granulated complex fertilizer [GF], slow nitrogen release complex fertilizer [SR], and liquid fertigation [LF]). Measurements included plant growth, total fresh yield, dry weight, and quality characteristics such as nutrient content, nitrate accumulation in lettuce and firmness, and total soluble solids [TSS] in tomatoes. Production safety was evaluated through tissue analyses of the edible parts for heavy metal residues. Results showed that 15 cm substrate depth improved the total yield for both vegetables, and the firmness of tomato fruit, compared to 7.5 cm depth. Fertilization type had no impact on tomato productivity but GF increased TSS in tomato fruits while LF enhanced lettuce yield. The deeper substrate increased N, P, K, Ca, Mg in lettuce leaves but only P and Mn in tomato fruits. In contrast, fertilization type affected leaf nutrient levels in lettuce, where N, P, and Mg were increased using LF. In tomato, the LF increased the fruit P, K, Ca, Mg, and Fe concentration. LF increased the concentration of nitrates in lettuce to levels exceeding the maximum safety limits according to the European Food Safety Authority (EFSA) but not according to EU regulation No 1258/2011. The leaf Mn concentrations in lettuce were higher than the health limits for young children in all measured samples irrespective of the experimental treatments. Selenium (Se) exceeded the health limits in tomato fruits for adults and toddlers using SR but only for toddlers using LF, while GF maintained the Se content below the safety levels. Lead (Pb) and aluminium (Al) were high for both vegetables but did not exceed the EFSA safety limits for adults. However, Pb exceeded the EU Commission Regulation No 420/2011 limit, for both vegetables and fertilization treatments. Washing the leaves of lettuce reduced only Fe and Al concentration but the remaining elements were unaffected. In conclusion, our results suggest that lettuce and tomato of adequately high quality for human consumption can be produced on urban green roofs with shallow substrate depths ranging from 7.5 to 15 cm but, further research is needed to ensure production safety.

Nitrate accumulation and physicochemical characteristics of lettuce as affected by sodium selenite and synthesized selenium nanoparticles

ABSTRACT Exogenous selenium (SE) application during lettuce (Lactuca sativa L.) production may prevent Se deficiency. Low concentrations of Se have beneficial effects on plant cell metabolism and application of Se can increase growth, yield, and quality and reduce nitrate concentration in lettuce. In the present study, biosynthetic synthesis of selenium (Se) nanoparticles (NPs) was carried out via Se ions reduction during exposure to rosemary (Salvia rosmarinus Spenn.) extract. Using scanning electron microscopy (SEM), the shape and size of the Se NPs were determined. To investigate effects of concentrations of sodium selenite (1, 2, and 4 mg∙L−1) and Se NPs (1, 2, and 4 mg∙L−1) on morphological and physiochemical traits of lettuce. Most nutrient solutions, especially 2 mg∙L−1 Se NPs, increased plant height, leaf number, fresh weight, chlorophyll a, total chlorophyll, and nitrate reductase activity of plants. Application of 2 and 1 mg∙L−1 Se NPs, caused a decrease in the level of nitrate accumulation. Under nutrient treatments, concentrations of P, K, Fe, Zn, and Se, and concentration of N decreased. Foliar application of biosynthetic synthesized Se NPs, may have potential for improved quantity and reduced nitrate accumulation in lettuce.

Potential Role of Foliar Application of Azotobacter on Growth, Nutritional Value and Quality of Lettuce under Different Nitrogen Levels.

Vegetables can be treated with biofertilizers as an alternative to chemical fertilizers because of their low toxicity. We investigated the effects of foliar spraying of Azotobacter under different levels of nitrogen (100, 150 and 200 mg/L in nutrient solution) on the growth, nutritional value, nitrate accumulation and antioxidant enzyme activities of hydroponically grown lettuce. The experiment was laid out in a completely randomized design with four replicates in a factorial combination. Plants treated with Azotobacter and 200 mg/L nitrogen had greater leaf area and photosynthetic pigments than plants treated with 200 mg/L nitrogen without spraying with Azotobacter. Increasing nitrogen levels increased leaf number, fresh and dry weights, leaf area and nitrate accumulation in lettuce plants. Peroxidase (POD) activity increased by 95.4% at a nitrogen level of 200 mg/L compared to a nitrogen level of 100 mg/L. Ascorbate peroxidase (APX) activity and leaf phosphorous (P) and potassium (K) concentrations were the highest in plants treated with a nitrogen source of 100 mg/L without foliar application of Azotobacter. As nitrogen levels increased in all treatments, nitrate reductase (NR) activity decreased and reached a minimum at the 200 mg/L nitrogen level. In general, foliar application of Azotobacter sp. can be used to promote plant growth and reduce nitrate accumulation in lettuce.

Alternation of temporally overlapped red and blue light under continuous irradiation affected yield, antioxidant capacity and nutritional quality of purple-leaf lettuce

The responses of yield, nutritional quality and antioxidant capacity of hydroponic purple-lettuce were investigated in a plant factory to different alternations of temporally overlapped red (R) and blue (B) light under continuous irradiation in which spectral photon flux density distribution changed with various combinations of R and B LEDs on a 24-h cycle. Two constant simultaneous rend and bleu (R-B) light treatments including one normal photoperiod (RB, 16/8 h, 225 μmol·m−2·s−1) and one continuous light (RB’, 24/0 h, 150 μmol·m−2·s−1) as well as three alternating temporally overlapped light treatments (B to R-B and then R, each part lasts the same time) with alternating frequencies of one time (T1), two times (T2) and four times (T4) respectively in 24 h were set up with the same daily light integral (DLI). The lettuce plants were exposed to the irradiation patterns for 14 days. In all treatments, plants received the same quantum number of red or blue light and the same daily light integral (12.96 μmol·m−2). In comparison with normal photoperiod (RB), the results showed that lettuce plants gained greater shoot fresh and dry weight under RB’ and T1 both on the 7 and 14 days after treatment, which depended on leaf area and biomass allocation. Among alternating light treatments, the shoot fresh weight of lettuce decreased with the increase of alternating frequencies. Lettuce plants under alternating temporally overlapped treatments had significantly higher MDA, H2O2 and O2− contents as well as higher SOD and CAT activities than that under RB and RB’. Alternating light treatments also induced phenols accumulation, and particularly lettuce under T2 had significantly higher anthocyanin, flavonoid and total phenol contents than RB. However, prolongation of light period under the same DLI condition might promote nitrate accumulation in lettuce while had no positive effects on starch and soluble sugar contents. Therefore, alternating temporally overlapped red and blue light under continuous irradiation could improve the antioxidant capacity of lettuce to a certain extent, and the responses of lettuce antioxidant system to alternating light exposure might depend on the duration and alternating frequency of two monochromatic light qualities.

The impact of different growth media and ammonium-nitrate ratio on yield and nitrate accumulation in lettuce (Lactuca sativa var. longifolia)

Ammonium (NH4+) to nitrate (NO3-) ratio and growth media significantly alter plant development and NO3- accumulation in lettuce. Nitrate accumulation is regarded harmful for environment and human health. The quality of lettuce is assessed by NO3-concentration, size, and weight. This study determined the impact of five different growth media (cocopeat, peat, bark, perlite and rockwool) and four different NH4+:NO3- ratios (0:100, 20:80, 40:60 and 60:40) on NO3- accumulation in lettuce, yield, and several growth attributes. The experimentation was conducted according to open feeding system of soilless agriculture. The ‘Cosmos’ variety of lettuce was used as experimental material in the study. Growth mediums and NH4+:NO3- ratios significantly altered NO3- accumulation, head, leaf, stem and root traits. The results revealed that instead of growing lettuce with NO3- only in peat and rockwool, addition of NH4+ (20:80 of NH4+:NO3-) into nutrient solution increased head weight. While head weight increased in perlite medium with the addition of NH4+, it decreased in cocopeat and bark media. It is concluded that growth media and NH4+:NO3- ratios pose significant impacts on NO3- accumulation in leaf and that the increase in NH4+ ratio decreased NO3- accumulation in all growing media. Therefore, it is recommended that NH4+ should be added in the nutrient solution to decrease NO3- accumulation, which will ultimately improve yield and quality of lettuce.

Effect of sulfate anions on nitrates accumulation and total Kjeldahl nitrogen content in lettuce plants cultivated in peat-perlite mixture

Intensive agriculture is characterized by increased use of synthetic nitrogen fertilizers. If the supplied nitrogen fertilizers are excessive, many vegetables, particularly leafy vegetables, accumulate high levels of nitrates. Increased vegetable nitrate content is related to possible implications for health, as nitrate metabolites can cause carcinogenesis, infantile methaemoglobinaemia and possibly even teratogenesis. The aim of the present study was to investigate the effect of sulfate ions on nitrate accumulation and total Kjeldahl nitrogen (TKN) content in lettuce (Lactuca sativa var. crispa ‘Lollo Bionda’), as an attempt to suggest possible ways to reduce nitrate levels. Plants were grown in peat-perlite mixture and fertilized by nutrient solutions under glasshouse conditions. Three levels of nitrogen concentrations (100, 200 and 400 mg L(‑1) N) have been applied in combination with three levels of sulfate anions (15, 150 and 300 mg L(‑1) S as SO(4)2-). At harvest stage, the fresh weight of leaves, nitrate accumulation, total sulfur concentration and TKN tissue content were determined. As expected, foliar biomass, nitrate accumulation and TKN content were enhanced when nitrogen concentration was increased. However, the increased levels of sulfates (150 and 300 mg L(‑1) S) had no effect on plant weight whereas a decline of nitrate accumulation was observed when sulfate fertilization increased. Moreover, a concentration of 150 mg L(‑1) S caused TKN increase while the reverse was the case for the concentration of 300 mg L(‑1) S. The results of the present study indicate that the enhanced concentrations of sulfates resulted in decreased nitrate accumulation in lettuce leaves. This may be due to interference in the uptake of nitrates by sulfates ions and may be proposed as a possible way to decrease nitrate content in lettuce, resulting in consumer safety.

Effects of different LED sources on the growth and nitrogen metabolism of lettuce

Using a light-emitting diode (LED) as the light source, the effects of eight different light treatments [white light (control, W), purple light (P), blue light (B), red light (R), green light (G), yellow light (Y), red–blue light in a 9:1 ratio (9R/1B), and red–blue light in a 4:1 ratio (4R/1B)] on the growth, quality and nitrogen metabolism of lettuce were studied. The results showed that compared with the white light, the purple light, blue light, red light, and the red-blue light combination could all increase the biomass of the aboveground part of lettuce to various degrees, while green light and yellow light inhibited lettuce growth. Under blue light, the contents of soluble protein and flavonoid in lettuce were the highest; under red light, the soluble sugar content was the highest, while the contents of soluble protein, free amino acids, and vitamin C (VC) were relatively higher under the 4R/1B light condition. Compared with white light, the sources of purple, blue, and red lights as well as the red–blue light combination all significantly reduced nitrate accumulation in lettuce, and the activities of the nitrogen (N) metabolism-related enzymes such as nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, and glutamate dehydrogenase were increased to varying degrees. In contrast, the contents of nitrate and ammonium N were significantly accumulated in lettuce under green light, and the activities of relative enzymes were significantly reduced. Therefore, the purple light, blue light, and red–blue combination light sources could promote N assimilation and improve the aboveground biomass accumulation in lettuce by improving the activity of the N metabolism-related enzymes in lettuce. Particularly under the 4R/1B light source, the biomass, soluble protein, VC, and total amino acid content were rather high in lettuce, which indicated that the 4R/1B light source could better effectively improve the nutritional quality and promote the growth of lettuce, while yellow light and green light are not suitable to serve as direct sources in a plant factory. These results provide a certain theoretical basis for the regulation of the light environment in cultivation facilities.

Noxious or nutritious? Progress in controlling nitrate as a contaminant in leafy crop species

AbstractHigh levels of nitrate in forage are a danger to the health of ruminant livestock consuming leafy crop species. Nitrate is also classed by food safety authorities as a contaminant in fresh produce. Since limits on nitrate levels in fresh produce marketed within the European Union were set, research has aimed to reduce nitrate, and to limit its variation. Progress is reported which focuses on the agronomy of a range of leafy salad crop species including lettuce (Lactuca sativa), spinach (Spinacea oleracea) and wild rocket (Diplotaxis tenuifolia). Genetic studies aiming to understand the sources of genetic variation in nitrate accumulation in lettuce are reviewed and considered in the context of research aiming to improve the nitrogen use efficiency of crop plants. Recent evidence is reported which suggests that nitrate could have significant health benefits that may contribute to the positive effects of eating a diet rich in fresh fruit and vegetables. Therefore, with the exception of babies and very young children, the risks from excess nitrate consumption are much greater for farm animals than they are for humans. Nevertheless, producers still need to carefully control nitrogen supply to leafy salad crops, to ensure that the risks of diffuse pollution from overuse of nitrogen fertilizer are minimized.

REDUCING NITRATE CONTENT IN LETTUCE BY PRE-HARVEST CONTINUOUS LIGHT DELIVERED BY RED AND BLUE LIGHT-EMITTING DIODES

Effect of pre-harvest continuous light with different red/blue ratio on photon flux density (R/B ratio) on reducing nitrate accumulation was studied by growing lettuce (Lactuca sativa L.) under continuous illumination delivered by light-emitting diodes (LEDs). Results show that nitrate concentration decreased by 1648.0–2061.1 mg kg−1 in leaf blade and 962.9–2090.3 mg kg−1 in petiole, accompanied by a dramatic increase in soluble sugar content. Compared with monochromatic red light treatment, the decrease in nitrate concentration and increase in soluble sugar content in lettuce under mixed red and blue light were more pronounced. The lowest nitrate concentration was observed in the treatment with R/B ratio of 4. It's concluded that pre-harvest exposure to 48 h continuous LED light could effectively reduce nitrate accumulation in lettuce and this process is strongly affected by R/B ratio of light. This study may provide new perspective for pre-harvest quality management of vegetable, especially in commercial leaf vegetable production under artificial lighting.

ISO-OSMOTIC REGULATION OF NITRATE ACCUMULATION IN LETTUCE

Concerns about possible health hazards arising from human consumption of lettuce and other edible vegetable crops with high concentrations of nitrate have generated demands for a greater understanding of processes involved in its uptake and accumulation in order to devise more sustainable strategies for its control. This paper evaluates a proposed iso-osmotic mechanism for the regulation of nitrate accumulation in lettuce (Lactuca sativa L.) heads. This mechanism assumes that changes in the concentrations of nitrate and all other endogenous osmotica (including anions, cations and neutral solutes) are continually adjusted in tandem to minimize differences in osmotic potential of the shoot sap during growth, with these changes occurring independently of any variations in external water potential. The hypothesis was tested using data from six new experiments, each with a single unique treatment comprising a separate combination of light intensity, nitrogen (N) source (nitrate with or without ammonium) and nitrate concentration carried out hydroponically in a glasshouse using a butterhead lettuce variety. Repeat measurements of plant weights and estimates of all of the main soluble constituents (nitrate, potassium, calcium, magnesium, organic anions, chloride, phosphate, sulphate and soluble carbohydrates) in the shoot sap were made at intervals from about 2 weeks after transplanting until commercial maturity, and the data used to calculate changes in average osmotic potential in the shoot. Results showed that nitrate concentrations in the sap increased when average light levels were reduced by between 30 and 49% and (to a lesser extent) when nitrate was supplied at a supra-optimal concentration, and declined with partial replacement of nitrate by ammonium in the external nutrient supply. The associated changes in the proportions of other endogenous osmotica, in combination with the adjustment of shoot water content, maintained the total solute concentrations in shoot sap approximately constant and minimized differences in osmotic potential between treatments at each sampling date. There was, however, a gradual increase in osmotic potential (i.e., a decline in total solute concentration) over time largely caused by increases in shoot water content associated with the physiological and morphological development of the plants. Regression analysis using normalized data (to correct for these time trends) showed that the results were consistent with a 1:1 exchange between the concentrations of nitrate and the sum of all other endogenous osmotica throughout growth, providing evidence that an iso-osmotic mechanism (incorporating both concentration and volume regulation) was involved in controlling nitrate concentrations in the shoot.

Nitrate production and accumulation in lettuce as affected by mineral Nitrogen supply and organic fertilization

Lettuce is one of the leading vegetables in Brazil. However, studies concerning its fertilization for the Northeastern region of the country are still scarce. This work was carried out to evaluate the effects of mineral Nitrogen supply and organic fertilization on the production and nitrate accumulation in lettuce, cultivar Elba, in the semi-arid zone of Paraíba, Brazil. A randomized block design, with four replications, was used to test five doses of cattle manure (30; 60; 90; 120, and 150 t ha-1), five doses of N supplied as urea (30; 60; 90; 120, and 150 kg ha-1), and a control without fertilization. Along with mineral N, the other nutrients were also supplied using mineral sources. No additional nutrients were supplied where organic fertilization was used. Lettuce yield was significantly higher when organic fertilization was used, compared to the mineral nutrient supply. Taking into account most of the fertilization doses recommended for growing lettuce in the Brazilian Northeastern States, N mineral fertilization in sandy soils with low organic matter contents can be fully replaced by organic fertilization with cattle manure. Plant total (PTY) and commercial yield (PCY) as affected by the mineral N supply fitted into a quadratic model, in which the doses 94.5 and 95.7 kg ha-1 corresponded to respectively the highest PTY (381.9 g plant-1) and PCY (321.6 g plant-1). Where organic fertilization was used, PTY and PCY fitted into a linear model. Therefore, the highest PTY (523.9 g plant-1) and PCY (449.48 g plant-1) were observed at the highest manure doses. Nitrate content on roots, stems, and leaves increased linearly with Nitrogen and manure application. In roots, the nitrate content (mg kg-1 of fresh matter) reached 93.96 (N mineral (Nm)) and 65.09 (organic fertilization (OF)); in stems, 183.45 (Nm) and 98.22 (OF); and in leaves, 121.98 (Nm) and 75.62 (OF). The maximum nitrate content observed in leaves was far below the risk level for human health.

Effect of organic and inorganic fertilizers applied during successive crop seasons on growth and nitrate accumulation in lettuce

Abstract A romaine-type lettuce ( Lactuca sativa L.) cv. Corsica was cultivated during three successive crop seasons (late-spring, late-autumn and late-winter) in the same soil of an experimental greenhouse in S.W. Peloponnese, Greece. Seven long-term fertilization treatments were tested for their effect on plant growth and nitrate concentration in the external lettuce leaves. Treatments included: three different doses of organic fertilization (composted sheep manure) applied at the start of each crop season, three different doses of inorganic N fertilization applied via fertigation during each crop season, and a control treatment in which no fertilizer was applied. A drip irrigation system was used to water all plants. The highest nitrate levels were observed in the medium and maximum inorganic fertilization treatments (572–664 mg kg −1 ) in all crop seasons. They were significantly higher compared to the respective organic fertilization treatments (253–435 mg kg −1 ) and all other fertilization treatments (148–435 mg kg −1 ). Crop season affected lettuce growth more than nitrate accumulation in the lettuce leaves: lettuce biomass production was the smallest and most uniform in the late-autumn season and did not respond to the fertilization treatments tested (ranging from 409 to 439 g plant −1 ), while in the late-spring season biomass production was the highest and most variable (561–841 g plant −1 ), it correlated with nitrate concentration in the leaves and in the medium and maximum inorganic fertilizer doses it significantly exceeded production from all other fertilization treatments (827–841 g plant −1 ). Following the three crop seasons the residual availability of N, P and K was clearly enhanced in the soil receiving the organic compared to the inorganic fertilization. Nitrate concentration in lettuce leaves was far below the upper limits set by the European Commission in all fertilization treatments throughout the three crop seasons, a result attributed mainly to the sufficient level of light intensity and duration throughout the year in Southern Greece.

Low Water Potential in Saline Soils Enhances Nitrate Accumulation of Lettuce

The relationship between nitrate accumulation of lettuce and soil salinity in plastic film house was investigated, and the effect of water stress on the nitrate accumulation in lettuce was further examined in nutrient solutions of different water potentials. Nitrate contents in fresh lettuce were in the range of 85–2510 mg kg−1, and the content increased as soil electrical conductivity increased up to 5 dS m−1. Nitrate content in lettuce increased significantly as water potential in the nutrient solution decreased under low light intensity and an ample supply of nitrate. Soluble sugar and oxalate contents also increased with water stress. However, the changes of nitrate contents in response to the water potential changes were more significant than those of organic compounds. These results confirm the osmotic regulatory function of nitrate in replacement of organic compounds in lettuce grown in plastic film house where light intensity and soil water potential are low.

The Effect of Light-dependent Application of Nitrate on the Growth of Aeroponically Grown Lettuce (Lactuca sativa L.)

With a high nitrate supply, and most frequently under low-light conditions, lettuce accumulates relatively large amounts of NO3-as a result of an excess of uptake over reduction. Different approaches, which are used to reduce leaf nitrate, often result in a yield loss. A computerized aeroponic system, which supplies different nitrate concentrations in accordance with the changeable light conditions (dynamic light-dependent application of nitrate), was used to reduce nitrate accumulation in lettuce (Lactuca sativa L.) var. Capitata cv. Vanity. Under unfavorable light conditions nitrate was supplied at limited rates (slight, medium, and strong reduction) to the plants. In response to given light conditions the nitrate supply was reduced close to one-half or one-fourth of the full nutrient solution (8 mmol·L-1 NO3-). Controlled nutrition resulted in efficient reduction in leaf nitrate. In the early-spring experiment the average nitrate content in outer leaves was decreased by 9%, 63%, and 92% and in the late-spring experiment the decrease was 23%, 58%, and 76% compared to control. At the same time, the controlled, light-dependent nitrate deprivation did not result in a loss of a lettuce yield (except in the treatment with strong nitrate reduction) and had limited effects on photosynthesis (PN-Ci measurements) and photosynthetic pigments.

Effect of nitrogen nutrition on growth and nitrate accumulation in lettuce (Lactuca sativaL.), under various conditions of radiation and temperature

Pour concevoir des itineraires techniques permettant d'obtenir une faible teneur en nitrate des laitues, il est necessaire de mieux comprendre les effets des conditions de culture sur la croissance et l'accumulation de nitrate dans la plante. L'objectif de cette etude est d'analyser l'effet de la nutrition azotee sur l'efficience d'interception et de conversion du rayonnement en matiere seche, et sur l'accumulation de nitrate et d'eau dans la laitue, pour differentes conditions de temperature et de rayonnement. La croissance des laitues et les concentrations en eau et en nitrate ont ete mesurees sur deux cultures hors-sol (en ete et en automne) avec deux niveaux de rayonnement incident et de nutrition azotee. L'efficience de conversion de l'energie lumineuse en matiere seche (RUE) etait comprise entre 2.12 et 3.5 g.MJ -1 . Elle etait plus elevee pour les faibles rayonnements et les forts niveaux de nutrition azotee. Une correlation positive entre les teneurs en nitrate et en eau a ete observee. Cette relation, dont la pente n'est pas affectee par les conditions de croissance, semblerait indiquer une forte interdependance des accumulations d'eau et de nitrate chez la laitue.

Nitrate accumulation reduction using chloride in the nutrient solution on lettuce growing by NFT in semiarid climate conditions

Nitrate accumulation in vegetable represents a serious threat to man's health. Different methods have been reported to reduce the nitrate accumulation in lettuce and other leafy vegetables with or without variation in the yield. The aim of this experiment was to show the different behavior of some cultivars of lettuce using the replacement of N‐NO3 for chloride in the nutrient solution during last week before harvest. The trial was carried out with three lettuce cultivars: Domino, Elvira, and Daguan and took place in southern Spain (Almería) under greenhouse by NFT. The results indicated a correct method to decrease nitrate content in leafy vegetables and perhaps sulphate could be an anion to study their effect in this way.

Effects of urease and nitrification inhibitors application on urea fate in soil and nitrate accumulation in lettuce

The effects of amendment of urea with the urease inhibitor N‐(n‐butyl)‐thiophosphoric triamide (NBPT) and the nitrification inhibitor dicyandiamide (DCD) on urea trasformation in soil and nitrate accumulation in winter lettuce were studied in a cold greenhouse experiment. The DCD inhibited remarkably the nitrification process for more then two months, maintaining significantly higher levels of ammonium in soil solution compared to normal urea. The DCD did not decrease the ammonia volatilization but determined a significant reduction of plant nitrate concentration at final harvest. The treatment urea + NBPT showed a behavior slightly different from normal urea with a higher level of soil nitrate three weeks after nitrogen (N) fertilization. No effects on NO3 plant accumulation were observed, but losses of NH3 were significantly reduced.

Effects of low light intensities at night on nitrate accumulation in lettuce grown on a recirculating nutrient solution

During winter, when light intensities are low, high nitrate concentrations often occur in leafy vegetables which are undesirable from the viewpoint of public health. The effects of low lighting during the night in combination with various nutrient solutions and solution temperatures on the nitrate concentration of lettuce cv. Panvit plants grown in a nutrient film technique system were studied. The experiment showed that lettuce heads could be grown during winter under glass with a nitrate concentration as low as 2900 mg/kg FW, which is much lower than the maximum permissible concentration for winter-grown lettuce in the Netherlands of 4500 mg/kg. The lowest nitrate concentration, without negative effects on yield or quality, was obtained by growing the crop on recirculating nutrient solutions with an initial ammonium:nitrate ratio of 0.25, with a change to a ratio of 1.0 for the last 2 weeks. The addition of chloride did not affect the nitrate concentration. Raising the temperature of the nutrient solution at night from 6 to 10 degrees C in combination with a minimum air temperature of 6 degrees stimulated growth, but also increased the lettuce nitrate concentration by an average of 360 mg/kg. Supplying light of a low intensity at a photon flux density of 27 micro mol m-2s-1 (400-700 nm) at night for up to 8 nights before harvest did not further reduce the nitrate concentration on a FW basis because the dry matter percentage increased.