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Boron Toxicity Alters Nitrate Reductase Activity, ProlineAccumulation, Membrane Permeability, and MineralConstituents of Tomato and Pepper Plants

To cite this Article: Eraslan, Figen, Inal, Ali, Gunes, Aydın and Alpaslan, Mehmet etal., 'Boron Toxicity Alters Nitrate Reductase Activity, Proline Accumulation,Membrane Permeability, and Mineral Constituents of Tomato and Pepper Plants',Journal of Plant Nutrition, 30:6, 981 - 994To link to this article: DOI: 10.1080/15226510701373221URL: http://dx.doi.org/10.1080/15226510701373221

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Journal of Plant Nutrition, 30: 981–994, 2007

Copyright © Taylor & Francis Group, LLC

ISSN: 0190-4167 print / 1532-4087 online

DOI: 10.1080/15226510701373221

Boron Toxicity Alters Nitrate Reductase Activity,Proline Accumulation, Membrane Permeability, andMineral Constituents of Tomato and Pepper Plants

Figen Eraslan, Ali Inal, Aydın Gunes, and Mehmet Alpaslan

Ankara University, Faculty of Agriculture, Soil Science and Plant NutritionDepartment, Ankara, Turkey

ABSTRACT

Boron (B) toxicity is an important disorder that can limit plant growth on soils of aridand semi arid environments throughout the world. Although of considerable agronomicimportance, our understanding of B toxicity is rather fragmented and limited. The effectsof increasing levels of B (0, 0.5, 5, 50 mg kg−1) on plant growth, proline accumulation,membrane permeability, nitrate reductase activity (NRA), and mineral nutrient interac-tions of tomato and pepper plants were investigated in greenhouse conditions. Increasinglevels of B increased the B contents of plants. Boron toxicity symptoms occurred at 5and 50 mg kg−1 levels. Fresh and dry weights of the plants clearly decreased with theapplication of the 50 mg kg−1 level of B. Membrane permeability and proline accu-mulation were significantly increased by the 50 mg kg−1 level of B. Nitrate reductaseactivity of tomato plants was increased with increasing levels of B. With the exception ofpotassium (K) and calcium (Ca) in pepper and magnesium (Mg) in tomato, B treatmentssignificantly affected nutrient concentrations of tomato and pepper. Except for sulfur(S) and Ca in tomato, the highest rate of B applied increased the N, phosphorus (P), andK concentrations of tomato and N, P, Mg, and S concentrations of pepper.

Keywords: boron toxicity, membrane permeability, NRA, proline

INTRODUCTION

Although boron (B) is an essential plant micronutrient, it is also phytotoxic ifpresent in excess amounts in growth medium. Reisenauer et al. (1973) noted

Received 15 December 2005; accepted 26 July 2006.Address correspondence to Ali Inal, Ankara University, Faculty of Agricul-

ture, Soil Science and Plant Nutrition Department, 06110 Ankara, Turkey. E-mail:inal@agri.ankara.edu.tr

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that the range between deficient and toxic levels of B is narrow for most plants,and that the ratio of adequate to toxic concentration is narrower than for anyother nutrient element.

While of lesser prevalence than B deficient soils, B-rich soils are of greatsignificance because of their association with B toxicity and decreased plantgrowth and crop yields in different regions, especially arid and semi-arid re-gions of the world. The highest naturally occurring concentrations of B havebeen found in soils derived from marine evaporates and marine argillaceoussediments. In addition, various anthropogenic sources of B excesses may in-crease soil B to toxic levels for plants. The most important source is irrigationwaters, but other sources such as wastes from surface mining, flying ash, andindustrial chemicals may also play a role (Nable et al., 1997). Because of theuse of borates and perborates in detergents, B is usually present in sewagesludges and effluents. Hence, the uncontrolled application of municipal com-posts, sewage sludges, and effluents on agricultural land has in some casesresulted in B toxicity (Neary et al., 1975). Finally, the poor drainage of salinesoils may be responsible for the excessive accumulation of B in the soil solution(Goldberg, 1997; Grieve and Poss, 2000).

Increasing evidence has been reported indicating the role of B in membraneintegrity and membrane functions such as formation and maintenance of mem-brane potentials and membrane permeability (Blevins and Lukaszewski, 1998;Wang et al., 1999). Under B deficient conditions, increased level of membranepermeability has been reported in sunflower (El-Shintinawy, 1999) and soybean(Liu and Yang, 2000). Furthermore, B toxicity also resulted in increases in mem-brane permeability, and possible role of membrane integrity and structure intolerance mechanism of B toxicity reported by Karabal et al. (2003).

The induction of proline accumulation in response to many other stressescommonly encountered in natural environments (e.g., heavy metal toxicity, nu-trient deficiency, photooxidative stress) suggests that genetic engineering ofproline overproduction in agriculturally-important crops might increase theiroverall environmental tolerance and thereby enhance productivity. Clearly, abetter appreciation of the extent to which proline accumulation is of protec-tive value to plants exposed to several different environmental extremes and anunderstanding of the mechanism(s) whereby proline accumulation may exerta beneficial effect under stressful conditions, are likely to be critical prerequi-sites for the success of such endeavors (Hare and Cress, 1997). Furthermore,the effects of various stresses on leaf proline have been reported (Roy andBera, 2002), but no data exist to our knowledge about the effects of high Bconcentrations on proline.

Nitrate reductase (NR) located in the cytoplasm converts nitrate to ni-trite; nitrite reductase localized in the plastids reduces the nitrite to ammonia(Yu et al., 1998). Thus ammonia can be delivered either from nitrate reduc-tion, uptake of ammonia, or photorespiration. Gene expression and enzymeactivity of the various proteins active in nitrogen assimilation is responsive to

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Boron Toxicity Effects on Tomato and Pepper 983

internal and environmental factors. There is a substantial volume of evidenceto show that NR activity is subjected to inhibition following the application ofa range of environmental stresses (Warner et al., 1977; Kleinhofs and Warner,1990).

Levels of B above the optimum range cause significant changes in the ac-tivity of numerous enzymes and consequently, the metabolism of higher plants(Shkolnik, 1974). Key enzymes of nitrogen (N) assimilation in plants (i.e., ni-trate reductase and glutamate dehydrogenase) were shown significantly affectedby environmental stresses such as heavy metals (e.g., cadmium), cold and salt(Atkin and Cummins, 1994; Lutts et al., 1999). However, to our knowledge,no study has been conducted about the effect of boron toxicity on activities ofnitrogen assimilating enzymes in pepper and tomato.

Among a wide variety of plant species, the typical visible symptom ofB toxicity is leaf burn-chlorotic and/or necrotic patches, often at the mar-gins and tips of older leaves. These symptoms reflect the distribution of Bin most species, with B accumulating at the end of the transpiration stream.The chlorotic/necrotic patches have greatly elevated B concentrations com-pared with the surrounding leaf tissues and some species (e.g., barley) showcharacteristic patterns for different genotypes (Nable et al., 1997).

The objectives of this investigation were to determine the effects of borontoxicity on proline accumulation, membrane permeability, nitrate reductaseactivity (NRA) and mineral nutrient interactions in tomato and pepper plantsgrown under greenhouse conditions.

MATERIALS AND METHODS

Growth Conditions and Experimental Design

Tomato (Lycopersicon esculentum L.) and pepper (Capsicum annuum L.) plantswere grown in a glasshouse under natural light conditions. Seeds were germi-nated in seedling trays filled with 1:1 v/v mixture of peat and perlite. Three-week-old seedlings were transplanted at a rate of one plant per pot filledwith 2000 g of air-dried soil. Some characteristics of the soil were as fol-lows: texture loam, calcium carbonate (CaCO3) 5.48%, pH (1:2.5 water) 7.72,EC 0.2 dS m−1, organic matter 0.76%, total N 0.21%. The concentration ofammonium acetate (NH4Oac)- extractable potassium (K) was 389 mg kg−1,sodium bicarbonate (NaHCO3)-available phosphorus (P) was 14.50 mg kg−1,and NaOAc-extractable B was 1.80 mg kg−1.

Before seedling transplantation, boron was applied at the rates of 0, 0.5,5, and 50 mg kg−1 from boric acid (H3BO3). A basal dose of N [Ca(NO3)2],K and P (KH2PO4) were applied to all the pots at the 140, 100, 125 mg kg−1

levels, respectively and mixed thoroughly with soil before transplanting theseedlings.

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Plant Measurements and Statistical Analysis

In vivo nitrate reductase activity (NRA, EC 1.6.6.1), proline accumulation, andmembrane permeability were assayed in fresh plant material sampled 60 dafter transplanting. Nitrate reductase activity was determined by the method ofKlepper et al. (1971) with some modifications. Leaf samples were collected andimmediately cooled in ice. Leaves were cut into small 3-4 mm pieces includingmid-rib, and 500 mg samples were placed in test tubes containing 10 mL ofbuffer solution, prepared by dissolving 0.2 M potassium nitrate (KNO3) in0.1 M mono potassium phosphate (KH2PO4) and adjusting the pH to 7.5. Theleaf pieces were vacuum infiltrated in vacuum desiccators. Thereafter sampleswere incubated in the dark at 25◦C for 1 h. By heating the test tubes in theboiling water bath for 1 min, the reaction was stopped. To estimate the amountof nitrite formed, 1 mL of each of 1% sulfanilamide in 1 N hydrochloric acid(HCl) and 0.02% naphtylethylene diamine dihydrochloride were added and thetest tubes vortexed. Absorbance of the resulting solution was recorded at 540nm with a Shimadzu UV-VIS 1201 spectrophotometer. Concentration of nitritewas calculated by drawing a calibration curve of nitrite. Enzyme activity wasexpressed as µmol of nitrite formed per gram fresh weight per hour.

For the proline assay, 500 mg leaf samples cut into small 3-4 mm pieces(excluding mid-rib) and cooled in ice, placed into tubes with 10 mL of distilledwater. The tubes were kept for 30 min in a boiling water bath to extract hot-water soluble compounds and then cooled to room temperature. Proline in thewater extracts was determined by the ninhydrin method (Troll and Lindsley,1955), improved to avoid interference with concentrated sugars (Magne andLarher, 1992). One mL ninhydrin reagent was added to an aliquot of extract.The mixture was boiled for 30 min in a water bath, and then cooled in an ice-water bath. The chromophore formed was extracted with 2.5 mL of tolueneby vigorous shaking for about 20 s. Absorbance of the resulting organic phasewas measured at 520 nm with a Shimadzu UV-VIS 1201 spectrophometer.Calibration curves were made with L-proline as a standard.

For the measurement membrane permeability the method of Yan et al.(1996) was followed. A portion of fresh material at the middle of the leaves fromthe plants was weighed into a glass beaker containing reverse osmosis water.The beaker was immersed at 30◦C for 3 h, and the conductivity of the solutionwas measured with a hand-held conductivity meter (Hanna Instruments Co.Ltd.). After boiling the samples for 2 min, their conductivity was then measuredagain when the solution was cooled to room temperature. The percentage ofelectrolyte leakage (EC) was calculated as Percent EC= (C1/C2) × 100, whereC1 and C2 are the electrolyte conductivities measured before and after boiling,respectively.

After the 63 d of growing period, all plants were harvested with a knife bycutting at the joint point of fine roots to the main stem. The rest of the plantswere weighed for the fresh weight determination. After washing, the plants

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dry blotted on paper towel and dried at 65◦C in an air-forced oven for 48 h.Then, the plants weighed again for the dry weights. For the measurement ofP, K, Ca, Mg, and B contents of the plants, the dried and finely ground (40mesh) plant tissue was dry-ashed in a muffle furnace at 500◦C for 5 h, dis-solved in 5 mL of 2 M nitric acid (HNO3), and finally diluted to 25 mL withreverse osmosis water. Extracts were filtered and stored in plastic vials until an-alyzed. In the extract solutions, K and P were determined by flame photometer(Jenway PFP7, ELE Instrumet Co. Ltd), and by the colorimetric phosphomolyb-date method of Murphy and Riley (1962) by using a Shimadzu UV-VIS 1201spectrophotometer, respectively. Calcium and Mg were determined by atomicabsorption spectrophotometer (Analytikjena, AAS Vario 6). Sulfur was deter-mined by using C-S determinator (Eltra CS 500). Boron in plant extracts wasanalyzed spectrophotometrically (Shimadzu UV-VIS spectrophotometer) usingthe azomethine-H reagent method (Wolf, 1974).

Kjeldahl N was determined by Kjeldahl digestion according to the methodof Bremner (1965). Dried plant samples (0.25 g) were wet-ashed on a blockdigester (Kjeldatherm, Gerhardt GmbH) with the aid of concentrated sulfuricacid (H2SO4), salicylic acid, and salt mixture. The wet ashed plant materialswere distilled on a distillation apparatus (Vapodest 20, Gerhardt GmbH). Thenthe distillates were titrated against 0.05 N H2SO4.

Treatments were replicated four times and arranged in a completely ran-domized design. Yield and chemical data were processed statistically by anal-ysis of variance (ANOVA) to determine if significant differences were presentamong means. The means of treatments were tested for statistically significantdifferences with Duncan’s Multiple Range Test (P = 0.05).

RESULTS

Boron Toxicity Symptoms

Boron toxicity symptom appeared at 5 and 50 mg kg−1 B treatments and wasmore severe in the high boron (50 mg kg−1) treatment in tomato and pepperplants. The first sign of boron toxicity symptoms was at the tips and leaf marginsof older leaves of the plants. In tomato plants, brown and necrotic lesionsappeared in leaflet tips, which curl inwards. In pepper plants, toxicity symptomsfirst appeared on the oldest leaves and spread to the younger leaves. The marginsbecame broader, developed necrotic blotches, turned faded brown and youngerleaves remained small.

Plant Growth and Boron Contents

Effects of increasing levels of boron on fresh and dry weights and boron con-centrations of tomato and pepper plant are shown in Table 1. Both in tomato

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Table 1The effects of increasing levels of boron on fresh and dry weights, and boron concen-trations of tomato and pepper plants

B treatment B Concentration(mg kg−1) Fresh Weights (g) Dry Weights (g) (mg kg−1)

Tomato0 112.23 a 14.87 a 121 c0.5 116.15 a 16.26 a 172 c5 98.09 a 13.40 a 433 b

50 40.18 b 4.85 b 2721 aF-test ∗∗ ∗∗ ∗∗

Pepper0 43.94 a 5.41 a 51 c0.5 51.41 a 6.77 a 51 c5 41.29 a 5.16 a 314 b

50 16.39 b 1.50 b 2483 aF-test ∗∗ ∗∗ ∗∗

∗∗P < 0.01, means followed by the same letter in the same column do not differsignificantly.

and pepper plants, boron toxicity stress caused to decrease plant fresh and dryweights creating significant differences compared with controls. The maxi-mum growth reduction in fresh and dry weights occurred at 50 mg kg−1 B andamounted to approximately 66% and 67% decreases, compared to control intomato and pepper plants, respectively.

Boron concentrations of tomato and pepper plants significantly increasedwith the increasing levels of boron (Table 1). The highest B concentrationswere recorded at 50 mg kg−1 B rates being 2721 mg kg−1 for tomato and2483 mg kg−1for pepper plants. Applied B at 5 and 50 mg kg−1 rates differedsignificantly the B concentration of both plants as compared to control and0.5 mg kg−1 rate.

Proline Accumulation, Nitrate Reductase Activity (NRA),and Membrane Permeability

Table 2 presents the effects of increasing levels of boron on proline accumu-lation, NR activity contents and membrane permeability of tomato and pepperplants. Proline accumulation increased significantly (P < 0.01) by the highestlevel of B both in tomato and pepper plants. The accumulation of proline intomato plant (7.56 µmol g−1 fw) was found higher than that of pepper plant(2.37 µmol g−1 fw) at 50 mg kg−1 B rate. Increasing levels of B treatments in-creased NRA of the tomato plants. The highest NRA was reached by 50 mg kg−1

level of B (P < 0.01). Small, but not significant, increases in NRA of the pepper

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Table 2The effects of increasing levels of boron on proline accumulation, NRA and membranepermeability of tomato and pepper plants

B treatment Proline NRA Membrane(mg kg−1) (µmol g−1fw) (µmol g−1fw h−1) permeability (EC, %)

Tomato0 0.49 b 0.187 b 43.55 b0.5 0.48 b 0.193 b 39.05 b5 0.72 b 0.240 b 47.68 b

50 7.56 a 1.039 a 62.58 aF-test ∗∗ ∗∗ ∗∗

Pepper0 1.21 b 0.118 31.67 b0.5 0.99 b 0.126 28.63 b5 1.02 b 0.131 30.60 b

50 2.37 a 0.138 50.68 aF-test ∗∗ ns ∗∗

ns: non-significant, ∗∗:P < 0.01, means followed by the same letter in the same columndo not differ significantly.

plants were also seen with the increasing levels of applied B. Membrane per-meability of both tomato and pepper plants were increased significantly by thehighest level of B (P < 0.01). Tomato plants gave higher membrane permeabil-ity than that of the pepper plants at each increment of applied B. As comparedto control, small but not significant decreases in membrane permeability werealso seen with the addition of B for tomato at 0.5 mg kg−1 and for pepper 0.5and 5 mg kg−1 levels. In other words, in the control (0 mg kg−1 B) treatmentsmembrane permeability showed higher values than 0.5 and 5 mg kg−1

Mineral Constituents of Plants

Effects of increasing levels of B on N, P, and K concentrations of tomato andpepper plants were given in Table 3. Application of 5 mg kg−1 B decreasedN concentration of the pepper plants significantly as compared to the controltreatment, while the toxic level of B (50 mg kg−1 B) was increased significantly.In spite of this, increasing levels of B increased N concentrations of tomatoplants as compared to the control treatment. But, B applied at the 5 mg kg−1

level did not change the B concentration of tomato as compared to the control.Phosphorus concentrations of both tomato and pepper plants were increasedsignificantly by toxic level of B. Potassium concentration of tomato plant wasdecreased by 5 mg kg−1 B treatment as compared to 0.5 mg kg−1 B, while toxiclevel of B increased significantly. Any significant changes were occurred in Kconcentrations of pepper plants by B treatments.

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Table 3The effects of increasing levels of boron on N, P and K concentrations of tomato andpepper plants

B treatment (mg kg−1) N (g kg−1) P (g kg−1) K (g kg−1)

Tomato0 41.52 c 3.63 b 24.13 bc0.5 45.90 a 4.00 b 25.53 b5 42.32 bc 3.25 b 19.48 c

50 44.92 ab 8.30 a 36.85 aF-test ∗ ∗∗ ∗∗

Pepper0 28.87 b 2.98 b 60.730.5 29.85 b 3.10 b 59.005 26.92 c 3.35 ab 58.78

50 37.85 a 3.98 a 63.53F-test ∗∗ ∗ ns

ns: non-significant, ∗: P < 0.05, ∗∗: P < 0.01, means followed by the same letter inthe same column do not differ significantly.

Calcium concentrations of tomato plant were decreased by 50 mg kg−1 Btreatment as the compared to the control and 5 mg kg−1 B treatments (Table 4).Boron treatments did not affect Ca concentrations of pepper plant. Magnesium

Table 4The effects of increasing levels of boron on Ca, Mg and S concentrations of tomato andpepper plants

B treatment (mg kg−1) Ca (g kg−1) Mg (g kg−1) S (g kg−1)

Tomato0 38.25 a 14.40 18.20 ab0.5 35.25 ab 18.15 22.15 a5 39.25 a 15.60 19.58 ab

50 30.25 b 18.00 13.58 bF-test ∗ ns ∗

Pepper0 19.50 13.65 a 3.25 b0.5 20.00 13.80 a 3.00 b5 24.00 12.60 a 3.43 b

50 20.75 9.80 b 5.05 aF-test ns ∗∗ ∗∗

ns: non-significant, ∗:P < 0.05, ∗∗:P < 0.01, means followed by the same letter in thesame column do not differ significantly.

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concentrations of tomato plant in our experiment did not significantly varywith the different rates of B applications (Table 4). However, Mg concentrationof pepper plants were decreased significantly (P < 0.01) by the toxic levelof B as compared to control treatment. Increasing levels of B treatmentswere significantly affected sulfur concentrations of tomato plant and 0.5 mgkg−1 B gave higher plant S concentration than 50 mg kg−1 B applied. TheS concentrations of pepper plants were increased significantly (P < 0.01) bytoxic level of B application.

DISCUSSION

In the present study, boron toxicity was seen at 5 and 50 mg B kg−1level of ap-plied B in the two tested plants, and the toxic effect of B was more pronouncedat 50 mg B kg−1level. The leaf injury symptom accrued in this study agrees withthose previously reported by Bergman (1992) for tomato and pepper, Alpaslanand Gunes (2001) and Gunes et al. (1999) for tomato plants. In this experiment,the dry and fresh weights of the tomato and pepper plants were notably reducedas a result of B toxicity. Increasing levels of B treatments increased the con-centration of B in the two tested plants. Fresh and dry weight decreases and Bconcentration increases are in agreement with the results of many researchersinvestigating the response of several plant species to B (Kastori et al., 1995;Bellaloui and Brown, 1998; Gunes and Alpaslan, 2000; Asad, 2002; Goldberget al., 2003; Molassiotis et al., 2006).

The effects of various stresses on leaf proline accumulations have beenreported (Aziz et al., 1999; Tarakcioglu and Inal, 2002; Roy and Bera, 2002),but no data exist to our knowledge about the effects of high B concentrationson proline accumulation in plants. Previous studies on the effect of excess Bon proline content have reported mixed results. As for example, Karabal et al.(2003), have reported that B toxicity (10 mM) did not cause significant changesin proline contents in the leaves and roots of hydroponically grown barleyseedlings, while Papadakis et al. (2004) found decreased proline accumulationin the leaves of Clementine mandarin plants grafted on ‘Sour orange’ but not inthose grafted on ‘Swingle citrumelo’. In the present study, proline accumulationin the both tomato and pepper plants significantly increased by B toxicity.

In this experiment, in vivo NRA measurements of tomato and pepper plantsshowed increase in the activity of this enzyme upon increasing B treatments. Inthe literature, there is some disagreement on the effects of B toxicity on NRA.According to Mahboobi et al. (2002), the activity of nitrate reductase tendedto decrease (15–17%) following B toxicity in root and leaf tissues of barleyand wheat cultivars compared to control. Furthermore, Camacho-Cristobal andGonzalez-Fontes (1999) reported that B deficiency was shown to reduce NRAin the leaves of tobacco plants. Boron is an essential element for higher plantsnot only due to its role in the maintenance of cell wall structure, but also because

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its deficiency causes a drastic decrease in leaf nitrate content, which leads toa decline in NRA. Kastori and Petrovic (1989) also reported that toxic B ratescaused NO3 accumulation in young sunflower plants.

Boron is known to play important roles in structure of cell-walls, mem-branes and membrane-associated functions in plants (Brown et al., 2002; Powerand Woods, 1997). Under boron deficient conditions an increased levels of mem-brane permeability have been reported in sunflower (Cakmak et al., 1995) andsoybean (Liu and Yang, 2000). In the present study, control treatments also gavehigher membrane permeability than lower rates of B. In this experiment, borontoxicity also resulted in increases in membrane permeability of leaves of bothof tomato and pepper plants. This is in agreement with the results of Karabal etal. (2003), who reported that membrane permeability was increased by borontoxicity in barley. Alpaslan and Gunes (2001) have concluded that membranepermeability of tomato and cucumber plants was significantly increased in thepresence of salinity by the increasing levels of applied B. Ismail (2003; 2004)also reported similar results for maize and sorghum plants.

In this study, increasing levels of applied B were significantly increased Nconcentrations of pepper and tomato plants, except 5 mg B kg−1 level in tomatoplant. Inal and Tarakcioglu (2001) also reported similar results. Lopez-Lefebreet al. (2002) have reported positive effect of B on N uptake and metabolism.According to Gupta et al. (1976), B applications had no significant effects onplant Kjeldahl N content. Sotiropoulos et al. (2002) reported that excess B hadno significant effect on the N concentration of the kiwifruit. In our experiment,P concentration were increased significantly by toxic level of B in both tomatoand pepper plants. Lopez-Lefebre et al. (2002) reported that B applicationencouraged P uptake in tobacco. Potassium concentration was decreased by5 mg B kg−1 whereas toxic level of B increased significantly K concentrationin tomato plant. Boron treatments did not affect K concentration of pepperplant. Alpaslan and Gunes (2001) reported that K concentration of tomato wasnot effected by B toxicity, and Wimmer et al. (2003) declared similar findingsof K concentration for wheat leaves. Sotiropoulos et al. (2002) concluded thatexcess B had no significant effect on the concentration of P and K in kiwifruit.

The data indicated that any significant changes occurred in Ca concentra-tion of pepper plants by B treatments. The highest level of applied B decreasedCa concentration as compared to the control and 5 mg kg −1 B treatment intomato plant. Magnesium concentration was not affected by B treatments intomato plant whereas in pepper plants concentration of Mg was decreased bythe toxic level of B. Sotiropoulos et al. (2002) reported that excess B in nutrientsolution decreased Ca concentration but had no significant effect on the con-centration of Mg in kiwifruit. Lopez-Lefebre et al. (2002) stated that increasingrates of B application diminished Mg uptake in tobacco plant. Any significantchanges was occurred control, 5 and 50 mg B kg−1 treatments, but 50 mg Bkg−1 treatment significantly reduced S concentration with respect to 0.5 mgB kg−1 treatment in tomato plants. The S concentration of pepper plant was

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increased significantly by toxic level of B application. But no data exist to ourknowledge about the effects of B on concentrations of this nutrient element.

As a conclusion, boron toxicity caused significant increases in membranepermeability and proline accumulation in both tomato and pepper plants. NRactivity was increased by increasing levels of B and significantly increasedby toxic level of B only in tomato plant. Except K and Ca in pepper and Mgin tomato, boron treatments significantly affected nutrients content of tomatoand pepper plants. And plants exhibited symptoms of toxicity to boron. Thoseeffects of B might be attributed to the effect of B on cell wall metabolism andmembrane integrity. The different effects of B on some parameters measuredin the present study were related to the response of cultivars to comparativelyhigher B supply. Our results was be helpful in preventing yield reduction andmineral imbalances in vegetables caused by excess B in rooting medium byimplying different strategies for managing B nutrition. However, the relationbetween excess B and plant physiological and mineral metabolism requiresfurther research.

REFERENCES

Alpaslan, M., and A. Gunes. 2001. Interactive effects of boron and salinitystress on the growth membrane permeability and mineral composition oftomato and cucumber plants. Plant and Soil 236:123–128.

Asad, A. 2002. Boron requirements for sunflower and wheat. Journal of PlantNutrition 25(4):885–899.

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