Phaseolus vulgaris L.

Journal of Horticultural Science & Biotechnology (2014) 89 (2) 185–192

Potassium fertiliser enhances the salt-tolerance of common bean (Phaseolus vulgaris L.) 2 By MONA G. DAWOOD1, MAGDI T. ABDELHAMID1* and URS SCHMIDHALTER 1 Botany Department, National Research Centre, 33 Al Behoos Street, 12622 Dokki, Cairo, Egypt 2 Department of Plant Sciences, Institute of Plant Nutrition, Technische Universität München, Emil-Ramann-Str. 2, D-85350 Freising-Weihenstephan, Germany (e-mail: [email protected]) (Accepted 12 October 2013)

SUMMARY Sodium chloride (NaCl) is the most abundant salt that contributes to soil salinity. The response of plants to excess NaCl is complex, involving changes in their morphology, physiology, and metabolism. Potassium (K) is not only an essential macronutrient for plant growth and productivity, but it is also a primary osmoticum for maintaining the low water potential of plant tissues. A pot experiment was conducted in the wire-house of the National Research Centre, Cairo, Egypt, during the 2010 – 2011 season, to examine the potential role of K fertiliser in alleviating the deleterious effects of NaCl-salinity on some physiological and biochemical traits of two recombinant inbred lines (RILs) of common bean (Phaseolus vulgaris L.; RIL 147 and RIL 115). The results showed that salinity levels of 25 mM (S1) and 50 mM NaCl (S2) caused significant decreases in the numbers of pods per plant, the fresh weight (FW) and dry weight (DW) of pods per plant, shoot DW per plant, as well as in the level of photosynthetic pigments, compared to plants irrigated with tap water (S0). A dose of 150 mg K2O kg–1 soil (K2) mitigated these harmful effects of salinity on common bean yield and on the content of photosynthetic pigments. Both salinity levels (S1 and S2) and treatment K2 caused significant increases in proline, free amino acid, and soluble carbohydrate concentrations, as well as peroxidase and polyphenol oxidase activities, relative to the corresponding control plants. In contrast, both RILs show a decrease in their phenolic compound concentrations due to salinity and/or the application of K2 compared to control plants (i.e., treatment S0K1; where K1 = 25 mg K2O kg–1 soil). The K+:Na+ ion ratio decreased significantly as the salinity level increased, and increased significantly under treatment K2. We conclude that treatment K2 mitigated the adverse effects of salinity (NaCl) through the effect of K+ ions enhancing the levels of photosynthetic pigments, anti-oxidant enzyme activities, osmoprotectant concentrations, and the K+:Na ion ratio, all of which were reflected in an improvement in plant performance.

C

ommon bean (Phaseolus vulgaris L.) is the most important grain legume cultivated for human consumption, with 23 million ha grown worldwide (Broughton et al., 2003). Approx. 12 million metric tons (MMT) are produced annually, of which approx. 8 MMT are from Latin America and Africa (FAO, 2005-2006). Soil salinity has become increasingly important, scientifically and politically. Over 6% of the total land area and 20% of the irrigated land in the World is adversely affected by salinity (FAO, 2008). Salinity problems are particularly relevant in arid and semi-arid areas such as Egypt. Approx. 33% of the cultivated land, and most extensions of agricultural land in Egypt are already salinised (Ghassemi et al., 1995). Reductions in the yields of different crops due to salinity in most of these areas are approx. 60%, when compared with normal, non-saline soil. Exploiting and increasing crop production in these areas is necessary to bridge the gap between the production and consumption of many food and feed crops such as common bean. Common bean is extremely sensitive to soil salinity and suffers yield losses, even when grown at soil electrical conductivity (EC) values ≤ 2 dS m–1, due to a limited uptake of water (Mass and Hoffman, 1977). Changes in the physiological and biochemical traits of *Author for correspondence.

plants due to salinity depend mainly on the effects of ions and solutes in the root zone on the activity of water in root cells, leading to reduced plant cell turgor, limited photosynthesis, and ion deficiencies due to inadequate transport mechanisms (Hasegawa et al., 1986). Plants that are naturally exposed to salt stress can adapt their metabolism in order to cope with the changed environment. The responses of plants to high soil salinity also vary at different stages of growth. Survival under such stressful conditions depends on the ability of the plant to perceive the stimulus, generate and transmit signals, and initiate biochemical changes that adjust its metabolism accordingly (Hasegawa et al., 2000). Salt stress, like many abiotic stress factors, induces oxidative damage in plant cells, catalysed by reactive oxygen species (ROS) such as the superoxide radical (O2•–), the hydroxyl radical (•OH) and hydrogen peroxide (H2O2). ROS cause damage to cell membranes and other essential macromolecules such as photosynthetic pigments, proteins, DNA, and lipids (Azevedo-Neto et al., 2006). ROS need to be scavenged in order to maintain normal plant growth. Plant cells therefore produce anti-oxidant enzymes such as superoxide dismutase (SOD), peroxidase (POX), and catalase (CAT), as well as several non-enzymatic antioxidant molecules such as ascorbate, glutathione, and tocopherol to protect themselves against oxidative stress.

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Potassium alleviates salt stress in common bean

Anti-oxidant enzyme activities and anti-oxidant molecule concentrations are frequently used as indicators of oxidative stress in plants (Mittler, 2002). Moreover, salt-stressed plants accumulate various organic molecules such as proline, glucose, or glycinebetaine for osmoregulation and to protect their enzyme activities (Munns and Termaat, 1986). Potassium (K+) ions play vital roles in most of the biochemical and physiological processes necessary for plant growth such as cell wall development, carbon assimilation, photosynthesis, and the synthesis and translocation of organic and inorganic nutrients from the soil to the plant (Thangavelu and Rao, 1997). K+ ions also maintain cell turgor, activate enzymes, and reduce excessive uptake of toxic ions such as Na+ from saline soils (Mengel and Kirkby, 2001). Several researchers have demonstrated the beneficial influence of K+ ions on plant tolerance to saline stress, and their roles in the efficiency of water use in plants (Abdelhamid et al., 2011). It is well-established that a plant which is adequately supplied with K+ ions can use soil moisture more efficiently than a K-deficient plant. The application of K+ mitigates the adverse effects of salinity through its roles in stomatal regulation, osmoregulation, maintaining the membrane ion charge balance, cellular energy status, and protein synthesis (Sanjakkara et al., 2001). Furthermore, the addition of K+ ions to cultivated saline soils avoided Na+ ion toxicity in rice plants by maintaining a high level of K+ uptake (ElKholy et al., 2003). As a consequence of the active uptake and accumulation of K+ ions in cells, the osmotic potential is decreased, then water enters and increases the turgor pressure of the cells which is responsible for growth. Potassium ions contribute more than Na+, Cl–, and glycinebetaine to osmotic adjustments under saline conditions (Ashraf and Sarwar, 2002). The addition of K+ ions to a saline medium significantly increased the K+:Na+ ratio in leaves, stems, and roots due to an increased uptake of K+ and direct competition between K+ and Na+ ions at the site of uptake in the plasmalemma (Epstein, 1966). However, some differences in the accumulation of Na+ ions existed between species and genotypes, and in the discrimination in favour of K+ transport to shoots (Gorham, 1990). Aslam et al. (1998) compared different methods of application of K+ ions to rice plants in saltaffected soils using a salt-tolerant and a salt-sensitive cultivar. They reported that tillering capacity, paddy and straw yields, and 1,000-grain weights increased following the application of K+ ions. Hence, an adequate supply of K+ ions to plants growing under saline or drought stress is believed to have an important role in inducing tolerance and plant survival (Chow et al., 1990; Kamel et al., 2010; Abdelhamid et al., 2011; Muhammad, 2013). This study aimed to measure the potential roles of K+ ions in alleviating the deleterious effects of salinity on some physiological and biochemical traits of two recombinant inbred lines (RILs) of common bean.

MATERIALS AND METHODS Plant material and growth conditions A pot experiment was conducted in a wire-house at the National Research Centre, Dokki, Cairo, Egypt (30º20' N; 31º53' E) from 25 October 2010 to 2 January

2011. During this period, daily temperatures ranged from 15.3º – 25.6ºC, with an average of 20.2º ± 3.4ºC. Daily relative humidities averaged 59.8 ± 8.4%, and ranged from 41.5 – 79.6%. Seeds of two RILs of common bean (RIL 147 and RIL 115) were provided by Dr. J. J. Drevon, Institut National de la Recherche Agronomique, Ecologie Fonctionnelle & Biogéochimie des Sols, Montpellier Cedex, France. Healthy common bean seeds (n = 840) were selected for uniformity by choosing those of equal size and of the same colour. The selected seeds were washed in distilled water, sterilised in 1% (v/v) sodium hypochlorite for approx. 2 min, washed thoroughly again in distilled water, and left to dry at room temperature (25ºC) for approx. 1 h. The common bean seeds were sown on 25 October 2010 and plants were harvested on 2 January 2011. Ten, uniform, air-dried common bean seeds of each RIL were sown along a centre row in each plastic pot (30 cm diameter) at a depth of 30 mm, in approx. 7.0 kg of clay soil. Seeds of each RIL were sown in 24 pots. The physical and chemical characteristics of the soil were determined according to Chapman and Pratt (1978). The soil had a clay texture [20% (v/v) sand, 20% (v/v) silt, and 60% (v/v) clay] with a pH of 8.52 in 0.01 M calcium chloride (CaCl2), an electrical conductivity (EC) of 0.70 dS m–1, an organic matter (OM) content of 1.36% (w/w), 1.43 mg kg–1 calcium carbonate (CaCO3), 25 mg kg–1 nitrate, 24.6 mg kg–1 phosphorus (P), 52 mg kg–1 K, 940 mg kg–1 magnesium (Mg), 130 mg kg–1 Na+, 1,500 mg kg–1 calcium (Ca), 9.2 mg kg–1 iron (Fe), 9.3 mg kg–1 manganese (Mn), 3.9 mg kg–1 zinc (Zn), and 2.5 mg kg–1 copper (Cu). To reduce compaction and improve drainage, the soil was mixed with yellow sand in a proportion of 3:1 (v/v). A granular commercial Rhizobium leguminosarum preparation, obtained from the Biofertilizer Inoculum Production Unit (Department of Microbiology, Soils, Water and Environment Research Institute, Agricultural Research Centre, Giza, Egypt) was incorporated into the top 30 mm of soil in each pot at the time of sowing. Granular ammonium sulphate [20.5% (w/w) N] was applied to each pot at a rate equivalent to 40 kg N ha–1, and single superphosphate [15% (w/w) P2O5] was added at a rate equivalent to 60 kg P2O5 ha–1. These N and P fertilisers were mixed into the soil in each pot immediately before sowing. Two weeks after sowing, the seedlings were thinned to four seedlings per pot. The experiment was set-up in a factorial arrangement with three levels of NaCl (S0, S1, or S2) and two levels of K fertiliser (K1 or K2). Four replicate pots were used for each treatment. To induce salt stress, NaCl was dissolved in distilled water and the plants were watered with an equal volume of 0, 25, or 50 mM NaCl 3 weeks after sowing (treatments S0, S1, and S2, respectively). The soil water capacity was estimated by saturating the soil in each pot with water, then weighing it after it had drained for 48 h. The water capacity of the soil in each pot was 0.36. Soil water contents were maintained at approx. 90% of pot water capacity. The level of soil moisture was controlled by weighing each pot and any loss of water was supplemented daily. Two levels of K fertiliser were applied (i.e., 25 or 150 mg K2O kg soil–1). These were equivalent to 56.2 kg K2O ha–1 (the recommended dose

M. G. DAWOOD, M. T. ABDELHAMID and U. SCHMIDHALTER for common beans; K1) and 337.5 kg K2O ha–1 (six-times the recommended dose; K2), respectively. Each level of K fertiliser was mixed thoroughly into the soil in each appropriate pot immediately before sowing the seeds. The treatments were maintained for 6 weeks. At the end of the experiment, 63 d after sowing (DAS), the above-ground portion of each plant (four per pot) was carefully removed and separated into leaves, stems, and pods. The number and fresh weight (FW) of pods per plant were recorded. Two grams of fresh leaves per plant were washed with distilled water to remove any surface dust and used to determine the concentration of photosynthetic pigments and the activities of two antioxidant enzymes [polyphenol oxidase (PPO) and POX]. The various plant organs (leaves, stems, and pods) were oven-dried for 72 h at 70ºC, and their dry weights (DW) were recorded. The dried leaves were ground to a powder and kept in a dessicator to determine their concentrations of phenolic compounds, total free amino acids, proline, total soluble carbohydrates, total carbohydrates, total nitrogen (N), P, K+, and Na+. Chemical analysis Photosynthetic pigment concentrations: Chlorophyll a, chlorophyll b, and carotenoid concentrations (in mg g–1 FW) were estimated according to Lichtenthaler and Wellburn (1983). Fresh leaf discs (0.2 g) were homogenised in 50 ml of 80% (v/v) acetone and centrifuged at 10,000 g for 10 min. The acetone solution was filtered through a sintered-glass filter, and the filtrate was made up to a known volume. The absorbance of the filtrate was determined using a Spekol spectrocolorimeter (VEB Carl Zeiss, Jena, Germany) at 663, 646, and 470 nm in order to calculate chlorophyll a, chlorophyll b, and carotenoid concentrations, respectively, expressed in mg g–1 FW, as described above. Phenolics concentrations: Phenolic compounds were extracted three-times with 100 ml cold 85% (v/v) methanol from each 1.0 g DW leaf sample. The methanol extracts were pooled, dried under vacuum, and made up to a known volume with cold 85% (v/v) methanol. A 0.5 ml aliquot of each methanol extract was added to 0.5 ml of Folin-Denis reagent (Snell and Snell, 1953), shaken, and allowed to stand for 3 min. One ml of saturated sodium carbonate was added to each tube, followed by 3 ml of distilled water, then shaken and allowed to stand for 60 min. The absorbance was read at 725 nm to calculate the concentrations of phenolic compounds (expressed in mg g–1 DW) according to Daniel and George (1972). Total free amino acid concentrations: Total free amino acids were extracted from each 0.25 g DW leaf sample using 25 ml of 80% (v/v) ethanol, then measured using the acidic-ninhydrin reagent method, as outlined by Yemm and Cocking (1955). One ml of acetate buffer, pH 5.4, and 1.0 ml of the chromogenic reagent were added to 1.0 ml of each ethanol extract. The mixture was heated in a boiling water bath for 15 min. After cooling under tap water, 3 ml of 60% (v/v) ethanol was added and the absorbance was read at 570 nm to calculate the concentration of total free amino acids, which was expressed in mg g–1 DW.

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Proline concentrations: Proline was extracted from 0.5 g of dry leaf tissue by grinding in 10 ml of 3% (v/v) sulphosalicylic acid. The mixture was then centrifuged at 10,000 g for 10 min. Proline concentrations were determined according to Bates et al. (1973). Two ml of the above supernatant was placed in a test tube and 2 ml of freshly prepared acidic-ninhydrin solution was added. The tubes were incubated in a water bath at 90ºC for 30 min, then the reaction was terminated in an ice-bath. Each reaction mixture was extracted with 5 ml of toluene and vortex-mixed for 15 s. The tubes were allowed to stand at room temperature for at least 20 min in the dark to separate the toluene and aqueous phases. The upper toluene phase was then carefully collected into a test tube and its absorbance was read at 520 nm. The concentration of proline in each sample was determined from a standard curve prepared using analytical grade proline, and was expressed in mg g–1 DW. Total carbohydrate and total soluble carbohydrate concentrations: Total carbohydrates were extracted from each 0.2 g of dry leaf material placed in a test tube, then 10 ml of 1.0 M sulphuric acid was added. The tube was sealed and placed in an oven at 100ºC overnight. The solution was then filtered into a 100 ml measuring flask and made up to the mark with distilled water. Total soluble carbohydrates were extracted overnight by submersing 0.1 g of dry leaf tissue in 10 ml of 80% (v/v) ethanol at 25ºC with periodic shaking. The extract was filtered through a Whatman No. 1 filter and the filtrate was oven-dried at 60ºC then dissolved in a known volume of distilled water. Total soluble carbohydrate and total carbohydrate concentrations were determined according to Yemm and Willis (1954) and expressed in mg g–1 DW. Peroxidase and polyphenol-oxidase activities: To measure these two anti-oxidant enzyme activities, 0.25 g of fresh leaf tissue was frozen and ground in 4 ml of 50 mM phosphate buffer, pH 7.0, containing 1% (w/v) polyvinylpolypyrrolidone. The homogenate was centrifuged at 15,000 g for 30 min at 4ºC and the crude supernatant was collected and used for all enzyme assays. Polyphenol-oxidase (PPO; EC 1.10.3.1) activity was measured using the method of Soliva et al. (2001) and expressed in Units PPO g–1 FW. Peroxidase (POX; EC 1.11.1.7) activity was measured according to Kumar and Khan (1982) and expressed in Units POX g–1 FW. Mineral ion concentrations: Total nitrogen (N) was determined using the micro-Kjeldahl method, as described by the AOAC (1970). Phosphorus (P), potassium ion (K+), and sodium ion (Na+) concentrations were measured according to the methods described by Chapman and Pratt (1978), and all were expressed as a percentage of the DW [% (w/w)]. Statistical analysis of the data All data were subjected to analysis of variance (ANOVA) for a randomised complete block design, after testing for homogeneity of error variances according to the procedure outlined by Gomez and Gomez (1984). Statistically significant differences


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TABLE I Effect of potassium ions on chlorophyll a, chlorophyll b, and carotenoid concentrations in the leaves of two recombinant inbred lines (RIL) of common bean grown under saline conditions Chlorophyll b (mg g–1 FW) Chlorophyll a (mg g–1 FW) NaCl treatment# Potassium treatment# RIL147 RIL115 S0 S1 S2

K1 K2 K1 K2 K1 K2

1.47b† 1.60a 1.28d 1.37c 1.01f 1.10e

2.01b 2.47a 1.81c 2.09b 1.53e 1.64d

Carotenoids RIL147 0.87b 0.92a 0.74d 0.82c 0.65f 0.70e

(mg RIL115

g–1 RIL147

FW) RIL115

0.79b 0.85a 0.73c 0.76b 0.65e 0.70d

0.46b 0.49a 0.39d 0.42c 0.25f 0.29e

0.56b 0.61a 0.50d 0.53c 0.43f 0.49e

†

Mean values (n = 4) in the same column for each trait and each RIL followed by the same lower-case letter are not significantly different according to Duncan’s multiple range test at P ≤ 0.05. S0 (tap water); S1 (25 mM NaCl); S2 (50 mM NaCl); K1 (25 mg K2O kg–1 soil); K2 (150 mg K2O kg–1 soil). Measurements were made 69 d after sowing (DAS). Recombinant inbred lines of common bean used were RIL147 and RIL115. #

between means were compared at P ≤ 0.05 using Duncan’s multiple range test.

synthesis (Sanjakkara et al., 2001), as well as lowering the production of ROS (Cakmak, 2005).

RESULTS AND DISCUSSION Concentrations of photosynthetic pigments Table I shows the marked differences in the concentrations of photosynthetic pigments between the two RILs of common bean. RIL115 had higher concentrations of chlorophyll a and carotenoids under both saline conditions, and following K fertilisation, compared to RIL147. Irrigation with 25 mM or 50 mM NaCl significantly reduced the concentrations of photosynthetic pigments in both RILs of common bean compared to the S0 treatment. The reduction increased with increasing salinity. Bekheta et al. (2009) also reported reductions in chlorophyll a, chlorophyll b, and carotenoid concentrations in faba bean due to salt stress. The decrease in chlorophyll concentration may be due to the production of proteolytic enzymes such as chlorophyllase, which is responsible for chlorophyll degradation (Dolatabadian and Jouneghani, 2009). Compared to treatment K1, treatment K2 caused significant increases in chlorophyll a, chlorophyll b, and carotenoid concentrations in both common bean RILs irrigated with tap water or with either saline solution (Table I). Hence, the K2 treatments (S1K2 or S2K2) mitigated the harmful effects of soil salinity on photosynthetic pigment concentrations. Numerous studies have shown that the application of K fertiliser decreased the adverse effects of salinity through the roles of K+ ions in stomatal regulation, osmoregulation, membrane charge balance, energy status, and protein

Phenolics, free amino acid, and proline concentrations Table II shows the significant decreases that occurred in the concentrations of phenolic compounds in both RILs under both saline treatments, or treatment K2, compared to the corresponding control (S0K1). Reduced concentrations of phenolic compounds in bean plants grown under salt stress have also been reported by Navarro et al. (2006) in Capsicum annuum and by Verma and Mishra (2005) in Brassica juncea. Salinity, and/or treatment K2, caused significant increases in proline and free amino acid concentrations in both RILs compared to their corresponding controls (S0K1;Table II). The maximum increase in proline and free amino acid concentrations were 78.6% and 52.5% in RIL115, and 36.1% and 73.4% in RIL147, respectively, in treatment S2K2 compared to S0K1. Muhammad (2013) showed that the application of K under saline conditions increased proline and total free amino acid concentrations in two maize hybrids. Some researchers have reported that a high proline concentration is a sign of stress (Rai et al., 2003), while others have suggested that high concentrations of proline act as a solute for intracellular osmotic adjustment (Silveira et al., 2003). Maqsood et al. (2008) stated that a large number of plant species accumulate proline in response to salinity stress in order to overcome such stress. Furthermore, the application of K plays a role in the transfer of nitrate ions from roots to shoots and leaves. Nitrate is first reduced to amines, then incorporated into amino acids to form proteins. Nitrate and protein contents are important

TABLE II Effect of potassium ions on total phenolic compounds, free amino acid, and proline concentrations in the leaves of two recombinant inbred lines (RIL) of common bean grown under saline conditions Phenolic compounds (mg g–1 DW) NaCl treatment# S0 S1 S2 †

Free amino acids (mg g–1 DW)

Proline (mg g–1 DW)

Potassium treatment#

RIL147

RIL115

RIL147

RIL115

RIL147

RIL115

K1 K2 K1 K2 K1 K2

11.1a† 10.2b 10.8a 8.5c 8.7c 6.8d

10.9a 9.6b 8.2c 7.9d 7.3e 6.5f

10.9f 11.7e 14.0d 15.5c 16.6b 18.9a

12.2f 12.9e 13.3d 14.4c 17.6b 18.6a

0.83d 0.86d 0.96c 1.00c 1.06b 1.13a

0.70f 0.82e 0.93d 0.98c 1.08b 1.25a


M. G. DAWOOD, M. T. ABDELHAMID and U. SCHMIDHALTER

189

TABLE III Effect of potassium ions on soluble carbohydrate and total carbohydrate concentrations and on the activities of peroxidase and polyphenol oxidase in the leaves of two recombinant inbred lines (RIL) of common bean grown under NaCl-salinity Soluble carbohydrates (mg g–1 DW) NaCl treatment#

Total carbohydrates (mg g–1 DW)

Peroxidase (Units g–1 FW)

Polyphenol-oxidase (Units g–1 FW)

Potassium treatment#

RIL147

RIL115

RIL147

RIL115

RIL147

RIL115

RIL147

RIL115

K1 K2 K1 K2 K1 K2

45.7e† 47.8d 48.3d 54.6b 50.4c 57.4a

37.2e 44.9d 46.4cd 47.7c 49.7b 54.0a

168b 180a 165b 176a 157c 179a

163b 176a 167b 170ab 156c 162bc

10.3f 11.0e 13.2d 14.7c 15.9b 17.1a

9.1e 11.8d 13.9c 15.8b 15.8b 17.5a

6.2f 7.4e 8.7d 9.6c 11.4b 12.9a

10.0e 12.9cd 12.7d 13.5c 14.8b 15.8a

S0 S1 S2 †

Mean values (n = 4) in the same column for each trait and each RIL followed by the same lower-case letters are not significantly different according to Duncan’s multiple range test at P ≤ 0.05. S0 (tap water); S1 (25 mM NaCl); S2 (50 mM NaCl); K1 (25 mg K2O kg–1 soil); K2 (150 mg K2O kg–1 soil). Measurements were made 69 d after sowing (DAS). Recombinant inbred lines of common bean used were RIL147 and RIL115. #

contributors to crop quality and yield (Maqsood, 2009). Without the application of K fertiliser, nitrate accumulates in the roots and a feedback mechanism in root cells prevents further nitrate uptake. Soluble and total carbohydrate concentrations Salinity (treatments S1 and S2) and/or treatment K2 caused significant increases in soluble carbohydrate concentrations in both RILs (Table III). Zheng et al. (2008) reported significant increases in soluble sugar contents in salt-tolerant and salt-sensitive cultivars of wheat. Total carbohydrate concentrations decreased in both RILs under increased salinity levels. Treatment K2 caused a marked increase in carbohydrate concentrations in both RILs at all salinity levels compared to K1-treated plants. Parida et al. (2004) showed that sugars accumulate under salt stress and play leading roles in osmoprotection, osmotic adjustment, carbon storage, and free radical scavenging. They also reported a decrease in starch content with an increase in reducing and non-reducing sugars in the leaves of mangrove, Bruguiera parviflora. Peroxidase and polphenol oxidase activities Irrigation of the two RILs of common bean with saline solutions S1 or S2, and/or the K2 treatment, caused gradual but significant increases in POX and PPO activities compared to the corresponding control plants (S0K1;Table III). Winston (1990) also reported increases in POX activity with increasing salinity. Rises in POX activity due to salinity have been shown in Glycine max (Ghorbanli et al., 2004), P. vulgaris (Telesinski et al., 2008), and in faba bean (Bekheta et al., 2009). Moreover,

K+ ions play important roles in increasing enzyme activity, improving the synthesis of carbohydrates and fats, and translocating photosynthetic products into sink organs (Soleimanzadeh et al., 2010). Zheng et al. (2008) reported enhancing the activities of anti-oxidant enzymes in wheat cultivars of differing salt tolerance by combinations of KNO3 and NaCl Mineral ion concentrations Salinity levels S1 and S2 caused significant reductions in the N, P, and K+ concentrations in leaves, and a significant increase in Na+ ions in both RILs of common bean compared to S0 (Table IV). Treatment K2 increased N, P, and K+ concentrations significantly under saline and non-saline conditions in both RILs of common bean. The K+:Na+ ion ratios in both common bean RILs decreased significantly with increasing salinity, while it increased following treatment K2. Table IV shows the negative relationship between Na+ and K+ ion concentrations in leaves due to salinity. Ashraf and Khanum (1997) reported that the selective uptake of K+ ions relative to Na+ ions was an important physiological mechanism contributing to salt-tolerance in many plant species. The application of K+ ions under saline conditions leads to a competition between K+ ions and other cations, especially Na+ ions, in the plant. The observed decrease in Na+ ion concentrations can be attributed to competition with K+ ions for limited binding sites on the plasma membrane, which suppresses the influx of Na+ ions from the external (soil) solution (Al-Uqaili, 2003). Our results confirmed the findings of Abdelhamid et al. (2010) who reported that adaptation to NaCl-salinity was due mainly to high accumulations of inorganic N, P,

TABLE IV Effect of potassium ions on nitrogen (N), phosphorus (P), potassium (K+), and sodium (Na+) ion concentrations, and the K+:Na+ ratio in the leaves of two recombinant inbred lines (RIL) of common bean grown under NaCl-salinity NaCl treatment# S0 S1 S2 †

Potassium treatment# K1 K2 K1 K2 K1 K2

N (%) RIL147 †

2.90b 3.20a 2.50d 2.77c 2.35f 2.61e

K+ (%)

P (%)

Na+ (%)

K+:Na+ ratio

RIL115

RIL147

RIL115

RIL147

RIL115

RIL147

RIL115

RIL147

RIL115

3.00b 3.33a 2.44d 2.69c 2.03f 2.26e

0.36a 0.42a 0.28b 0.32b 0.20c 0.24c

0.32a 0.38a 0.26b 0.30b 0.18c 0.22c

2.25b 2.41a 1.96d 2.16c 1.68f 1.88e

1.93b 2.11a 1.69d 1.83c 1.18f 1.43e

0.36e 0.30f 0.49c 0.42d 0.64a 0.56b

0.39e 0.35f 0.49c 0.45d 0.61a 0.55b

6.26b 8.03a 4.00d 5.13c 2.63f 3.35e

4.95b 6.04a 3.46d 4.08c 1.93f 2.60e



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TABLE V Effect of potassium ions on the number of pods per plant, pod FW, pod DW, stem DW, leaf DW, and shoot DW in two recombinant inbred lines (RIL) of common bean grown under NaCl-salinity NaCl Potassium treatment# treatment# S0 S1 S2

K1 K2 K1 K2 K1 K2

Pod number (plant–1)

Pod FW (g plant–1)

Pod DW (g plant–1)

Stem DW (g plant–1)

Leaf DW (g plant–1)

Shoot DW (g plant–1)

RIL147

RIL115

RIL147

RIL115

RIL147

RIL115

RIL147

RIL115

RIL147

RIL115

RIL147

RIL115

2.4bc† 3.9a 2.6bc 2.9b 0.7d 2.1c

3.5b 4.6a 2.4d 3.0c 2.0e 2.4d

6.56bc 9.68a 4.40c 7.30b 0.50d 1.12d

9.28b 11.41a 4.70d 5.81c 3.13f 3.40e

1.19c 1.87a 0.99d 1.46b 0.09f 0.20e

1.46b 1.96a 0.92d 1.22c 0.61f 0.75e

1.17b 1.37a 0.94d 1.11c 0.72e 0.88e

1.20b 1.38a 0.91d 1.08c 0.62f 0.79e

1.49b 1.66a 1.26d 1.39c 1.05f 1.17e

2.18b 2.45a 1.97d 2.08c 1.24f 1.45e

3.83b 4.90a 3.19d 3.96c 2.02e 2.09e

4.84b 5.79a 3.80d 4.38c 2.47f 2.99e

†

Mean values (n = 4) in the same column for each trait and each RIL followed by the same lower-case letters are not significantly different according to Duncan’s multiple range test at P ≤ 0.05. # S0 (tap water); S1 (25 mM NaCl); S2 (50 mM NaCl); K1 (25 mg K2O kg–1 soil); K2 (150 mg K2O kg–1 soil). Measurements were made 69 d after sowing (DAS). Recombinant inbred lines of common bean used were RIL147 and RIL115.

K+, Ca2+, and Mg2+, and lesser quantities of Na+ and Cl–, as well as a higher K+:Na+ ion ratio. Common bean yields and yield components Table V shows the interactive effects of NaCl-salinity and the application of K+ ions in two RILs of common bean. RIL115 had higher numbers of pods per plant, higher FWs and DWs of pods per plant, as well as shoot DWs per plant, under both saline conditions with additional K fertilisation (K2) compared to RIL147. Species and genotypes differ in their ability to adapt to salt stress, therefore differences between genotypes under salinity have been observed in rice (Zeng et al., 2000), faba bean (Abdelhamid et al., 2010), and soybean (Gaballah et al., 2011). Irrigation of the two RILs of common bean with saline solutions S1 or S2, significantly reduced the number of pods per plant, the FW and DW of pods per plant, as well as shoot DW per plant, compared to plants irrigated with tap water (S0). The adverse effects of salinity on common bean yields increased with increasing salt stress. These results agree with those of Stoeva and Kaymakanova (2008) in common bean plants. Berstein et al. (1995) claimed that an inadequate supply of Ca2+, K+, and Mg2+ ions to meristems may restrict cell division and/or expansion when plants are grown under high NaCl concentrations. Furthermore, Chinnusamy et al. (2005) reported that, under salt stress, the predominant cause of reduced plant growth appeared to be ion toxicity rather than osmotic stress. Treatment K2 increased the number of pods per S0, S1, and S2 plant by 1.31-, 1.25-, 1.20-fold in RIL115, and by 1.62-, 1.11-, and 3.00-fold in RIL147; while the increases in pod FW per plant were 1.23-, 1.23-, 1.08-fold in RIL115, and 1.47-, 1.66-, 2.24-fold in RIL147,

compared with treatment K1 with the 0, 25, or 50 mM NaCl solutions, respectively. Chow et al. (1990) showed that spinach grown under saline conditions required two-times more K, and Khayyat et al. (2007) found that supplementary K fertilisation mitigated the negative effects of NaCl on strawberry plants and that fruit numbers and yields increased with increasing K+ ion concentrations from 5 – 10 mM. Moreover, Maqsood (2009) reported significant reductions in growth parameters in two genotypes of maize grown under saline conditions, while a significant increase in growth was observed following the addition of 200 mg K kg–1 soil.

CONCLUSIONS It can be concluded that salinity had a negative impact on the yield and quality of common bean plants. One application of 150 mg K2O kg–1 soil resulted in increased concentrations of photosynthetic pigments, higher antioxidant enzyme activities, higher osmoprotectant concentrations, and higher K+:Na+ ratios, all of which were reflected in an improved plant performance under NaCl stress. This work was part of Research Project No. 9050105 supported by The National Research Centre, Cairo, Egypt. The authors would like to thank Dr. J. J. Drevon (Institut National de la Recherche Agronomique, Ecologie Fonctionnelle & Biogéochimie des Sols, INRAIRD-SupAgro, Montpellier Cedex, France) for providing seeds of the two recombinant inbred lines of common bean (RIL 147 and RIL 115).

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