ABSTRACT

The high concentrations of salts in water, especially in semiarid regions, increase salt concentration in the soil and plant with the consequent decrease in NPK contents. Current study evaluates the influence of irrigation with water of different salinities, associated to nitrogen (N) fertilization, on NPK contents in castor bean cv. BRS Energia. An experiment was conducted for four months, in lysimeters under the  field condition, in the municipality of Pombal, Paraíba - Brazil, using an  Eutrophic Entisol of sandy loam texture ina randomized 5 × 4 factorial block design with three replications. Five levels of electrical conductivity of irrigation water (ECw=0.3, 1.2, 2.1, 3.0 and 3.9 dS m^-1) and four doses of N (70, 100, 130 and 160 mg kg^-1) were tested. The highest contents of potassium (K) in roots and leaves were obtained respectively with ECw of 0.3 and 2.2 dS m^-1. The N content in leaves was not influenced by factors under analysis. Increase in N dose decreased the plant contents of phosphorus (P) and K.

Key words: Ricinus communis L., electrical conductivity, regressions.

The experiment was conducted under the field condition in an experimental area of Agricultural Sciences and Food Technology Center of the Federal University of Campina Grande (CCTA/UFCG) in Pombal, Paraíba - Brazil, at 6°48’16” S and 37°49’15” W, average altitude 144m, during September 2011 and January 2012 using plasticcontainers as drainage lysimeters. Five levels of electrical conductivity of irrigation water (ECw = 0.3; 1.2; 2.1; 3.0 and 3.9 dS m^-1) were used combined with four N rates (70; 100; 130 and 160 mg N kg^-1 of soil). These doses were based on the recommendations of Novaiset al. (1991) for assays in pots (100 mg kg^-1). Experimental design consisted of randomized blocks (5x4), with three replications. The experimental units were distributed in simple rows spaced 0.90 m apart, with 0.70 m between plants in the row.

 

The seeds were sown in the 100 L plastic containers. After thinning, one plant per pot was kept and formed the experimental unit. Each recipient had two holes in its lower segment for drainage of excess water and plastic bottles for the collection of drained water were placed beneath the holes to estimate water consumption per plant. Recepients were filled with 2 kg of pebbles number zero which covered the bottom of the recipient. Further, 107.5 kg of Eutrophic Entisol of sandy loam texture, non-saline and non-sodic, collected at a depth between 0 - 30 cm, were placed on the pebbles. Soil, retrieved from the municipality of Pombal, PB - Brazil, was ground and sieved (<0.02 mm)and its physical and chemical attributes (Table 1) were analysed by methodology described by Claessen (1997).

 

 

Fertilization was performed for potassium and phosphorus based on the recommendations of Novais et al. (1991), using 12.0 g of potassium chloride and 162.5 g of monoammonium phosphate. In addition 2.5 kg of vermicompost (6.3 g N kg^-1; 1.28 g P kg^-1; 0.53 g K kg^-1) was also applied to increase fertility and water retention capacity of soil.

 

Nitrogen was applied in split doses, one third was applied at the start of the experiment and the other two thirds in four equal applications along with irrigation water at 10-day intervals starting 25 days after sowing (DAS). In treatment N2 (equivalent to 100% recommendation), 33.34 g mono-ammonium phosphate (MAP) and 8.88 g urea were applied. Two foliar applications with Albatroz (N - 10%; P₂O₅ - 52%; K₂O - 10%; Ca - 0.1%; Zn - 0.02%; B - 0.02%; Fe - 0.15%; Mn - 0.1%, Cu - 0.02%; Mo - 0.005%) in the proportion of 1 g L^-1of water, at the start of flower emission (29 and 37 DAS), were also undertaken.

 

Considering the contents of N, P and K found in the vermicompost and the albatroz solution treatments received additional amounts of 6390, 21900 and 1070 mg kg^-1 of N, P₂O₅and K₂O, respectively, however, these additional amounts on top of recommended dosages of Novais et al. (1991) for nitrogen, phosphorus and potassium were not considered for analyzing the effect of N since all treatments had received the same amount of vermicompost and albatroz solution and the same being negligible compared to pre-established doses of N.

 

Different salinity levels were obtained by addition of NaCl in water from the local supply system and quantity was determined by employing relation C (mg L^-1) = 640 × ECw (dS m^-1), (Rhoades et al., 2000). After soil conditioning in the recipient, water content was raised to field capacity by capillary saturation, followed by free drainage with the respective type of water. After preparation and ECw calibration, using a portable conductivity meter, the waters were stored in plastic recipients, adequately protected in order to avoid evaporation. The subsequent irrigations occurred daily at 17.00 h and volume of water to be applied was calculated according to irrigation requirements determined by water balance, or rather, volume applied minus volume drained in the previous irrigation.

 

The contents of N, P and K in leaf, stem and root were assessed after 120 DAS. The plants were harvested and their respective parts separated and dried in a forced air oven at 65°C till constant weight. The plant samples were then weighed, milled and analyzed, following methodologies recommended by Silva (1999). Data were evaluated by analysis of variance with F-test at 0.05 probability using SISVAR-ESAL (Ferreira, 2003) and regression analysis were performed when effects were significant. Choice of model of regression was based on value of coefficient of determination (R²) taking into consideration a plausible biological explanation.

Analysis of variance (Table 2) showed that levels of water salinity (ECw) had a significant effect on N and P contents in stem and root tissues, whereas K contents of all organs of the castor bean plants under analysis were significantly influenced by this treatment. For N rates, there was a significant effect on N accumulation in stem and root tissues. Nitrogen rates also affected (p<0.05) P and K contents in leaf and stem tissues. The interaction between ECw versus N rates was significant for concentrations of N in the roots and P and K in the stem tissues, respectively.

 

 

The foliar N content showed a mean value of 11.98 g kg^-1 which was not affected by salinity in irrigation water, but there was a 67.71% increase in N content of stem with per unit increase (p<0.01) of ECw (Figure 1a). Results for N content in the stem corroborate with that of Al-Harbi (1995) who reported the non-dependence between N contents and salinity levels for the period in which the plant underwent stress and especially for the species and genotypes with different salt tolerance.

 

The N content in the roots showed a quadratic response (p < 0.05) for plants with doses of 70 and 100 mg kg^-1 N (Figure 1b). However, there was no significant effect for doses of 130 and 160 mg Nkg^-1, respectively, which showed average contents of 2.45 and 2.66 g Nkg^-1. Decrease in stem and root N contents in plants submitted to NaCl stress may have been caused by the competition between chloride (Cl^-) and nitrate (NO₃^-) ions transporters (Ogawa et al., 2000) and/or the inactivation of NO₃^- transporters due to the toxic effects of the ions (Lin et al., 2002). Salinity also causes the rupture of the root membrane and inhibits NO₃^-absorption (Parida and Das, 2004).

 

 

Further, saline stress interferes in enzymes similar to ammonium (NH₄⁺), with a greater accumulation of the ion in the tissue, besides the accumulation from the degradation of proteins caused by the stress (Zhou et al., 2004). Costa et al. (2008) also reported that increase of salt concentration of the irrigation water (0.15 to 4.5 dS m^-1) caused deleterious effect on N concentration in root tissues of Amaranthus spp.

 

Phosphorus content in leaf tissues was negatively affected by N doses (Figure 2a). There was a 5.33% decrease in P for each interval of 30% in N doses. When plants were submitted to the maximum N dose (160 mg kg^-1), P content in leaves decreased by 0.9 g kg^-1 (16%) when compared to those with a lower dose (70 mg kg^-1). According to Malavolta et al. (1997), P is a highly mobile element in plants and reaches the leaves or the meristhematic locations by long distance transport and together with N is immediately distributed in the above ground plant material. With the aging of leaves, up to 60% of P may also be transported by the phloem to other segments of the plant, especially to new organs and developing fruit, with a decrease in P content of the leaves.

 

 

There was a quadratic response of plants with 100% N dose with regard to P contents in stem tissues (Figure 2b). High accumulation of P (3.65 g kg^-1) was reported under water salinity of 3.9 dS m^-1. However, N dose of 160 mg kg^-1 caused a linear increase in the concentration of P in the stem, with a 12.35% increase with per unit increase in ECw. There was a 44.49% (1.02 g kg^-1) increase in P amounts in plants submitted to ECw=3.9 dS m^-1 when compared to plants irrigated with lower salinity level (0.3 dS m^-1). It may be observed (Figure 2b) that the application of doses of 70 and 130 mg kg^-1 N did not affect P content in the stem tissues of the castor bean plant but registered mean concentrations of 2.61 and 2.81 g kg^-1 P, respectively.

 

Several studies demonstrated that the interaction between water salinity and P uptake in plants is a highly complex issue and depends on plant species, salt concentration, type of salt and salinity level of irrigation water (Grattan and Grieve, 1999). However, the accumulation of P in the stem of plants under salt stress may be a consequence of the decrease of translocation associated with a possible reduction in growth rate (Lacerdaet al., 2006). Results corroborate those observed in gliricidia (Farias et al., 2009) and cowpea (Silva et al., 2011) plants cultivated under saline stress.

 

Plants irrigated with ECw=2.2 dS m^-1 caused a high accumulation of K contents in leaves (27.03 g kg^-1) with a decrease in ion concentration above this ECw level (Figure 3a). The low uptake of K in plants under high salinity levels has been attributed to the competition between Na⁺ and K⁺ by the absorption sites of the plasmalema and to the great efflux of K⁺ from the roots as a direct result of osmotically induced exchanges in the permeability of the plasmalema and the substitution of Ca²⁺ by Na⁺ in the membrane (Ferreira et al., 2001). 

 

 

The first explanation is mainly due to the fact that the two cations (Na⁺ and K⁺) have a similar atomic radius and K⁺ transporters are less specific in toxic concentrations of Na (Castillo et al., 2007). Results in current research confirmed results obtained by Sousa et al. (2012) when they evaluated the influence of ECw in macro-and micronutrient contents in leaves of Jatropha curcas. Authors reported a quadratic effect and a highest K content (32.63 g kg^-1) in plants under ECw=2.2 dS m^-1.

 

With regard to the effect of N dose on the K content of leaves (Figure 4b), there was a decrease of 4.79% in the concentration of K⁺ with each 30% increase in the N dose, which amounts to a decrease of 14.37% of K when data of plants cultivated under the highest (160 mg kg^-1) and lowest N dose (70 mg kg^-1) were compared. According to Rosolem (2005), K is a strong competitor with other cations owing to the high efficiency of the plants´ absorption system. Absorption of the other cationic ions increases when K⁺ is absent in the solution since competition is less. The divergences in results in current study and those observed by Pacheco et al. (2008) may be explained considering that the latter studied the effects of NPK fertilization on castor bean IAC 226 and reported high contents of K⁺ when combined to high N doses.

 

The interaction between salinity and N dose interfered in the accumulation of Kin the stem tissues of the castor bean plant (Figure 4a), showing a quadratic response for all N doses. Maximum K⁺ contents (13.57, 11.41, 11.87 and 11.21 g kg^-1) occurred when the plants underwent irrigation with ECw of 3.9, 3.3, 3.9 and 3.1 dS m^-1, respectively. The lowest content of the nutrient (4.84, 6.13, 5.50 and 4.10 g Kkg^-1) occurred in plants irrigated with water at ECw=0.3 dS m^-1for N rates of 70, 100, 130 and160 mg kg^-1, respectively. Selectivity increase of the K membrane is relevant to reduce the effect of salinity in plants since it is the main ion in osmotic adjustment and in the maintenance of cell turgor (Munns, 2002).

 

 

Potassium content was also significantly affected by the interaction of water salinity of irrigation water and N doses (Table 2). Figure 4b shows that plants with 70 and 100 mg kg^-1 of N decreased quadratically, with maximum contents of K (6.89 and 7.36 g kg^-1 respectively), obtained in plants irrigated with ECw of 0.3 dS m^-1. There was a growing linear response with dose 160mg kg^-1N which caused a 10.3% increase in K content with per unit increase in ECw (Figure 4b) or K content in plants irrigated with ECw=3.9 dS m^-1 increased by 1.40 g kg^-1 (37.10%) when compared to plants under ECw=0.3 dS m^-1. There were no significant statistical effects for dose of130 mg kg^-1 N, averaging 4.89 g kg^-1 K in the roots (Figure 4b).

 

Decrease in K content in the root is possibly due to the direct exposure of roots to saline solution which causes changes in the integrity and selective permeability of the plasma membrane (Grattan and Grieve, 1999). Viana (2007) analyzed the combination of N and K doses in wheat, in a controlled area, and reported that K concentration in the dry matter of the roots decreased as N supply increased.

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