Abstract: Pot experiment was conducted in the greenhouse of the National Research Centre to study the effect of two doses (50 and 100 mg L-1) of PK foliar fertilization in the form of K2PO3 on leaf nutrient concentrations and growth parameters of cowpea (Vigna unguiculata L.) grown under two diluted Mediterranean seawater levels (3.0, 6.0 dS m-1) in the irrigation water in addition to tap water (0.4 dS m-1) as control. Diluted seawater as irrigation led to significant decreases of macro and micronutrients concentrations in the leaves of cowpea plants. Plant height, number of green leaves, fresh and dry weights were also negatively affected with high significance (p0.05) as the plants irrigated with saline water. PK-foliar fertilization in the form of K2PO3 could increase P, K and other macro- and micronutrient concentrations in the leaves of the salt-stressed plants. The most effective dose was the 100 mg L-1 K2PO3 with the lower salinity level (3.0 dS m-1). Making the plants more tolerant to salinity stress, PK-foliar fertilization could improve plant growth parameters and increase plant heights, number of green leaves, fresh and dry weights.
INTRODUCTION
Salt stress condition is an osmotic which is apparently similar to that brought by water deficit (Almoguera et al., 1995). Most of crop plants are classified as glycophytes cannot tolerate high concentrations of salts in the root medium (Hussein et al., 2007a). Injurious ions such as Na+ and Cl– negatively affect nutrient uptake and balance (Sairam and Tyagi, 2004; Hussein et al., 2007b). As the saline soils generally have higher concentrations of Na+ than K+ and Ca2+, a passive accumulation of Na+ in shoots and roots is occurred (Bohra and Doerffling, 1993). Accumulation of Na+ in roots reduces K+ uptake (Cramer et al., 1985; Lacerda et al., 2003) and Na+ presence in higher concentrations in xylem restricts K+ translocation from root to shoot (Engels and Marschner, 1992) which resulting in low K+ shoot content. Transport of Ca, Mg and total nitrogen were also reported to be inhibited with NaCl-salinity (Tremaat and Munns, 1986).
Cowpea is a predominately hot weather crop widely grown in eastern Africa and southeast Asia primarily as leafy vegetable. Steele et al. (1985) estimated that protein content of the leafy cowpea parts consumed annually in Africa and Asia is equivalent of 5 million tons and that this represents as mush as 30% of the total food legume production in the lowland tropics. Cowpea is inherently more drought and salinity tolerant than other crops but it still suffers considerable damage due to frequent drought and salinity stresses in different regions where rainfall is scanty and irregular (Singh et al., 2003).
Foliar fertilization can increase tolerance of plants to salinity by compensating the deficient nutrients in the plant tissues (El-Fouly et al., 2002; Shaaban et al., 2004). Sugar beet plants grown under high stress conditions could grow well and produce higher yields when sprayed with potassium (Tehrani and Malakouti, 1997).
The present research studied the response of cowpea plants irrigated with diluted seawater under greenhouse conditions to PK-foliar fertilization.
MATERIALS AND METHODS
Pot experiment was conducted in the greenhouse of the National Research Centre, Dokki, Cairo, Egypt during the two successive winter seasons 2004 and 2005 to study the response of cowpea (Vigna unguiculata L.) grown under two levels of salinity stress.
Seeds were sown in December, 1st in metallic tin pots 35 cm diameter and 50 cm depth. The inner surface of the pots was coated with three layers of bitumen to prevent direct contact of the metal with soil. Every pot contained 30 kg clay loam soil. Two kilograms of gravel (particles 2-3 cm in diameter) were placed in the bottom to make the movement of water from the base upward. Plants were thinned twice (8 and 12 days after sowing) to leave 5 uniform plants per pot. Calcium super phosphate (15.5% P2O5) and potassium sulfate (48.5% K2O) in the rate of 6.0 and 3.0 g pot-1, respectively were added in two equal splits (before sowing and two weeks later). The pots received N-fertilization in the rate of 6.86 g pot-1 as ammonium sulfate (20.6% N) in two equal splits (2 weeks after sowing and 2 weeks later).
Treatments
Irrigation with diluted Mediterranean seawater (Table
1) in two concentrations (3.0 and 6.0 dS m-1) started at
20 days after sowing, while tap water (0.4 dS m-1) is considered
as control. Every treatment contained 6 replicates. Potassium di-Hydrogen
Phosphate (KH2PO3) in the concentrations of 50 and
100 mg L-1 was sprayed at 20 and 30 days after sowing.
Sampling and Analysis
Soil
A representative soil sample was taken just before sowing, air dried,
ground and sieved through 2.0 mm sieve and analyzed (Table
2). Mechanical analysis was carried out using the hydrometer method
(Bauyoucos, 1954), pH and Electric Conductivity (EC): Water extract (1
soil: 2.5 water) method (Jackson, 1973), CaCO3: Calcimeter
method (Black, 1965), Organic Matter (OM): potassium dichromate method
(Walkley and Black, 1947). Phosphorus (P) was extracted using sodium bicarbonate
(Olsen et al., 1954). Potassium (K) and Magnesium (Mg) were extracted
using ammonium acetate method (Chapman and Pratt, 1978). Iron (Fe), manganese
(Mn), zinc (Zn) and copper (Cu) were extracted using DTPA method (Lindsay
and Norvell, 1978).
| Table 1: | Cation and anion components of sea water |
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| Table 2: | Soil chemical and physical characteristics |
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| *Adequate, **Low | |
Shoots
Shoot samples were taken at the age 45 days after sowing. The shoots
were separated into stems and leaves, washed with tap water, 0.01 N HCl
and bi-distilled water, respectively, dried at 70 °C for 24 h, weighed
and ground. A part of the plant material was dry-ashed in a Muffel furnace
at 550 °C for 6 h. The ash was digested in 3 N HNO3 and
the residue was then suspended in 0.3 N HCl (Chapman and Pratt, 1978).
A part of the sample was weighed and oven dried at 105 °C for 24 h,
then weighed again and the dry weight was calculated.
Nutrient Measurements
Nitrogen was determined using Kjeldahl-method; phosphorus was photometrical
determined using the molybdate-vanadate method according to Jackson (1973).
Potassium and Ca++ were measured using Dr. Lang-M8D Flame-photometer.
Magnesium, Mn++, Zn+ and Cu+ were determined
using the Perkins-Elmer Atomic Absorption Spectrophotometer.
Evaluation of the Nutrient Status
Soil nutrient status was evaluated according to the sufficient concentrations
of Ankerman and Large (1974).
Statistical Analysis
Data were statistically analyzed using the method described by Snedecor
and Cochran (1980).
RESULTS AND DISCUSSION
Effect of Salinity
Salinity of irrigation water dramatically affected nutrient concentrations
in cowpea leaves. Concentrations of N, P, K, Mg and Ca were decreased
with salinity dose increment (Fig. 1). Micronutrients
concentrations were also declined as salinity level of irrigation water
increased (Fig. 1). The most effected were nitrogen,
phosphorus, potassium, calcium manganese and zinc. Osmotic potential created
by saline ions at the root medium restricted water and nutrient elements
flow into roots (Munns, 2002). Accumulation of Na+ in roots
found to reduce K+ uptake and translocation from root to shoot
(Cramer et al., 1985; Engels and Marschner, 1992; Maiti et al.,
2006; Graboov, 2007). Calcium, magnesium and nitrogen transport was also
reported to be inhibited due to salinity (Tremaat and Munns, 1986; Maiti
et al., 2006). Due to nutrient deficiency and/or harmful effects
of the saline toxic ions, growth parameters of cowpea plants were negatively
affected. Plant heights, number of green leaves per plant, fresh and dry
weight were significantly decreased, especially with the high salinity
level (6.0 dS m-1) (Table 3) which is characteristic
to plants irrigated with saline water
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| Fig. 1: | Macro and micronutrients concentration in cowpea leaves as affected by salinity of the irrigation water |
| Table 3: | Effect of PK-foliar fertilizer on growth parameters of cowpea plants grown under two levels of salinity in the irrigation water (n = 12) |
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| Mean values with the same letter(s) are not statistically significant | |
(Meiri and Shalhevet, 1973; Izzo et al., 1991; Munns, 1993; Maiti et al., 2006). However, other investigations suggested that this effect might be due to disturbance in growth regulators (Brenant et al., 2007), photosynthesis and protein building (Debuba et al., 2006), enzymes activity (Thapon et al., 2008) or antioxidant defense (Xie et al., 2008).
Effect of PK-Foliar Fertilization
Nutrient concentrations in the leaves of cowpea plants grown under
salinity stress of irrigation water were significantly increased as the
plants received twice dosing of 100 mg L-1 K2PO3
as foliar supplements (Fig. 2, 3). As
P and K uptakes and translocation were inhibited by salinity stress conditions
in the root medium, PK-foliar fertilization may become the most suitable
remedy for their deficiency in the shoot tissues (Shaaban et al.,
2004). Furthermore, adequate concentrations of both elements in the foliar
fertilized leaves improved uptakes of other nutrients to reach the sufficiency
levels for plant growth and development (El-Fouly et al., 2002).
Consequently, sufficient concentrations of nutrients, especially nitrogen
enabled the plant mechanism regulations to synthesize considerable concentrations
of metabolites to overcome the harmful effects of the saline ions (Kao,
1997). As salinity caused dehydration which inhibits photosynthesis of
stressed plants, K-foliar supplementations can partially reverse such
dehydration effects (Pier and Berkowitz, 1987).
PK-foliar fertilization positively affected growth parameters of cowpea grown under salinity stress conditions. Plant heights, number of green leaves per plant and both fresh and dry weights were significantly increased as the plants received PK-foliar fertilization (Table 3). The most effective dose was 100 mg L-1 with the lower salinity level (3.0 dS m-1). As potassium is mostly responsible about cell turgor, PK-foliar fertilization led to better increases of fresh weight than dry weight (Lindhauer, 1989). Lindhauer (1985) showed also that K+-fertilization besides increasing the dry matter production and leaf development, greatly improved the retention of water in the plant tissues even under severe stress conditions. Kabir et al. (2007) support this finding, but Hussein and El-Greatly (2007)
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| Fig. 2: | Macronutrients concentrations in cowpea leaves as affected by salinity level in the irrigation water and PK-foliar fertilization |
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| Fig. 3: | Micronutrients concentrations in cowpea leaves as affected by salinity level in the irrigation water and PK-foliar fertilization |
attributed this phenomenon to the effect of PK on endogenous harmony. Meanwhile, Williams and Kafkafi (1995) demonstrated that K-shortage in the root medium can be alleviated by K-foliar fertilization, especially at the higher demands during fruit-stage.
CONCLUSIONS
Salinity of diluted seawater as irrigation water led to significant decreases of macro and micro-nutrients concentrations in the leaves of cowpea plants. Plant height, number of green leaves, fresh and dry weights were negatively affected significantly as the plants irrigated with saline water. PK-foliar fertilization in the form of K2PO3 could increase P, K and other macro- and micronutrient concentrations in the leaves. Making the plants more tolerant to salinity stress, PK-foliar fertilization could improve plant growth parameters.
ACKNOWLEDGMENTS
The authors wish to thank staff members of the project Micronutrients and other Plant Nutrition Problems in Egypt and its principle investigator Prof. Dr. Mohamed M. El-Fouly for their support during the course of this study.