Abstract: A pot experiment with wheat plants (Triticum aestivum L. cv. Giza 167) grown on 2.0% or 10.0% CaCO3 in the soil was conducted to study the effect of foliar fertilization of boron, zinc and their combinations on zinc/boron ratio, elemental status and shoot growth. Growth of plants grown on low lime levels was not affected. The best treatments led to highest growth of shoots grown on soil of high lime content were 25 ppm B or 25 ppm B + 50 ppm Zn in the spray solution. Boron concentrations were increased in the shoot tissues of plants grown on both CaCO3 levels as boron or zinc was applied. Meanwhile, zinc concentrations were slightly increased when only boron was sprayed; however it decreased when combinations of boron and zinc were applied. Boron or zinc uptake by the shoots grown on low lime level was less affected compared to those grown on high lime level. Best uptake of boron and zinc was obtained by the low dose of the two elements combination (25 ppm B +50 ppm Zn). Uptake increases over control obtained by this treatment reached about 230% for boron and 650% for zinc in the plant shoots grown on high lime level. Uptake percentage of other elements by the shoots grown under lime stress conditions was higher than that of plant shoots grown under low lime level in the soil. Highest uptake of Fe, Mn, Cu, K and Ca were obtained by boron treatments alone or low dose of boron and zinc combinations, which suggested that boron stimulates metabolic functions facilitate absorption and translocation of these nutrients, especially in plants grown under lime stress conditions
INTRODUCTION
Boron deficiency causes many disorders in plant growth and development[1, 2]. Growth of both shoots and roots was found to be inhibiting due to less supply of boron[3,4]. Acting on the membrane level, boron deficiency was reported to inhibit membrane uptake of number of nutrients[5, 6] and affects nutrient transport capacity due to lowering ATPase and NAD(P)H-development redox activity[7]. Negative effects of boron deficiency were also reported to enhance susceptibility of plants to environmental stress such as marginal deficient supplies of nutrients and water deficit in the soil[3].
It is nearly evident that Monocots are less sensitive than Dicots to boron-deficiency. However, unfavorable soil conditions such as high pH values and/or high CaCO3 levels were, generally, found to restrict the uptake and translocation of micronutrients[8,9]. On the other hand, under favorable soil conditions, only around 10% of boron added to soil found to absorbed by plant roots[10]. Nutrient uptake and yield increases were obtained by boron foliar application[11,12].
The present work objected to study the effect of foliar application by boron, zinc and their combinations on growth and nutrients concentrations and uptake in the shoots of wheat grown on soil contains a relatively high level of CaCO3 compared to plants grown on soil contains low CaCO3 level.
MATERIALS AND METHODS
Sowing and practices: Wheat (Triticum aestivum L. cv. Giza 167) grains were sown in July 2000 in the greenhouse of the Plant Nutrition Institute, University of Bonn, Germany, in black-plastic pots filled with 5.0Kg sandy loam soil. Before sowing, half of the pots received pure CaCO3 to increase its content to around 10.0%. Then, each pot received 0.4 g ammonium sulfate (20.6% N), 0.2 g potassium sulfate (50% K2O) and 0.1 g super mono-phosphate (15.5% P2O5) Kg-1 soil. Irrigation took place daily to keep soil moisture at 60 % capacity. Two weeks after sowing, the plants were thinned to leave 10 plants pot-1 .
Experimental design and treatments: Eight treatments with 3 replicates under 2 levels of CaCO3 in the used soil were arranged in a complete randomized block design. The treatments were carried out as foliar sprays at 21 days after sowing as follows:
Control
| • | B+ (25 ppm boron) |
| • | B++ (50 ppm boron) |
| • | Zn+ (50 ppm zinc) |
| • | B+Zn+ (25 ppm boron + 50 ppm zinc) |
| • | B++Zn+ (50 ppm boron + 50 ppm zinc) |
| • | Zn++ (100 ppm zinc) |
| • | B+Zn++ (25 ppm boron + 100 ppm zinc) |
Boron treatments were prepared from pure boric acid (H3BO3), while Zn-EDTA was used for zinc treatment preparations.
Besides, 50 ppm Fe (Fe-EDTA) + 50 ppm Mn (Mn-EDTA) + 25 ppm Cu (Cu-EDTA) were added to the spray solution in all treatments.
Analysis
Soil: Representative soil samples were taken after soil preparation and before fertilization. The samples were air-dried and passed through a 2.0 mm sieve pores. Soil physical and chemical characteristics are shown in Table 1. Soil mechanical analysis was carried out using hydrometer method[13]; pH and E.C (electric conductivity): soil/water suspension (1:2.5)[14]; Total calcium carbonate (CaCO3): Calcimeter method[15]; Organic matter (O.M.): potassium dichromate method[16].
Soil phosphorus was extracted using sodium bicarbonate[17]. Potassium (K) and magnesium (Mg) were extracted using ammonium acetate at pH 7.0 (14), while Fe, Mn, Zn and Cu were extracted using DTPA[18]. Boron was extracted and measured according to Wolf[19].
Plant material: Ten days after spraying, wheat plants were harvested. Shoots were weighed for fresh weight, then washed with tap water, 0.01 N HCl and boron-free distilled water, oven dried at 70°C in plastic containers for 24 h and ground using a stainless steel humor mill. One-gram sample was dry-ashed in a muffle furnace at 550°C for 6 hrs using 3.0 N HNO3. The residue was, then, suspended in 2.0 N HCl.
Measurements: Total nitrogen content of the samples was determined using Bauschi digestion and distillation apparatus. Phosphorus was photometrically determined in the dry ashed residue using the Molybdate-Vanadate method and measured using the UVNIS spectrophotometer (Perkin-Elmer Lambda2). Potassium and Ca were measured in the extract using Jenway PFP7 flamephotometer. Magnesium, Fe, Mn, Zn and Cu were measured using an atomic absorption spectrophotometer (Zeis PMQ3). Boron was extracted according to Wimmer and Goldbach[21] and measured using UVNIS spectrophotometer (Perkin-Elmer Lambda 2).
| Table 1: | Physical and chemical characteristics of the soil |
| L = Low, vL = very low, M = Moderate H =High, vH = very high according to Ankerman and Large[20] | |
Data Analysis: Data were statistically analyzed using Costate Statistical Package[22].
RESULTS AND DISCUSSION
Boron/zinc relationship: Boron and/or zinc foliar applications increased boron concentrations in the tissues of plant shoots grown on both CaCO3 levels (Fig. 1). Zinc concentrations were slightly increased as a result to boron application when added alone and decreased when combinations of boron and zinc were applied. Uptake of both elements by the plants grown on low lime levels was less affected compared to those grown on high lime levels in soil (Fig. 2). Best uptake of boron or zinc was obtained by a low dose of the two element combinations (+B+Zn). Uptake increases over control obtained by this treatment reached about 230% for boron and 650% for zinc in the shoots grown on high CaCO3 level.
This means that both nutrients enhance the uptake of each other and only optimal concentrations of them in the shoot tissues are obtained by applying a proper dose of their combination. This relationship may attributed to the essentiality of boron and zinc for the optimal function of plasma membrane ATPase[7] and the proposed role of boron in membrane stability and optimum ion uptake[5].
Other nutrients: Concentrations of the other nutrients Fe, Mn, Cu, K and Ca followed the same trend of zinc (Fig. 3).
| Fig. 1: | Boron and Zinc concentrations in shoots of wheat plants grown on low or high CaCO3 level in the soil as affected by foliar fertilization of boron, zinc and their combination. |
| Fig. 2: | Boron and zinc uptake by shoots of wheat plants grown on low or high CaCO3 level in the soil as affected by foliar fertilization of boron, zinc and their combinations |
They increased as boron added alone. In the same time, concentrations of Fe, Cu and K were also increased when only low dose of zinc (50 ppm) was applied. However, low B + low Zn combination treatments led to a medium concentration of all nutrients in the shoot tissues.
Figure 4 shows nutrient uptakes by shoots of wheat plants grown under high lime level stress conditions as affected by boron and zinc treatments compared to those grown under low lime level in the soil. Uptake increase by the shoots grown under lime stress conditions was higher than that of shoots grown under low lime level in the soil. Highest uptake of Fe, Mn, Cu, K and Ca was recorded for Boron treatments alone or low dose of B/Zn combinations. This may also support the suggestions of Blaster-Grill et al.[5] and Cseh and Fodor[23] that boron has the key role in membrane stability, which leads to optimum ion uptake and consequently, optimum ion concentrations in the plant tissues.
| Fig. 3: | Iron, Mn, Cu, K and Ca concentrations in the shoots of wheat plants grown on low or high CaCO3 level in the soil as affected by foliar fertilization of boron, zinc and their combinations |
Growth: Growth expressed as fresh and dry weights of wheat shoots grown on soil contains low lime level was not affected with different B and Zn treatments. However, boron/zinc treatments could significantly increase the biomass accumulation in wheat shoots grown on soil contains high lime (Fig. 5). The best treatments were the low B-dose (25 ppm) and the combination of low doses of both B and Zn (25 ppm B + 50 ppm Zn). It was also found that combination of low B (25 ppm) + high Zn (100 ppm) led to a significant growth inhibition. Positive growth increases in the plants grown under high lime stress conditions indicate that boron may plays a role in increasing nutrient availability to be taken up by plants[12] through membrane stability, which leads to optimum ion uptake[5] and enhancing of ATPase and redox system[7]. This leads to the suggestion that boron may be considered as master of nutrient regulation and uptake mechanisms in plants.
| Fig. 4: | Iron, Mn, Cu, K and Ca uptake by the shoots of wheat plants grown on low or high CaCO3 level in the soil as affected by foliar fertilization of boron, zinc and their combinations |
| Fig. 5: | Fresh and dry weights of wheat plants grown on low or high CaCO3 level in the soil as affected by foliar fertilization of boron, zinc and their combinations (bars with same letters are not significantly different, P=0.05) |
From the present work, it can be concluded that:
| • | Boron foliar treatment can increase B and Zn concentrations and uptake in shoots of wheat plants grown on soil contains low or high lime level. However, uptake increase percentage by the shoots grown on soil contains high lime level is much higher than that of those grown on soil contains low lime level. |
| • | A dose as low as 25 ppm boron or 25 ppm Boron +50 ppm zinc in the spray solution can increase nutrient ion concentrations and uptake, plant tolerance to lime stress conditions which leads to a good growth. |
| • | Since boron was found to control most of elemental uptake and assimilation in the shoot tissues, it can be suggested that boron is the master key of nutrient regulation and uptake mechanisms in the plants. |
ACKNOWLEDGMENTS
The authors wish to thank Prof. Dr. Heiner Goldbach, head institute of Plant Nutrition, Bonn University, Germany and his staff for their sincere help during the course of the study. Thanks are also due to the staff members of the Micronutrients Program, NRC, Egypt for facilitating samples analysis Deep gratitude from the first author to the German Academic Exchange Service (DAAD) for financing his stay in Germany during the course of the study.