EFFECT OF PLANT HORMONES ON THE GROWTH AND NUTRIENT UPTAKE OF MAIZE IN ACIDIC SOILS OF THE HUMID TROPICS

Thesis Overview

<p><b>1.0 INTRODUCTION&nbsp;</b></p><p><b>1.1 BACKGROUND STUDY</b></p><p>Besides genetic and climatic factors, the growth and yield of crops are mainly determined by the amount of nutrients available in the soil (Jungk and Rademacher, 1983). The vast area of tropical soils of the humid tropics are acidic. A number of studies have shown that high proportions of soils which belong to the great soil groups of Oxisols and Ultisols have a marked ability to fix applied inorganic P, and usually have low extractable P (Sanchez and Salinas, 1981). Higher soil acidity is associated with increases in precipitation, leaching, weathering, hydrolysis, organic matter, nitrification, fertiliser application, oxidation of sulphides, uptake of ions, and more (Adams, 1984). Phosphate can readily be rendered unavailable to plant roots because it is the most immobile of the major plant nutrients and whose efficiency can be affected by P fertiliser distribution and distance of application from the plant (Eghball and Sander, 1989). The quantity of P in soil solution is in the range of 0.3 to 3 kg P2O5 ha-1 as growing crops absorb about 1 kg P2O5 ha-1 per day. The labile fraction in the topsoil layer of 20 cm is in the range of 150 to 500 kg P2O5 ha-1, which could replenish soil solution P (Mengel and Kirby, 1996). The phosphate concentrations of the soil solution and its buffering capacity are the most important parameters governing the P supply to plant roots. Thus, the rate of desorption is higher in soils with a higher phosphate buffer capacity (Dear et al., 1992). The consideration of significant varietal and species differences in tolerating low available P2O5 and low pH effect is also important. At similar yield levels, upland rice usually requires less P than maize. The general recommendation for these crops, in acidic soils, ranges from 100 to 150 kg P2O5 ha-1 for maize and 0 to 60 kg ha-1 for upland rice (Sanchez, 1976). The low productivity and yields of crops, in acidic soils, can mainly be attributed either to the deficiency of nutrients such as P, Ca and Mg or to low pH and toxicity of Al, Fe and Mn (Soon, 1991; Marschner, 1995). Consequently, the application of lime to acid soils can displace P2O5 from precipitates of Al and Fe-phosphate, making the exchange sites more active through the improvement in physico-chemical properties of such soils and culminates in the replenishment of Ca and Mg (Sommer, 1979; Crizaldo, 1981). The first observable effect of Al on plants is a limitation in root growth. Root tips and lateral roots become thickened and turn brown and the uptake and translocation of P to the upper plant parts are affected. The toxicity in the tops is often characterised by symptoms similar to those of phosphate. In the plant Al may interfere with the P metabolism by the formation of stable Alphosphate complexes (Sommer, 1979; Marschner, 1995). Plant growth regulators make plants use nutrients more efficiently by exploiting their genetic and physiological potentials on a higher level (Jungk and Rademacher, 1983). Plant hormones are able to influence growth and differentiation in plants without having a nutritive character. By promoting, inhibiting or modifying the physiological processes of plants, results might be gained which directly or indirectly lead to higher yields (Koter et al., 1983). Caldiz et al. (1991) found that foliar application of N and benzyladenine (BA) on wheat delayed chlorophyll loss and increased grain protein but not yield. Similarly, the application of Gibberellins (GAs) has remarkable effects on the elongation of primary stalk, on the growth of dwarf plants and development of side branches (Nickell, 1983; Ross et al., 1993). This effect occurs in the young tissues and growth centres and is caused by an increase in the rate of cell division (Nickell, 1983). Therefore, the objectives of this study were to optimise the availability of phosphate fertiliser using buffering material and plant hormones for acidic and P deficient soils, and to investigate the effect of exogenous plant growth regulators on the relative dry matter yield and nutrient uptake of maize.&nbsp;</p><p><b>1.2 MATERIALS AND METHODS&nbsp;</b></p><p>Greenhouse experiment An experiment was conducted in a greenhouse at the Institute of Agricultural Chemistry, University of Bonn, Germany to study the response of maize Zea mays L. (cv. Golda FAO-240) to phosphorus fertiliser and plant hormones on acidic soils of Liberia. The temperature of the growing room was kept constant at 20°C. Sub-samples of 1.7 kg of air dried soil, screened through a 4 mm sieve, were placed into plastic pots of 18 and 10 cm upper and lower diameter size, respectively. Each pot was watered to 70% water holding capacity of the soil. The treatments included 7 structurally selected combinations of 3 levels of phosphorus (0, 26 and 52 mg P 1.7 kg-1 soil) in the form of Monoammonium dihydrogen phosphate (NH4H2PO4) with 1 mg benzyladenine (BA) and 2 mg gibbberellin (GA) applied as depot 10 cm deep in the soil after mixing with 10 g neutral compost. The experiment was laid out in completely randomised design with four replications. The pH of the experimental soil was 5.0 in the 0.05 M K2SO4 which was attained by liming before planting on the basis of 200 mg exchangeable Ca 100 g-1 soil by adding 1.05 g Ca(OH)2 and 12.2 g CaSO4 × 2H2O 1.7 kg-1 soil according to Jensen curve. Nitrogen (N), potassium (K) and magnesium (Mg) were applied uniformly to all pots in two split applications, i.e., 0.200 g N 1.7 kg-1 soil at planting in the form of CO(NH2)2 and ammonium from NH4H2PO4 and 0.200 g N 1.7 kg-1 soil 3 weeks after planting as Ca(NO3)2. Likewise, 0.249 g K and 0.09 g Mg 1.7 kg-1 soil were applied at planting as K2SO4 and MgSO4 x 7H2O, respectively, and 0.166 g K and 0.06 g Mg 1.7 kg-1 soil in the same form 3 weeks after planting (Table 1). The required micronutrient elements, namely copper (Cu), manganese (Mn), zinc (Zn), boron (B) and molybdenum (Mo), were also applied uniformly to all pots after mixing 3 ml of the original solution from each element. Fifteen ml per pot was applied from the mixture prepared in the proportion of 7.21 g CuSO4.5H2O, 24.60 g MnSO4.H2O, 35.19 g ZnSO4, 11.43 g H3BO3 and 3.68 g (NH4)6Mo7O24.4H2O each in 2000 ml water. Eight seeds were sown in each pot at a depth of 2.5 cm. The date of emergence, in which 50% of the seedlings appeared above the surface of soil and plant height scores as the mean of 4 randomly selected plants from each pot were recorded just prior to harvesting. Fifty-five days after planting the above ground portion of each plant was harvested, and the dry weight was recorded. <br></p><p> Table 1. Plant nutrients applied as elemental and fertiliser form. Nutrient elements* Amount (g pot-1) Fertiliser form Amount (g pot-1) P 0.026 NH4H2PO4 0.0972 P 0.052 NH4H2PO4 0.194 N 0.194 CO(NH2)2 0.416 N 0.188 CO(NH2)2 0.404 K 0.249 K2SO4 0.560 Mg 0.090 MgSO4.7H2O 0.920 Ca 0.565 Ca(OH)2 1.05 Ca 2.840 CaSO4.2H2O 12.20 N 0.200 Ca(NO3)2.7H2O 1.686 K 0.166 K2SO4 0.38 Mg 0.060 MgSO4.7H2O 0.61 *15 ml pot-1 mixture of micronutrient elements were added. <br></p><p> <b>Plant and soil analysis&nbsp;</b></p><p>After harvesting, the plant materials were dried in an oven at 70ºC to 95ºC up to a constant weight. Oven dried samples were ground to pass a 1 mm sieve for macronutrient analysis. One gram of finely ground sub-samples were ashed at 550ºC for 4 hours in a muffle furnace. After dry oxidation, the samples were treated with concentrated NH4NO3, evaporated to dryness at 100ºC and placed back carefully in the muffle furnace at 550°C for 2 hours. After cooling, the samples were boiled by adding 5 ml HCl and transferred to a 100 ml measuring flask. Then, the samples were filled up to 100 ml with distilled water, mixed thoroughly and filtered with ash free filter paper. The aliquots from this were used for the determination of P, K, and Mg after suitable dilution. Then, phosphorus was determined colorimetrically following the ascorbic acid method (John, 1970). Potassium was determined by flame photometer and Mg by atomic absorption spectrophotometer. The total uptake of nutrients was calculated by multiplying the percent nutrient concentrations with dry matter yields on oven dried weight basis. After harvesting the aboveground portions of plants at soil level, the roots were picked from the soil and samples of all soils were collected. Air dried soil samples were ground to pass through a 2 mm sieve and kept for analysis. The measurements of pH were conducted in H2O, 0.01 M CaCl2 and 0.1 N K2SO4 solutions with a liquid to solid ratio of 2.5:1 by using glass electrode. The concentrations of P and K in the soil were determined following CAL (Calcium Acetate Lactate) method (Schüller, 1969). Then, P and K were determined by colorimeter and flame photometer, respectively. Likewise, the concentration of Mg was determined according to Schachtschabel (1976) by using atomic absorption spectrophotometer. The concentrations of P and K 100 g-1 soil were evaluated by deducting the measured values of the checks from the samples.&nbsp;</p><p><b>1.3 Data analysis</b>&nbsp;</p><p>The data were subjected to analysis of variance using the SAS statistical package version 8.2 (SAS Institute Inc., 1999–2001) and presented as means separated by the Duncan’s Multiple-Range Test (DMRT). Coefficients of correlation and orthogonal contrasts were performed using the standard procedures from SAS programs at P &lt; 0.01 probability level. To evaluate the effects of treatments on plant growth and nutrient uptake six single degrees of freedom (df) orthogonal contrasts were partitioned from seven structured treatments.&nbsp;</p><p><b>1.4 RESULTS AND DISCUSSION</b>&nbsp;</p><p>Dry matter yield and nutrient uptake The placement of phosphorous as mineral fertiliser with 10 g neutral compost at a depth of 10 cm in the soil resulted in a substantial increase in plant height and shoot dry matter yield (Table 2). The use of neutral compost to apply P-fertiliser reduces the absorption of phosphate thereby improves the root-fertiliser contact and optimises P availability to plants. Analysis of variance indicated that plant height, dry matter yield and uptake of nutrients by plants were significantly (P &lt; 0.01) affected by phosphorus application but not by plant hormones. Plant height and shoot dry matter yields were higher by about 32 and 38%, respectively, due to P application than the control. However, a lower increase in plant height (19%) and dry matter yield (26%) occurred due to the treatment of both phosphorous and plant growth regulators as benzyladenine (BA) and gibberellin (GA) over the control as compared to only the same rate of P application. Similarly, Koter et al. (1983) reported that the application of kinetin on maize plants prolonged vegetative growth, which resulted in decreased grain yield. The investigation of Caldiz et al. (1991) also indicated that foliar application of BA and nitrogen on wheat plants delayed chlorophyll loss in the flag leaf but modified neither yield nor yield components. Benzyladennine (BA) increased only grain protein. Plant Puptake efficiency from fertiliser increased when phosphate was applied as depot close to the plants, because this increased the contact between P and roots of plants at an early stage of growth. <br></p>