Abstract: Field experiments were conducted at the Delta State University, Asaba Campus Teaching and Research Farm and at Delta State Polytechnic, Ozoro during the 2003 and 2004 cropping seasons to evaluate the effects of time of application of crude oil to soil on the growth of seven maize varieties viz., composite suwam 1, Hybrid 3x-yx, AMATZBR w, TZBRSYN w, AMATZBR y, TZBRSYN y and Ozoro local. 0, 5.2, 10.4, 20.8 and 41.6 mL of the oil per stand of maize served as the treatments. The experiment was laid out in a split-split-plot design with four replications. Results obtained showed that soil treatment with crude oil at four Weeks after Planting (4 WAP) died within 24 h while the plants without crude oil treatment remained intact. At 6 WAP, only the maize varieties subjected to 41.6 mL of the oil withered while composite (suwan 1), Hybrid 3x-yx and the Ozoro local withered and died on exposure to 20.8 mL of the oil. No significant differences (p≥0.05) were observed in the plants height, leaf area and stem diameter of the maize varieties tested at 8 WAP. This study has shown that the time of application of crude oil to soil has a significant effect on the growth of maize.
INTRODUCTION
Since the discovery of oil in Nigeria, the nation prospered through the petroleum industry, while for the oil producing communities, it is a story of untold socio-economic and environmental devastation through spills, pipeline explosion, destruction of the ecosystem and pollution (Agbogidi et al., 2005). Nigeria is basically an agricultural country whose rural population is engaged largely in farming.
Maize is commonly grown in the Niger Delta-the hub of Nigeria’s oil industrial activity. Though frequent spills have been reported (Awobajo, 1981; Ekekwe, 1981; Nwankwo and Ifeadi, 1983; Freedman, 1991), very few systematic studies have been conducted to evaluate the effects of crude oil on cereals especially maize. The objective of this study was to determine the effect of time of application of crude oil to soil on the growth of maize with a view to recommending the same to maize growers in oil producing areas of Nigeria.
MATERIALS AND METHODS
The study was carried out in the Teaching and Research Farm of the Department of Agronomy, Delta State University, Asaba Campus, Delta State, in 2005. Asaba is located at latitude 06°14’N, longitude 06°49’E of the equator (Asaba Meteorological Bulletin, 2003). Asaba lies in the tropical rainfall zone. The rainy season is usually between April and October, with an annual rainfall range of 1505 to1849.3 mm. The mean temperature is 28±6°C. The relative humidity is 69-8% and the monthly sunshine is 4.8 bars (Asaba Meteorological Bulletin, 2003). The other location was Ozoro (Delta State College of Agriculture Research Farm) in Isoko-North Local Government Area of the State. The area lies between latitude 6°131E and longitude 5°331N and it is under the rainforest ecological zone. Ozoro experiences double peak periods of rainfall i.e., between June/July and September /October, respectively. The annual rainfall is 2800 mm while the mean annual temperature is 31°C and relative humidity 76-90% (College of Agriculture Meteorological Station, Ozoro, 2003). Ozoro is an oil producing community with reported case of oil spillages.
A split-split-plot design with four replicates was used at each location for each set of maize varieties. Between row spacing was 0.75 m and within row spacing was 0.25 m. Two seeds were planted and the maize seedlings were thinned to one plant per hill or stand when seedlings were 12-15 cm tall or at knee level.
The experimental area was hoe-weeded as and when due to enable the plants develop under non-limiting conditions. In order to determine the minimum level of contamination that will cause damage to the plants, low levels of 0.0 (control), 5.2, 10.4, 20.8 and 41.6 mL of crude oil were applied per stand. Six stands of maize plants per sub-sub-plot per treatment were used. The oil was slowly poured from a beaker into the soil around the maize stand (ring application) at 3 WAP. Applications were carried out at 5 and at 7 WAP on fresh sets of maize plants. The volumes of crude oil used were 5.2, 10.4, 20.8 and 41.6 mL/plant. There was a control without crude oil. Visual observations of growth and development followed. Data were taken both from the contaminated and uncontaminated sub-sub-plots.
Parameters assessed were plant height (cm), leaf area (cm2) and stem diameter (collar girth) (cm). Plant height was measured from soil level to terminal bud using a measuring tape. Leaf area was derived from the length and breath measurements of the longest leaf per plant per sub-plot and correction factor value of 0.75 was used to multiply the value of the length times breathe following the procedure of Curnard (1971). The stem diameter was determined with the use of calibrated veneer caliper. Data collected were subjected to analysis of variance and the treatment means were separated with the Duncan’s multiple range tests using SAS (1996).
RESULTS AND DISCUSSION
The exposure of the maize seedlings to the crude oil at three weeks after planting showed mortality even at low concentration All the three weeks old maize seedlings from the seven varieties died within 1-24 h of the oil application with no yellowing or wilting symptoms for about 36 h after the fall. The rate and time of plant drooping were dependent on the oil concentrations. Plants subjected to higher levels of 20.8 and 41.6 mL of oil drooped earlier than those exposed to 5.2 and 10.4 mL of the oil at both locations. At 48 h after oil application, wilting of collapsed seedlings was observed and it was almost completely severe at the zone of disrupted tissue. At 72 h after crude oil application, there was observed withering (yellowing and death of the seedlings).
Five weeks old maize plants exposed to crude oil did not succumb to the injurious effect as rapidly as those exposed to the experimental spill at 3 WAP. At higher treatment levels (20.8 and 41.6 mL), their drooping and eventual death stretched over a period of 2-4 days. Although maize varieties AMATZBR w, TZBRSYN w, AMATZBR y and TZBRSYN y did not droop and die at 20.8 mL oil application, yellowing of the lower leaves was observed about two weeks after oil application. Other morphological observations included root rot, stem rot, drying of lower leaves and leaf burnt. Crude oil was observed to have noticeable effects on the morphology, growth and vigour of the plants.
The mean plant height values with the application of 5.2 mL of crude oil at 6 WAP were significantly greater than those with the treatments including the control plants at p≤0.05 (Table 1).
| Table 1: | Effects of different crude oil levels on plant height (cm) of maize at 6 WAP within Anwai and Ozoro locations |
| Means in the same column with same letter(s) are not significantly different (p≥0.05), using DMRT | |
| Table 2: | Effects of different crude oil levels on leaf area (cm2) of maize at 6 WAP within Anwai and Ozoro locations |
Means in the same column with same letter(s) are not significantly different (p≥0.05), using DMRT | |
Death of Composite (suwan1), Hybrid 3x-yx and Ozoro local was also observed on exposure to 20.8 mL of the crude oil while none of the maize varieties survived when subjected to 41.6 mL of the crude oil at both locations (Table 1). The leaf area (Table 2) and the stem diameter (Table 3) of the maize seedlings at 6 WAP followed the same trend in response to the application of different levels of crude oil as that obtained for the plant height (Table 1). The effects of different crude oil pollution levels on plant height, leaf area and stem diameter of the seven varieties of maize at 8 WAP are indicated in Table 4-6, respectively. No significant differences (p≥0.05) were recorded in the growth variables of the maize varieties exposed to crude oil treatments in both Anwai and Ozoro locations at 8 WAP. At 8 WAP, crude oil treatment of soil had no significant effects on the growth, morphology and vigour of the maize plants.
Three weeks old maize plants were more susceptible to crude oil effects than five and seven week old plants in the varieties studied at Anwai and Ozoro locations. The maceration of tissues of the basal stem segment of three weeks old maize plants where they had contact with experimental spill, possibly resulted in leakage of cell materials, loss of turgor and inevitable collapse of the tender plants. It also suggests damage to cell membrane by penetrating hydrocarbon molecules. Baker (1970a) and Gill et al. (1992) reported penetration of living cells/tissues by oil while Van Overbeck and Blondeau (1975) maintained that hydrocarbons dissolve the plasmalemma and open it up by displacing fatty molecules.
| Table 3: | Effects of different crude oil levels on stem diameter (cm) of maize at 6 WAP within Anwai and Ozoro locations |
| Means in the same column with same letter(s) are not significantly different (p≥0.05), using DMRT | |
| Table 4: | Effects of different crude oil levels on plant height (cm) of maize at 8 WAP within Anwai and Ozoro locations |
|
Means in the same column with same letter(s) are not significantly different (p≥0.05), using DMRT | |
| Table 5: | Effects of different crude oil levels on the leaf area (cm2) of maize varieties at 8 WAP at Anwai and Ozoro |
| Means in the same column with same letter(s) are not significantly different (p≥0.05), using DMRT | |
| Table 6: | Effects of different crude oil levels on the stem diameter (cm) of maize varieties at 8 WAP at Anwai and Ozoro |
| Means in the same column with same letter(s) are not significantly different (p≥0.05), using DMRT | |
The young and tender tissues at the base of the three weeks old maize plants seemed particularly susceptible to this injurious effect. These tissues could have ruptured accordingly and leakage of cell contents that resulted could have accounted for the slimy feel of this softened zone. In the same vein, severe disintegration of cells could have taken place in the soft, slimy basal stem segment. This finding corresponds with the report of Baker (1970a). The observed faster and earlier drooping and collapsing of three weeks old maize plants exposed to higher levels of oil is in accordance with the findings of Baker (1970a) and Anoliefo (1998) that the age of plant and level of oil pollution seem to be exerting the most influence an agricultural lands and crops. Baker (1970a) further maintained that high doses of oil are more damaging than low doses due to increased anaerobic processes because there is increased utilisation of oxygen during soil degradation and this limits normal diffusion processes. This further confirmed the reports of Udo and Oputa (1984) and Agbogidi and Eshegbeyi (2006) that such poor growth could be attributed to suffocation of plants caused by the presence of inadequate air created by oil pollution.
The relative tolerance shown by the older maize plants (five week old and seven weeks old plants) may be due to their already cutinised root system. Oily scum on soil surfaces would impede oxygen and water. The anaerobic conditions created in subsoil as a result, would aid the persistence of oil. This condition makes water and essential nutrients for growth unavailable to plants (Odu, 1981). This also causes some toxic elements to be more readily available to plants thereby causing reduction in plant growth and or death (Awobajo, 1981). Similarly, death of the three weeks old maize plants subjected to all the levels of the oil including 5.2 mL and those of the five weeks old plants exposed to 41.6 mL of oil in the two locations could be attributed to the phytotoxic, hydrophobic and other stress imposing properties of crude oil. Nelson-Smith (1974) stated that the low boiling points of unsaturated hydrocarbons such as benzene, toluene, xylene and naphthalene were the most toxic components in crude oil. Similar reports have been made by Spreight (1991) and Sheretz (1998). Nelson-Smith (1974) further maintained that toxicity of crude oil was a function of the presence of these substances. The eventual death of the maize plants exposed to 41.6 mL of the oil as observed in this study could have stemmed from the toxicity of sulphides and heavy metals including manganese consequent upon the displacement of air by crude oil. Similar reports have been made by Garner (1971) and Bamidele and Agbogidi (2006). It could be probably due to a softened zone of macerated tissue at the base of seedlings that had contact with the crude oil. A weakened and usually slimy feel was typical of every collapsed seedling where the young stem had contact with the applied crude on the soil surface. Overton et al. (1994) had also observed that aromatic compounds in crude oil appeared to be more toxic to higher plants than aliphatic. While Odu (1972) and McCown and Deneke (1992) also reported poor performance of plants growing in oil-polluted soils, Baker et al. (1993) reported that oil spillage caused death of seedlings and defoliation in mangroves. The death of the maize plants may also be linked to the absorption into the tissues of high aromatic hydrocarbon components, which are very toxic. This could have led to the disturbance of osmotic balance within the plant tissue. This observation agrees with prior findings of Wilkins (1985) and Kinghorn (1989) that an alteration in the osmotic balance of plants results in poor metabolic activities and subsequent death. Crude oil penetration into the lower stem segment of five weeks old maize plants and seven weeks old maize plants could have been extremely slow or tissues in the region could be tougher and more tolerant than the three-weeks old seedlings. It may also be suggested that the combined effects of the residual phyto-toxicity of the applied crude oil and the progressively stressful conditions imposed in their root system by the oil gradually broke the resistance of the three weeks old maize plants hence they easily succumbed.
The observed poor growth in five-weeks old maize plants (AMATZBR w, TZBRSYN w, AMATZBR y and TZBRSYN y) exposed to 20.8 mL of the crude oil may also be due mainly to the difficulties in the absorption of water and nutrients by roots and also to excess of heavy metals for example manganese and iron. Udo and Oputa (1984) maintained that poor growth of plants in oil-polluted soils could be attributed to suffocation or consumption of oxygen by increased microbial activity. The disturbance of the plant-water relationship bringing about interference in the nutrient supply to plants could be considered as the primary cause of the poor plant growth in oil/gas polluted soils. Ogri (2001) posited that petroleum products impact an inhibitory effect on plant growth and development in various ways including inhibition of photosynthesis brought about by cellular impermeability. Growth reduction in crude oil polluted soils as observed in this study may also be attributed to a disruption in aeration and biological properties of the soil. This observation is in line with the findings of Dean (1968). Poor growth has also be related to adverse changes in soil-plant-water relationships and some workers including Adams and Ellis (1960), Schwendinger (1968), Baker (1970a), Terge (1984), Udo and Oputa (1984), Anoliefo and Vwioko (1994), Overton et al. (1994), Bamidele and Agbogidi (2000) and Odjegba and Sadiq (2002) regarded these relationships as the over riding factor accounting for poor growth of plants in oil polluted ecosystems. The researchers also attribute it to an adulterated structure of the soil. There was increase effect of oil pollution in the soil. This observation further confirmed earlier report of Gill et al. (1992) that root stress reduces leaf growth via stomatal conductance.
The observed chlorosis in five weeks old maize plants apart from the three varieties that died when subjected to 20.8 mL of the crude oil two weeks after the crude oil application may be due to chlorophyll destruction and cell injury. Baker (1970b) and Odjegba and Sadiq (2002) have separately demonstrated that oil affects photosynthesis and starch formation because hydrocarbons in oil tend to accumulate in the chloroplast where there is a higher lipid content than in the rest of the cytoplasm. Baker (1970b) further maintained that oil constituents could dissolve in the lipoid phase of the grana, thereby causing an increase in distance between individual chlorophyll molecules and other disruption of sub-microscopic structures required for photosynthetic activities. Yellowing of leaves may also be interpreted as a synergistic effect of some heavy metals present in the soil as a result of crude oil application to soil. This report is in accordance with the work of Bossert and Bartha (1984). Heavy metal build up in soils following oil pollution has also been reported by Gudin and Syratt (1975) Udo and Oputa (1984), Freedman (1991), Nicolotti and Eglis (1998) and Siddiqui and Adams (2002). The observed luxurious growth in five weeks old maize plants, which received 5.2 mL of oil indicated enhancement of growth (fertilizer effect). Growth stimulation of a variety of plants as a result of the presence of crude oil seems to be a fairly usual phenomenon. Schwendinger (1968) reported that plants tolerate up to 3% crude oil pollution by weight while nodule formation, development and growth in soya bean was enhanced in the soil treated with 0.75% of crude oil. This study has shown that time of application of crude oil to soil has a significant effect on the growth of maize.