An Assessment of the Growth of Irvingia gabonensis (Aubry-Lecomte Ex O’Rorte) Bail Seedlings as Influenced by Crude Oil Contamination of Soil

MATERIALS AND METHODS

The study was conducted in the teaching and research farm of the Delta State University, Asaba Campus, Delta State, Nigeria. Asaba (latitude 6° 14’ N, longitude 6° 49’ E, temperature 28 ± 6°C, rainfall 1505-1849 mm, relative humidity 69-80% and monthly sunshine 4.8 h) is located in the rainforest agro-ecological zone (Asaba Meteorological Bulletin, 2003).

The mature fruits of Irvingia gabonensis were harvested from selected tree in Delta State University, Asaba Campus. The seeds were mechanically depulped and sun dried for two days. The seeds were later presoaked in water for four hours following the method of Elias et al. (1999). The viable seeds, determined by simple flotation technique were then sown in the nursery.

The crude oil used (with specific gravity of 0.8768 g cm^-1) was obtained from the Nigerian National Petroleum Corporation (NNPC), Warri, Delta State.

The soil mixture used was a 2.1 ratio of the topsoil to well decomposed organic manure. The mixture was air-dried and passed through 2 mm sieve. The soil mixture 1.00 kg (1,000 g) set aside for each treatment was thoroughly mixed with the appropriate volume of crude oil before the poly-pots were each filled with 1.00 kg weight of the oil contaminated soil. The crude oil pollution contamination levels used were 0.00% (control), 2.19, 4.38, 6.58 and 8.77% of the oil per wet weight (% w/w) of soil respectively.

The experimental set-up comprised five treatments; each consisted of 14 poly-pots with three replications. The experimental design adopted was a randomized complete block design. A seedling of Irvingia gabonensis in the nursery was transplanted into the poly-pots at 11 weeks old and watered to field capacity immediately and afterwards, every other day until the end of the trial following the procedure of Agbogidi and Ejemete (2005) and Agbogidi and Eshegbeyi (2006). The set-up was monitored for 10 Weeks After Transplanting (WAP) while growth characters were measured fortnightly with effect from the second week after transplanting. Plant height was measured with a meter rule at the distance from the soil level to terminal bud. The number of leaves was determined by counting while the leaf area was determined by tracing the leaves on a graph paper and the total leaf area per plant was obtained by counting the number of 1cm squares. Collar girth was measured with veneer calipers. At the end of the experiment (10 WAT), the seedlings were harvested and separated into leaves, stems and roots. They were oven-dried at 85°C for 22 h following the method of Anonymous (1966) after which, the dry weights were taken. Plant variables were analysed with analysis of variance while the Duncan’s multiple range test was used to evaluate statistical differences among the significant means.

RESULTS

The height measurements of Irvingia gabonensis seedlings are indicated in Table 1. Significant reductions (p = 0.05) in height were observed for all treatments relative to the control at each time interval. At the 2.19% oil treatment, a height of 36.68 cm was recorded compared to the control of 38.44 cm. This result showed that the height of the control plants was significantly greater than those sown in soil treated with crude oil (p = 0.05). The rate of increase in the height of plants was also less in the soil treated with 4.38, 6.58 and 8.77 % oil compared to that of the control. Growth stunting was observed for seedlings subjected to 6.58 and 8.77% treatments as from the 8th weeks after transplanting.

The effects of crude oil contamination level on the number of leaves of I. gabonensis seedlings are presented in Table 2. Crude oil application to soil at all intensities resulted in a significant reduction (p = 0.05) in the number of leaves when compared with the seedlings grown in the control soils. Defoliation or leaf drop, leaf burnt, leaf withering and suppression of leaf initiation were evident in Irvingia gabonensis seedlings grown in soils amended with higher oil levels.

Crude oil contamination of soil significantly reduced (p = 0.05) total leaf area in the seedlings exposed to crude oil amendment compared to their counterparts in the control. The leaf area of the control plants was significantly greater (p = 0.05) than that of plants grown in soils amended with 2.19% crude oil (59.81 cm²) and 4.38 and 6.58% oil treated soil (54.10 cm²) and (48.81 cm²) respectively at p = 0.05 (Table 3). No significant differences existed in the leaf area of I. gabonensis seedlings grown in 6.58 and 8.77% oil at the 5% probability level.

There was a progressive increase in the collar girth of I. gabonensis seedlings grown in the uncontaminated soil (0.00%) and soils treated with 2.19% crude oil (Table 4). No significant differences were observed in seedlings grown in the control and those subjected to 2.19% oil at p = 0.05. A negative relationship was however observed in the collar girth of the seedlings with increasing level of oil in the soil.

There was a significant decrease (p = 0.05) in the dry weight accumulation of the seedlings with an increase in oil levels (Table 5). This result showed that a negative relationship has been observed between the dry matter yield of I. gabonensis seedlings and the concentration of the oil applied to the soil.

Table 1:	Height (cm) values of Irvingia gabonensis seedlings as influenced by various levels of crude oil contamination of soil
Means with different superscripts are significantly different at p = 0.05 using Duncan’s Multiple Range Test (DMRT)

Table 2:	Number of leaves of Irvingia gabonensis seedlings as influenced by various levels of crude oil contamination of soil
Means with different superscripts are significantly different at p = 0.05 using Duncan’s Multiple Range Test (DMRT)

Table 3:	Leaf area (cm2) of Irvingia gabonensis seedlings as influenced by various levels of crude oil contamination of soil
Means with different superscripts are significantly different at p = 0.05 using Duncan’s Multiple Range Test (DMRT)

Table 4:	Mean collar girth (cm) values of Irvingia gabonensis seedlings as influenced by various levels of crude oil contamination of soil
Means with different superscripts are significantly different at p = 0.05 using Duncan’s Multiple Range Test (DMRT)

Table 5:	Mean dry weight (g) values of Irvingia gabonensis seedlings as influenced by various levels of crude oil contamination of soil
Means with different superscripts are significantly different at p = 0.05 using Duncan’s multiple range test (DMRT)

DISCUSSION

Results obtained from this study indicated that crude oil of different concentrations adversely affected the performance of the test plant. Mallallah et al. (1995) observed a 64.8% decrease in the growth of Vicia faba in the presence of crude oil while Anoliefo (1991) reported that the seeds of Citrullus vulgaris exposed to crude oil for five minutes and above did not develop beyond radicle and plumule emergence. Nasamu (1996) reported a general depression in the root growth of Colocasia esculenta and Xanthosoma sagittifolium. Gill et al. (1992) and Bamidele and Agbogidi (2000) reported that a reduction in shoot growth following oil pollution is a direct result of a reduction in root growth probably because roots are important organs for the absorption and translocation of water and mineral nutrients (Gill et al., 1992). Asuquo et al. (2002) maintained that the growth of okra and fluted pumpkin decreased significantly with an increase in crude oil concentrations. The observed leaf drop and leaf withering followed by a reduction in all the assessed growth variables including dry matter yield could be attributed to heavy metal toxicity. This finding agrees with the report of Khan and Frankland (1983). Ochiai (1995) maintained that heavy metal toxicity as well as the difficulty of plants to obtain sufficient water and mineral nutrients from oil polluted soils normally account for their reduced growth characters and poor performance. Absorption of toxic oil fractions through the roots could cause poisoning of plant by disruption cell membranes and cellular organelles (Getter et al., 1984) required for vital metabolic processes. Reduced oxygen exchange between the soil and the atmosphere could have affected root function thereby leading to reductions in the growth responses assessed.

Reduction in the dry matter yield of I. gabonensis seedlings subjected to higher pollution levels was expected as the plants had stopped growing normally. The observed reduced biomass production with increasing oil intensity confirms the findings of Baker (1968, 1970) who showed that as hydrocarbons from oil polluted soil accumulate in the chloroplast of leaves, photosynthetic ability of the leaves becomes reduced, affecting translocation in affected plants probably due to obstruction of the xylem and phloem vessels hence reduction in photosynthate and dry matter content. Reduced dry weight of plants including forest species following crude oil pollution of soil has earlier been reported by Culter et al. (1991), Agbogidi (2003) and Agbogidi and Ofuoku (2005).

The present study has demonstrated that crude oil contamination of soil has a highly significant effect of reducing the growth responses of Irvingia gabonensis seedlings. It also indicated that I. gabonensis has some bioremediation potentials especially at low concentrations in crude oil impacted areas.