Abstract: Effects of crude oil application on the growth of Rhizophora racemosa seedlings were conducted at undisturbed tidal inundated mangrove wetland, beside the Chevron yard along Escravos River in Warri, Delta State. Seedlings of R. racemosa were subjected to four crude oil treatments viz., control (0 mL), moderate (20 mL), chronic (40 mL) and acute (100 mL), in a randomized complete block design with five replicates. Treatments were applied weekly for moderate and chronic treatments while acute treatment was a single dose using ring method. The growth indices assessed after 12 weeks were plant height, stem girth, number of leaves, leaf length, leaf width, biomass and total chlorophyll of R. racemosa in soil containing varying treatments. The results showed that mangrove seedlings, R. racemosa were sensitive to and were negatively impacted by moderate, chronic and acute exogenous application of escravos crude oil when compared with the control. The results showed that the control treatment gave the highest mean value for plant height and stem diameter of seedlings (40.98±1.783, 0.80±0.058), respectively when compared with other treatments. The number of leaf drop, biomass and chlorophyll content showed a similar trend, respectively. There were significant difference among treatments at p<0.05. This study provides insight on the potential risk associated with R. racemosa seedlings development exposed to crude oil contamination. The study recommends that crude oil pollution should be stop or minimized to the barest minimum during oil exploration and extraction in the mangrove environment.
INTRODUCTION
Rhizophora racemosa G. Meyer belongs to the red mangrove family Rhizophoraceae which comprises other species such as Rhizophora mangle and Rhizophora harrisonii, widely distributed across the East Atlantic regions of the world and they constitute over 60% of the mangrove vegetation of Nigeria Niger-delta region (NDES, 1996). Mangrove vegetation provides shoreline protection to coastline communities and provide critical habitat for diverse marine and terrestrial flora and fauna. R. racemosa grows mostly in saline coastal habitats in the tropics and sub tropics. Mazda et al. (2005) reported that R. racemosa tolerate wide range of saline conditions, ranging from brackish water with moderate salinity of ocean or sea water, where the salt becomes concentrated by evaporation. Choudhry (1997) reported that R. racemosa thrives mainly in the mangrove environment because of its ability to accommodate high salt content and compensate low oxygen levels available in the mangrove ecosystem.
Crude oil plays an important role in Nigeria economy and over 70% of oil exploration and exploitation activities takes place in the mangrove areas of the Niger-Delta region as reported by Chindah et al. (2007). The effect of crude oil pollution on plant, have been shown to be directly proportional to the volume of spill (Baker, 1981). Amakiri and Onofeghara (1983) also reported that oil tends to act as a physical barrier preventing entry of essential factors and nutrients such as water and oxygen that is required for plant growth and development. However, when the environment is perturbed, the flora and fauna stand the risk of being affected. Hence, the study, therefore, seeks to determine some effects of crude oil application on the growth of mangrove seedling of R. racemosa and scientifically deduce the potential to tolerate some levels of pollution and thus promote its awareness.
MATERIALS AND METHODS
Sampling Site and Sample collection: The sampling sites included a relatively undisturbed tidal inundated mangrove wetland, beside the Chevron yard along Escravos River in Warri, Delta State. The method of Chindah et al. (2007) was adopted for the collection of “chikoko soil” and establishment of R. racemosa seedlings. These seedlings were allowed to acclimate for 30 days. The seedlings were arranged in four rows of parallel with five replicates laid at 1m intervals for each treatment (control, moderate, chronic and acute).
Application of treatment: Treatment was by applying the crude oil (Escravos Crude oil) that commenced at the end of 30 day acclimation period. The Escravos Crude oil constitutes of asphaltenes (0.03%), 1190 ppm Nitrogen, 4.55 ppm Nickel, 0.51 ppmVanadium, 11.7 characteristic factor K, 33.7° API gravity, 0.86 specific gravity and contains 0.16% sulphur content (Crude Oil Marketing, 2008). Seedlings of R. racemosa were subjected to four treatments (control, moderate, chronic and acute). The acute treatment consisted of a one-time application of 100 mL crude oil added on the surface of the soil. The moderately chronic treatment consisted of weekly application of 20 mL while the highly chronic treatment consists of weekly application of 40 mL of the same crude oil. The control was without addition of any oil.
Growth indices measured: The growth parameters assessed were plant height, stem girth at the first inter-node, number of leaves, leaf area and biomass of R. racemosa in soil containing varying treatments. Total chlorophyll of R. racemosa was determined using the method of Allen et al. (1984). The growth of seedlings was monitored weekly for 12 weeks and yellowing of leaves was recorded.
Statistical analysis: Data obtained were subjected to statistical analysis at the probability of 95% confident limit using Analysis of Variance (ANOVA).
RESULTS
The effects of exogenous application of crude oil on R. racemosa seedlings are shown in Table 1. The results varied according to treatments as follows: plant height: control (40.98±1.783)>chronic (35.46±0.656)>moderate (34.72±0.845)>acute (27.38±0.526); stem diameter: control (0.80±0.058)>moderate (0.58±0.040)>acute (0.52±0.024)>chronic (0.48±0.00); leaf length: control (9.18±0.086)<moderate (9.48±0.150)<acute (9.66±0.139)<chronic (10.44±0.063); leaf width: control (3.12±0.04)>moderate (3.10±0.068)>chronic (3.10±0.080)>acute (2.94±0.051); number of leaves: control (32.2±3.980)>moderate (31.0±2.786)>acute (25.2±2.040)>chronic (19.6±2.040).
| Table 1: | Effect of Escravos crude oil on some growth indices of R. racemosa after 12 weeks |
| Mean±standard error | |
The number of leaf drop, biomass and chlorophyll content showed a similar trend. The plant height and stem diameter of seedlings of the control treatment gave the highest mean value when compared with other treatments. The number of leaf drop, biomass and chlorophyll content showed a similar trend. The study showed that mangrove seedlings, R. racemosa exposed to varying crude oil pollution gave rise to hampered growth with respect to the plant height, stem diameter, leaf development when compared with the control. The results obtained did not show a particular trend for the growth indices assessed.
DISCUSSION
The plant height showed a decreasing order of growth performance as the quantity and concentration of crude oil application increases as reflected in the results obtained: control (17.52%)>moderate (14.34%)>chronic (13.93%)>acute (13.51%). These results indicated that the quantity of crude oil applied impacted negatively on the plant height of R. racemosa. The number of leaves also showed a similar trend with the plant height: control (39.75%)>moderate (30.97%)>chronic (42.86%)>acute (30.16%). Stem diameter showed varied trend as plant height: control (40%)>moderate (26.92%)>acute (26.92%)>chronic (20.83%). It indicated a progressive reduction with increasing concentration of crude oil treatments. Baker (1981) and Duarte et al. (1998) in their separate works observed retardation effect of crude oil on seedling and plant development. Amadi et al. (1996) reported that crude oil application on the soil reduces sediment porosity and gaseous exchange that in turn have negative effect on the physiological function of the plant. Similarly, Page et al. (1985) found out that petroleum hydrocarbon induces stress in salt-extracting plants like R. racemosa, by disrupting the ability of the roots to exclude ions from sea or brackish water. Chindah et al. (2007) also reported that the seawater cation would be elevated in the tissues of mangrove plants unable to exclude salt efficiently in their roots and this situation could be attributed to the polycyclic aromatic components associated with crude oil. Proffitt et al. (1995) earlier reported that increasing aromatic content of the crude oil confers on it, its toxicity hence its herbicidal effect.
However, other studies on crop plants have indicated similar negative growth pattern on plant survival and biomass production (Merckl et al., 2005). Okonwu and Amakiri (2009) reported that crude oil application affects the growth and development of IT84S-2246 variety of cowpea, Vigna unguiculata. Okonwu et al. (2010) also reported that crude oil application on Zea mays either by spraying or spilling caused reduction of plant chlorophyll content, growth retardation, defoliation, damage of plant tissue and eventual death. Plants show sensitivity to crude oil pollution treatment. These scenarios suggest that crude oil have negative consequences both on R. racemosa seedlings, mangrove plants and other crops.
CONCLUSION
The study showed that mangrove seedlings, R. racemosa exposed to varying crude oil pollution gave rise to hampered growth with respect to the plant height, stem diameter, leaf development when compared with the control. Hence crude oil pollution has severe effects on the growth and development of R. racemosa seedlings.