Physiological responses of Avicennia marina seedlings to the phytotoxic effects of the water-soluble fraction of light Arabian crude oil

Stomatal behavior, growth performance and the accumulation of polynuclear aromatic hydrocarbons (PAHs) were evaluated in seedlings of the mangrove Avicennia marina (Forssk.) Vierh., treated with a water-soluble fraction (WSF) of Abu-Dhabi light Arabian crude oil through foliar spraying or soil application. Irregular stomatal behavior and weak stomatal control over transpiration were observed during the first 24 hours, where stomatal resistances of plants sprayed with 150 and 300 μg PAHs plant^-1 were significantly lower than that of the control plants. After six weeks, all treated plants showed no significant difference in their relative growth rate (RGR) or in the net assimilation rate (NAR) compared with the control plants. Tri-aromatic hydrocarbons were the most accumulated in tissues of the treated plants. Penta- and hexaaromatics, on the other hand, were undetectable in the WSF and consequently in the treated plants. A linear relationship was observed between the dose applied to plants and the amounts of tissue accumulated PAHs (r² = 0.515 for soil application and r² = 0.984 for foliar spray). In plants sprayed with 300 μg PAHs plant^-1, the total PAHs accumulated were more than that accumulated in plants treated through soil application. These findings suggest that: aqueous extraction of crude oil tends to signify the percentage of the low molecular weight PAHs, e.g. naphthalene, to the total PAHs; disturbed stomatal behavior in the first day of the treatment may be due to the venting of the volatile low molecular weight aromatic hydrocarbons (e.g. benzene, toluene, and xylenes) through the stomata; and uptake of water-soluble hydrocarbons by plants is equally possible through both of the root system and the foliage. The ecological implications of these finding are discussed in relation to oil pollution of mangrove stands under field conditions.