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ŠUMARSKI LIST 5-6/2012 str. 50     <-- 50 -->        PDF

con­taminated site may be a reliable criterion for selection of tolerant genotype or species. For instance, using this criterion, Paspalum scrobiculatum L. and alfalfa are good candidates for remediation of crude oil contaminated sites (Kirk et al., 2002; Ogbo et al., 2009).
Proline content – Sadržaj prolina
Treatments with increasing crude oil concentrations af­fect­­ed proline accumulation in poplars with respect to control (Table 2). Proline content in poplars varied from 9.90 to 58.50 µg/g. Considerable changes were observed at 50 and 100 % treatment in all clones. Increased accumulation of free proline in plants exposed to crude oil contamination may be the consequence of disturbed water regime. Many organisms, including higher plants, accumulate free proline in response to osmotic stress due to drought, high salinity and chilling (Nanjo et al., 1999). Water availability could be critical since the oil-impregnated soil does not take up water homogeneously. As a consequence, water drains rapidly through the containers, and limiting water supply of the root system (Merkl et al., 2005a).
Nitrate reductase activity (NRA) – Aktivnost nitrat-reduktaze
Negative effect of soil contamination with crude oil on activity of nitrate-reductase was well pronounced in investigated poplar clones (Table 2.). A lack of considerable changes and significant stimulation were observed at 5 % treatment, respectively. Effect of further increase of oil content in soil mainly decreased activity of the enzyme in poplars.
Inhibition of nitrate reductase activity was reported previously in other plant species. In Amaranthus hybridusplants treated with engine oil, decrease of the activity was proportional to applied oil concentrations (Odjegba and Atebe, 2007). Nitrate reductase catalyses the first step in nitrate assimilation, the reduction of nitrate to nitrite, which has been considered as the rate-limiting step of this metabolic pathway (Campbell, 1999). Inactivation of nitrate reducatse occurs in response to stress conditions including the loss of light, a decrease in CO2 levels, an increase in cytosolic pH or variations in photosynthetic activity (Kaiser et al., 1999). Nitrate reductase was the first recognized substrate inducible enzyme in plants (Lexa et al. 2002). Therefore, changes in availability of essential nutrients, particularly nitrate, can also cause changes in gene expression and nitrate reductase activity. Increased oil levels caould decrease total nitrogen and available nitrate due to their temporal immobilization by microbes (Agbogidi et al., 2007). Furthermore, the inhibitory effect of oil could partly be attributed to the toxic effect of some of its constituents on the enzyme activity (Odjegba and Atebe, 2007). For instance, polycyclic aroma­tic hydrocarbons (PAH) present a toxic and recalcitrant com­pounds of engine oil (Wang et al., 2000).
Photosynthetic parameters – Fotosintetski parametri
Rate of photosynthesis (Table 3) in poplar clones affected by oil contamination was either increased or unchanged with respect to control. Contrary to expectations, the 100% treatment stimulated photosynthetic rate in poplars. Light energy absorbed by chlorophyll molecules can be used to drive photosynthesis (photochemistry), while excess energy can be dissipated as heat or it can be re-emitted as light (chlorophyll fluorescence), and these three processes occur in competition (Pellegrini et al., 2010).
The chlorophyll fluorescence parameter Fv/Fm (Table 3.) has been indicator of the maximum quantum efficiency of PSII photochemistry (Butler, 1978). In healthy plants, this value ranges between 0.800 and 0.860 (Björkman and Demming, 1987). Maximum quantum yield of PSII was not considerably affected by crude oil pollution in poplar clone ´Pannonia´, while in clone ´Bora´ and clone ´9111/93´ Fv/Fm was significantly changed at 75 and 100% treatment (Table 4.). Chlorophyll fluorescence has been a useful diagnostic tool for the assessment of plant stress and photosynthesis rate (Krause and Weis, 1991). Results of Ralph and Burchett (1998) showed that PSII photochemical efficiency (Fv/Fm ratio)