Μικρομετεωρολογικές συνθήκες και δυνατότητα απορρύπανσης του περιβάλλοντος με καλλιεργούμενα φυτά: η περίπτωση της 3,4-διχλωροανιλίνης

Abstract

The massive use of herbicides in intensive agriculture has caused serious problems as regards to the lowering of the quality of the components of the environment because of its contamination. The application of herbicides for a long period, from which chloroanilines are produced and especially 3,4-dichloroaniline (3,4-DCA), make obvious the noticeable aggravation of the soil with this substance in the regions where the herbicides are applied. Purpose of this work is the investigation of the possibilities of cultivated plants to be used on the remediation of contaminated soils. Two and three plant species of spring (maize, rice) and winter cereals (wheat, barley, triticale) were used, respectively. These species were grown hydroponically under controlled environmental conditions (temperature, relative humidity, light intensity, photoperiod), for the elimination of the effects of soil parameters which constrain the root uptake of 3,4-DCA, in order to obtain a better evaluation on the remediation potential with the use of the aforementioned plant species. At the same time, the effect of increased concentration of 3,4-DCA was investigated on its uptake ability for a broadleaf plant species (cowpea) under constant growth conditions. Also, a comparative examination on the sorption capacity of 3,4-DCA, took place for typical Greek agricultural soils with distinct properties. Specifically, the examined soil types were a sandy clay loam (KAL), a clay (HM), a loam (KAR) and a silt clay (KOR) including river sand (AMMOS) for comparison. For the clarification of the role of soil sorption of 3,4-DCA on the ability of plants to absorb and metabolize this substance, the cowpea plant was selected which presented a remarkable capacity to uptake high concentrations of 3,4-DCA. This plant was developed in soils with significant differences as regards to their sorption capacity (soil types KAL and KOR) and in AMMOS. The analysis of the hydroponic experiments showed that all the experimental plant species (maize, rice, wheat, barley, triticale) removed remarkably large amount of 3,4-DCA from the growth solution. Taking into account the average recommended planting density, it was found that the expected removal of 3,4-DCA from the soil is greater in the case of triticale. It was furthermore found that the uptake of 3,4-DCA from the spring cereals (maize and rice) was accelerated significantly (almost doubling of the total plant uptake of 3,4-DCA) in low relative humidity, regardless of lighting conditions. The increased uptake of 3,4-DCA at low relative humidity was attributed to the increased transpiration of plants, taking into account that in the aforementioned conditions the plants removed significantly greater volume of solution (almost two-fold) compared to the conditions of high relative humidity. The growth of cowpea under high concentrations of 3,4-DCA (replenishment with the initial concentration of 3,4-DCA) resulted in the increase (almost three-fold) of its uptake, up to almost its complete removal from the solution, regardless of the rate of plant growth and transpiration. From the analysis of the sorption experiments and more specifically from the comparative examination of the ability of the soil types KAL, HM, KAR, KOR and AMMOS to absorb 3,4-DCA for a period of 48 hours (sorption equilibration time), it was demonstrated that the soil with the highest amount of organic matter (KAL) sorbed significantly greater amounts of 3,4-DCA in the whole range of its applied concentrations, followed by the soils HM, KAR, KOR and AMMOS. The correlation analysis of the sorbed quantity with basic soil properties confirmed the principal role of the organic matter as an influencing factor of the sorption capacity of soils. It was furthermore found that the percentage of sorbed amount of 3,4-DCA decreases with the increasing initial concentration and especially to the soil KAL which presents high sorption capacity. From this it can be concluded that the available sorption sites are not unlimited. Thus, the description of the phenomenon with the Freundlich equation was satisfactory and all sorption isotherms were of type L, given that the available sorption sites are limited and even more as the initial concentration of 3,4-DCA increases. The addition of a small amount of CaCO (1%) increased significantly the pH of the acidic soil types, KAL and KAR, resulting simultaneously in a significant reduction (50%) of their sorption capacity for 3,4-DCA, compared with the respective controls (without liming). A significant effect of the temperature increase (up to 35 0 C) was also noticed, which substantially improved the sorption capacity of the examined, in this case, soil types KAL, HM and KOR. The effect of temperature was more evident in the organic soil KAL, with a high sorption capacity, followed by the HM (intermediate sorption capacity, less organic) and the KOR (low sorption capacity, even less organic) soils. From the analysis of uptake and metabolism of 3,4-DCA in cowpea plants, grown in the soil types KAL, KOR, and AMMOS, it was demonstrated that sorption has an essential role on the availability of 3,4-DCA for its uptake by plants. This was strengthened by the fact that plants grown in soil with a high sorption capacity, KAL, absorb less amount of 3,4-DCA, compared to plants grown in KOR and AMMOS. The metabolism, however, took place in plants, either after the addition of 3,4-DCA when transplanting to the experimental soil, or after adding 3,4-DCA 48 hours before transplantation. It was furthermore demonstrated that significantly greater amounts of the metabolites of 3,4-DCA 3 were detected in the plants transplanted to soil in which the addition of 3,4-DCA was preceded by 48 hours. In this case, the ranking of the three metabolites, based on their concentration detected in the roots of the plants grown in soils KAL and KOR for 48 hours, followed the descending order MGA (conjugate of 3,4-DCA with malonic acid and glucose) , MA (conjugate of 3,4-DCA with malonic acid) and GA (conjugate of 3,4-DCA with glucose). These conjugates are not detected in the case of the 3,4-DCA, when transplanting plants in AMMOS, which was attributed to the possible deactivation of the metabolism mechanisms due to phytotoxicity. The metabolism of almost all absorbed 3,4DCA to the GA,

MA and MGA conjugates in the roots of cowpea showed that the plants are means of degradation and of removal of 3,4-DCA from the soil, and that they can be used in applications where phytoremediation of contaminated soil takes place as they bind and metabolize 3,4-DCA