The effect of C2-chlorohydrocarbons and their phytotoxic metabolites on photosynthesis of crop plants and natural vegetation

Abstract

The effect of C2-chlorohydrocarbons and their phytotoxic metabolites on photosynthesis of crop plants and natural vegetation: The emission of the highly volatile ~-chlorohydrocarbons tetrachloroethylene (TECE) and 1, 1, 1-trichloroethane (TCE) into the atmosphere, and their subsequent photochemical or biological degradation to trichloroacetic acid (TCA), proceeds predominantly in industrialised countries. Due to continued industrialisation, increasing use of TECE and TCE as solvent and cleaning agent in the metal and textile industry leads to the build-up of these chloroorganic substances in the atmosphere. Increased urban activities and veldfire catastrophes increase the oxidation potential of the atmosphere which leads to an increase in atmogenically produced TCA in the regions on the lee side of the emitting source. The uptake of TECE into the plant mesophyll may proceed directly via the stomata or after accumulation in the cutide, leading to inhibition of growth. On the other hand, TCA can enter the plant either through the stomata, or via the root system after deposition from the atmosphere onto the ground and dissolving in ground- or rainwater. The vegetation of semi arid and arid zones are affected in particular. As formation of phytotoxic TCA poses a serious threat to sensitive ecosystems such as the grasslands of all continents, determination of its effect is of great scientific significance. The present study falls within the scope of the research project on the Scientific and Technical Cooperation agreement between South Africa and Germany on the ecotoxicological effect of Cr chlorohydrocarbons and the deposition of their phytotoxic metabolites on the vegetation around the important industrial region of Johannesburg (EVA project). Crop plants, representing ½ and C4 crop plants, namely Phaseolus vulgaris and Zea mays respectively, \Nere grown under controlled conditions and exposed to a range of TCA concentrations, as v.,ell as to TECE vapour. The effect of these treatments on the photosynthetic metabolism of the plants were determined using the following techniques: - Fast phase kinetics of chlorophyll a fluorescence for assessing the effects on the primary processes of photochemistry and assessing plant vitality; - Infra red gas analysis to determine the effects on photosynthetic gas exchange by quantifying the important parameters of CO2 assimilation; and - In vivo assessment of the activity of the key-enzyme for CO2 fixation, Rubisco, including Western-blot analysis.

It could be shown that the sensitivity of P. vulgaris (C:3) and Z. mays (C4) towards TCA and TECE differed. The physiological and biochemical effect on these plants also differ. It is postulated that the difference in response is due to the large morphological and biochemical differences between C:3 and C4 plants. Low concentrations of TCA had a stimulatory effect on the growth and metabolism of P. vu/garis, but inhibition occurred at higher concentrations as a result of limitation of mesophyll processes, i.e. primary photochemistry and/or enzymatic reactions, as was deduced from A:~ response curves, the JIP-test and determination of Rubisco-activity. No initial stimulation occurred in Z. mays, and inhibition was evident even at low levels of TCA exposure, as a result of mesophyll limitation. In P. vulgaris treatment with TECE vapour was responsible for inhibition of mesophyll processes. Marked decreases were evident in efficiency of CO2 fixation, as shown by the decrease in initial slope of A:Ci response curves. However, the overall vitality of the plant was not markedly affected, according to the values of PIAss obtained by the JIP-test. Z. mays seemed to be more resistant to TECE, showing no marked effects due to the treatment. This was probably because of its specialised anatomy and CO2 concentrating mechanism. Two year old pine needles were utilised as bioindicator to assess the occurrence of TCA in natural vegetation over presumed pollution gradients in South Africa. The vitality of the needles was determined in parallel by means of fast fluorescence kinetics and the JIP-test to assess the effect of TCA. A direct correlation existed between TCA content and vitality of the needles. For the transect stretching from the highly industrialised areas of Gauteng in South Africa northwest through Upington to the coast of Namibia, no clear relationship between vitality and TCA content existed, which was probably due to the variation in environmental factors. A direct correlation between the TCA content and plant vitality could be established for the transect from Potchefstroom to Sasolburg. This indicated that TCA could be responsible for the decrease in vitality of natural vegetation. Also it was clear that the levels of TCA in plants near highly industrialised areas like Sasolburg are much higher than in rural areas. Crchlorohydrocarbons like TECE and compounds like TCA, have very definite effects on the photosynthetic metabolism of both crop plants and natural vegetation, and when these effects act in combination with additional environmental factors like drought, it could lead to severe damage and possible desertification in sensitive ecosystems like the grasslands of the world.