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Possible effects of climate change on crop-weed interactions Professor Andreas J. Karamanos Faculty of Crop Science Agricultural University of Athens Definition of climate change the long-term change in the average weather conditions prevailing in a region they are expressed as changes: - in the average air temperature - in precipitation - in winds etc. Causes External ( changes in solar or cosmic radiation, earth’s orbit etc.) Human activities (greenhouse gases) Greenhouse effect Global warming: - increase in average air temperature from 1.5 – 5.5οC between the years1990-2010 Parallel impacts: -distortion in the movement of barometric systems - distortion in the hydrologic cycle - ice melting - increase in the frequency of extreme incidents Climate change aspects affecting agriculture Increase in air temperature Increase in CO2-concentration Change in the amount and distribution of precipitation Change in evapotranspiration Increase in the frequency of extreme weather incidents (droughts, floods etc.) Precipitation/evapotranspiration forecasts in the Mediterranean Time-series of rainfall intensity in Athens Increase in [CO2] and plant physiology Positive effects on photosynthesis Increase in [CO2] and plant physiology Decrease in photorespiration (only in C3-plants) Increase in net photosynthesis Namely, increase in overall carbon balance Increase in [CO2] and plant physiology Decrease in transpiration by 25-50% (partial stomatal closure) Increase in water use efficiency (WUE, g of dry matter / g of transpired water) Elevated [CO2] and WUE Increase in temperature and plant physiology Acceleration of most physiological processes Increase in temperature and plant physiology Increase in respiratory dry matter consumption Increase in temperature and plant traits Decrease in average grain weight of temperate crops Increase in temperature and plant traits Increase in growth rate Effects of temperature on phenology Increase in temperature and plant traits Vernalization in temperate crops Combined temperature and [CO2]-effects on photosynthesis Combined temperature and [CO2] - effects Water shortage effects Less intense under higher [CO2] Evident on all physiological processes Different degrees of sensitivity among physiological processes Plant differentiation according to their adaptability to drought Reasons for change in the weed / crop competition patterns Differential effects of [CO2] on C3 and C4 plants: more beneficial to C3 plants - 14 of the 18 “worlds worst weeds” are C4 plants - 12 of the 15 major crops are C3 plants perennial weeds more difficult to control due to the stimulation of growth of rhizomes and storage organs Ranges of increase in biomass of C3 and C4 weeds and crops grown under doubled [CO2] (Patterson, 1995) C-assimilation pathway C3 Weeds Crops 95 – 272 % 107 – 494 % C4 56 – 161 % 98 – 182 % Reasons for change in the weed / crop competition patterns Differential effects of temperature increase (2 to 5oC) on weeds and crops: - expansion of warm season weeds to higher latitudes and cooler areas (e.g. Cassia, Amaranthus, Sesbania, Crotalaria, Rottboellia, Imperata, Panicum, Striga, etc.) - competitive advantage (higher growth rates) of warm season weeds against temperate crops Reasons for change in the weed / crop competition patterns Differential effects of water availability to crops and weeds: - less severe water stress-effects in raised [CO2] levels (reduction in transpiration, development of osmotic adjustment, higher root / shoot ratio) - they depend on the drought resistance mechanisms to be developed by crops and weeds Interventions on weed control Choice of the appropriate crop species/cultivar Adjustment of planting time/season Possible effects of the elevated [CO2], temperature, and water availability on chemical efficacy Adjustment of the irrigation/fertilization schemes Conclusions The increase in [CO2] and temperature will affect the weed-crop competitive interactions. Water availability might be a less decisive factor. The effects on crop production will depend on: - the local weed flora - the C-assimilation pathway and the temperature requirements of the crop and weed species The interacting effects of many factors influence weed/crop competition in a complex way and make an overall prediction on the consequences on crop production risky. Thank you for your attention