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 the average air temperature - in precipitation - in precipitation - in winds etc. - 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 – 5.5 ο C between the years increase in average air temperature from 1.5 – 5.5 ο C between the years  Parallel impacts: -distortion in the movement of barometric systems -distortion in the movement of barometric systems - distortion in the hydrologic cycle - distortion in the hydrologic cycle - ice melting - ice melting - increase in the frequency of extreme incidents - increase in the frequency of extreme incidents

Climate change aspects affecting agriculture  Increase in air temperature  Increase in CO 2 -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 [CO 2 ] and plant physiology  Positive effects on photosynthesis

Increase in [CO 2 ] and plant physiology  Decrease in photorespiration (only in C 3 -plants)  Increase in net photosynthesis Namely, increase in overall carbon balance

Increase in [CO 2 ] 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 [CO 2 ] 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 [CO 2 ]-effects on photosynthesis

Combined temperature and [CO 2 ] - effects

Water shortage effects  Less intense under higher [CO 2 ]  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 [CO 2 ] on C 3 and C 4 plants:  more beneficial to C 3 plants - 14 of the 18 “worlds worst weeds” are C 4 plants - 14 of the 18 “worlds worst weeds” are C 4 plants - 12 of the 15 major crops are C 3 plants - 12 of the 15 major crops are C 3 plants  perennial weeds more difficult to control due to the stimulation of growth of rhizomes and storage organs

Ranges of increase in biomass of C 3 and C 4 weeds and crops grown under doubled [CO 2 ] (Patterson, 1995) C-assimilation pathway WeedsCrops C3C3C3C3 95 – 272 %107 – 494 % C4C4C4C4 56 – 161 %98 – 182 %

Reasons for change in the weed / crop competition patterns  Differential effects of temperature increase (2 to 5 o C) 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 [CO 2 ] 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 [CO 2 ], temperature, and water availability on chemical efficacy  Adjustment of the irrigation/fertilization schemes

Conclusions  The increase in [CO 2 ] 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 local weed flora - the C-assimilation pathway and the temperature requirements of the crop and weed species - 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.

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