Title: Quantifying the effect of drought on carbon dioxide-induced changes in competition between a C3 crop (tomato) and a C4 weed (Amaranthus retroflexus)
Abstract: Recent and projected increases in atmospheric carbon dioxide concentration ([CO2]) and subsequent effects on climate are likely to alter competitive outcomes of weeds and crops. Rising [CO2] per se could increase the competitive ability of C3 crops relative to C4 weeds. However, such an outcome may depend on other climatic variables. In this study, tomato, a C3 crop species, was grown from emergence to anthesis using replacement series mixtures with Amaranthus retroflexus, a C4 weed species at three different [CO2], 400, 600 and 800 μmol mol−1, with and without water stress. Under well-watered conditions, leaf photosynthetic rates and plant height, leaf area and biomass all increased with elevated [CO2] for tomato relative to A. retroflexus, consistent with the kinetics of C3 photosynthesis. However, if water was limiting, a significant positive effect of [CO2] was noted for plant height and biomass of A. retroflexus with increased competition. This result may be related to a greater increase in leaf water potential with rising [CO2] for A. retroflexus relative to tomato under water stress. Overall, these are the first data to suggest that increases in atmospheric CO2 could still exacerbate crop losses from a C4 weed, even with a C3 crop, if drought occurs. [VALERIO, M., TOMECEK, M. B., LOVELLI, S. and ZISKA, L. H. (2011), Quantifying the effect of drought on carbon dioxide-induced changes in competition between a C3 crop (tomato) and a C4 weed (Amaranthus retroflexus). Weed Research. doi: 10.1111/j.1365-3180.2011.00874.x]