Quantifying the effect of drought on carbon dioxide‐induced changes in competition between a C₃ crop (tomato) and a C₄ weed (<i>Amaranthus retroflexus</i>)

Abstract

V alerio M, T omecek MB, L ovelli S &amp; Z iska LH (2011). Quantifying the effect of drought on carbon dioxide‐induced changes in competition between a C 3 crop (tomato) and a C 4 weed ( Amaranthus retroflexus ). Weed Research 51 , 591–600. Summary Recent and projected increases in atmospheric carbon dioxide concentration ([CO 2 ]) and subsequent effects on climate are likely to alter competitive outcomes of weeds and crops. Rising [CO 2 ] per se could increase the competitive ability of C 3 crops relative to C 4 weeds. However, such an outcome may depend on other climatic variables. In this study, tomato, a C 3 crop species, was grown from emergence to anthesis using replacement series mixtures with Amaranthus retroflexus , a C 4 weed species at three different [CO 2 ], 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 [CO 2 ] for tomato relative to A. retroflexus , consistent with the kinetics of C 3 photosynthesis. However, if water was limiting, a significant positive effect of [CO 2 ] 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 [CO 2 ] for A. retroflexus relative to tomato under water stress. Overall, these are the first data to suggest that increases in atmospheric CO 2 could still exacerbate crop losses from a C 4 weed, even with a C 3 crop, if drought occurs.