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

Authors: VALERIO, MARIA - University Of Basilicata; Tomecek, Martha; LOVELLI, STELLA - University Of Basilicata; Ziska, Lewis

Submitted to: Weed Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/19/2011
Publication Date: 7/13/2011
Citation: Valerio, M., Tomecek, M.B., Lovelli, S., 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. 51(6):591-600. DOI:10.1111/J.1365-3180.2011.00874.

Interpretive Summary: Globally, weeds remain the largest detriment to achieving maximum crop yields. Although carbon dioxide is the sole source of carbon for plant growth, recent and projected increases in atmospheric carbon dioxide concentration ([CO2]) associated with climate change are likely to stimulate the growth of weeds and crops differentially. Part of this difference may reflect photosynthetic pathway, where many crops have one kind of pathway (C3) that should respond to rising CO2 more than many weeds which have a pathway (C4) that should respond less to rising CO2 concentration. However, it is unclear if other climatic variables, likely to change with rising CO2 will alter the projected response based on photosynthetic pathway alone. In this study, tomato, a C3 crop, was grown from emergence to anthesis at different ratios of redroot pigweed, a C4 weed, at three different concentrations of carbon dioxide anticipated for this century, 400, 600 and 800 parts per million (ppm), with and without water stress. Under well-watered conditions, leaf photosynthetic rates and plant height, leaf area and biomass all increased with elevated carbon dioxide for tomato relative to pigweed, consistent with what is known about the different photosynthetic pathways. However, if water was limiting, a significant positive effect of carbon dioxide was noted for plant height and biomass of pigweed with increased competition. This result may be related to a greater increase in leaf water potential with rising carbon dioxide for pigweed relative to tomato under water stress. Overall, these are the first data to suggest that increases in atmospheric carbon dioxide could still exacerbate crop losses from a C4 weed in competition with a C3 crop, if drought occurs. This information will be of interest to growers, weed scientists, and land managers.

Technical 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 redroot pigweed (Amaranthus retroflexus L.), 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.