Assessing the impact of increasing carbon dioxide and temperature on crop-weed interactions for tomato and a C3 and C4 weed species

Based on the carboxylation kinetics of the C3and C4photosynthetic pathway, it is anticipated that C3crops may be favored over C4weeds as atmospheric CO2increases. In the current study, tomato (Lyco-persicon esculentum), a C3crop species, was grown at ambient ( 400 mol mol−1) and enhanced carbondioxide ( 800 mol mol−1) with and without two common weeds, lambsquarters (Chenopodium album), aC3weed, and redroot pigweed (Amaranthus retroflexus), a C4weed, from seedling emergence until mutualshading of crop-weed leaves. Because growth temperature is also likely to change in concert with risingCO2, the experiment was repeated at day/night temperatures of 21/12 and 26/18◦C. For both day/nighttemperatures, elevated CO2exacerbated weed competition from both the C3and C4weed species. Amodel based on relative leaf area following emergence was used to calculate potential crop losses fromweeds. This analysis indicated that potential crop losses increased from 33 to 55% and from 32 to 61%at the 21/12 and 26/18◦C day/night temperatures, for ambient and elevated CO2, respectively. For thecurrent study, reductions in biomass and projected yield of tomato appeared independent of the photo-synthetic pathway of the competing weed species. This may be due to inherent variation and overlap in thegrowth response of C3and C4species, whether weeds or crops, to increasing CO2concentration. Overall,these results suggest that as atmospheric CO2and/or temperature increases, other biological interac-tions, in addition to photosynthetic pathway, deserve additional consideration in predicting competitiveoutcomes between weeds and crops.